GDB Internals

A guide to the internals of the GNU debugger

John Gilmore Cygnus Solutions Second Edition: Stan Shebs Cygnus Solutions

Cygnus Solutions Revision: 1.127 TEXinfo 2.257

Copyright \begin{matrix}c \\ ◯ \end{matrix} 1990-1999 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Scope of this Document

This document documents the internals of the GNU debugger, GDB. It includes description of GDB’s key algorithms and operations, as well as the mechanisms that adapt GDB to specific hosts and targets.

1 Requirements

Before diving into the internals, you should understand the formal requirements and other expectations for GDB. Although some of these may seem obvious, there have been proposals for GDB that have run counter to these requirements.

First of all, GDB is a debugger. It’s not designed to be a front panel for embedded systems. It’s not a text editor. It’s not a shell. It’s not a programming environment.

GDB is an interactive tool. Although a batch mode is available, GDB’s primary role is to interact with a human programmer.

GDB should be responsive to the user. A programmer hot on the trail of a nasty bug, and operating under a looming deadline, is going to be very impatient of everything, including the response time to debugger commands.

GDB should be relatively permissive, such as for expressions. While the compiler should be picky (or have the option to be made picky), since source code lives for a long time usually, the programmer doing debugging shouldn’t be spending time figuring out to mollify the debugger.

GDB will be called upon to deal with really large programs. Executable sizes of 50 to 100 megabytes occur regularly, and we’ve heard reports of programs approaching 1 gigabyte in size.

GDB should be able to run everywhere. No other debugger is available for even half as many configurations as GDB supports.

2 Overall Structure

GDB consists of three major subsystems: user interface, symbol handling (the “symbol side”), and target system handling (the “target side”).

Ther user interface consists of several actual interfaces, plus supporting code.

The symbol side consists of object file readers, debugging info interpreters, symbol table management, source language expression parsing, type and value printing.

The target side consists of execution control, stack frame analysis, and physical target manipulation.

The target side/symbol side division is not formal, and there are a number of exceptions. For instance, core file support involves symbolic elements (the basic core file reader is in BFD) and target elements (it supplies the contents of memory and the values of registers). Instead, this division is useful for understanding how the minor subsystems should fit together.

2.1 The Symbol Side

The symbolic side of GDB can be thought of as “everything you can do in GDB without having a live program running”. For instance, you can look at the types of variables, and evaluate many kinds of expressions.

2.2 The Target Side

The target side of GDB is the “bits and bytes manipulator”. Although it may make reference to symbolic info here and there, most of the target side will run with only a stripped executable available – or even no executable at all, in remote debugging cases.

Operations such as disassembly, stack frame crawls, and register display, are able to work with no symbolic info at all. In some cases, such as disassembly, GDB will use symbolic info to present addresses relative to symbols rather than as raw numbers, but it will work either way.

2.3 Configurations

Host refers to attributes of the system where GDB runs. Target refers to the system where the program being debugged executes. In most cases they are the same machine, in which case a third type of Native attributes come into play.

Defines and include files needed to build on the host are host support. Examples are tty support, system defined types, host byte order, host float format.

Defines and information needed to handle the target format are target dependent. Examples are the stack frame format, instruction set, breakpoint instruction, registers, and how to set up and tear down the stack to call a function.

Information that is only needed when the host and target are the same, is native dependent. One example is Unix child process support; if the host and target are not the same, doing a fork to start the target process is a bad idea. The various macros needed for finding the registers in the upage, running ptrace, and such are all in the native-dependent files.

Another example of native-dependent code is support for features that are really part of the target environment, but which require #include files that are only available on the host system. Core file handling and setjmp handling are two common cases.

When you want to make GDB work “native” on a particular machine, you have to include all three kinds of information.

3 Algorithms

GDB uses a number of debugging-specific algorithms. They are often not very complicated, but get lost in the thicket of special cases and real-world issues. This chapter describes the basic algorithms and mentions some of the specific target definitions that they use.

3.1 Frames

A frame is a construct that GDB uses to keep track of calling and called functions. FRAME_FP in the machine description has no meaning to the machine-independent part

of GDB, except that it is used when setting up a new frame from scratch, as follows:

           create_new_frame (read_register (FP_REGNUM), read_pc ()));

Other than that, all the meaning imparted to FP_REGNUM is imparted by the machinedependent code. So, FP_REGNUM can have any value that is convenient for the code that creates new frames. (create_new_frame calls INIT_EXTRA_FRAME_INFO if it is defined; that is where you should use the FP_REGNUM value, if your frames are nonstandard.)

Given a GDB frame, define FRAME_CHAIN to determine the address of the calling function’s frame. This will be used to create a new GDB frame struct, and then INIT_EXTRA_ FRAME_INFO and INIT_FRAME_PC will be called for the new frame.

3.2 Breakpoint Handling

In general, a breakpoint is a user-designated location in the program where the user wants to regain control if program execution ever reaches that location.

There are two main ways to implement breakpoints; either as “hardware” breakpoints or as “software” breakpoints.

Hardware breakpoints are sometimes available as a builtin debugging features with some chips. Typically these work by having dedicated register into which the breakpoint address may be stored. If the PC ever matches a value in a breakpoint registers, the CPU raises an exception and reports it to GDB. Another possibility is when an emulator is in use; many emulators include circuitry that watches the address lines coming out from the processor, and force it to stop if the address matches a breakpoint’s address. A third possibility is that the target already has the ability to do breakpoints somehow; for instance, a ROM monitor may do its own software breakpoints. So although these are not literally “hardware breakpoints”, from GDB’s point of view they work the same; GDB need not do nothing more than set the breakpoint and wait for something to happen.

Since they depend on hardware resources, hardware breakpoints may be limited in number; when the user asks for more, GDB will start trying to set software breakpoints.

Software breakpoints require GDB to do somewhat more work. The basic theory is that GDB will replace a program instruction a trap, illegal divide, or some other instruction that will cause an exception, and then when it’s encountered, GDB will take the exception and stop the program. When the user says to continue, GDB will restore the original instruction, single-step, re-insert the trap, and continue on.

Since it literally overwrites the program being tested, the program area must be writeable, so this technique won’t work on programs in ROM. It can also distort the behavior of programs that examine themselves, although the situation would be highly unusual.

Also, the software breakpoint instruction should be the smallest size of instruction, so it doesn’t overwrite an instruction that might be a jump target, and cause disaster when the program jumps into the middle of the breakpoint instruction. (Strictly speaking, the breakpoint must be no larger than the smallest interval between instructions that may be jump targets; perhaps there is an architecture where only even-numbered instructions may jumped to.) Note that it’s possible for an instruction set not to have any instructions usable for a software breakpoint, although in practice only the ARC has failed to define such an instruction.

The basic definition of the software breakpoint is the macro BREAKPOINT.

Basic breakpoint object handling is in ‘breakpoint.c’. However, much of the interesting breakpoint action is in ‘infrun.c’.

3.3 Single Stepping

3.4 Signal Handling

3.5 Thread Handling

3.6 Inferior Function Calls

3.7 Longjmp Support

GDB has support for figuring out that the target is doing a longjmp and for stopping at the target of the jump, if we are stepping. This is done with a few specialized internal breakpoints, which are visible in the maint info breakpoint command.

To make this work, you need to define a macro called GET_LONGJMP_TARGET, which will examine the jmp_buf structure and extract the longjmp target address. Since jmp_buf is target specific, you will need to define it in the appropriate ‘tm-xyz.h’ file. Look in ‘tm-sun4os4.h’ and ‘sparc-tdep.c’ for examples of how to do this.

4 User Interface

GDB has several user interfaces. Although the command-line interface is the most common and most familiar, there are others.

4.1 Command Interpreter

The command interpreter in GDB is fairly simple. It is designed to allow for the set of commands to be augmented dynamically, and also has a recursive subcommand capability, where the first argument to a command may itself direct a lookup on a different command list.

For instance, the set command just starts a lookup on the setlist command list, while

set thread recurses to the set_thread_cmd_list.

To add commands in general, use add_cmd. add_com adds to the main command list, and should be used for those commands. The usual place to add commands is in the _initialize_xyz routines at the ends of most source files.

4.2 Console Printing

4.3 TUI

4.4 libgdb

libgdb was an abortive project of years ago. The theory was to provide an API to GDB’s functionality.

5 Symbol Handling

Symbols are a key part of GDB’s operation. Symbols include variables, functions, and types.

5.1 Symbol Reading

GDB reads symbols from “symbol files”. The usual symbol file is the file containing the program which GDB is debugging. GDB can be directed to use a different file for symbols (with the symbol-file command), and it can also read more symbols via the “add-file” and “load” commands, or while reading symbols from shared libraries.

Symbol files are initially opened by code in ‘symfile.c’ using the BFD library. BFD identifies the type of the file by examining its header. symfile_init then uses this identification to locate a set of symbol-reading functions.

Symbol reading modules identify themselves to GDB by calling add_symtab_fns during their module initialization. The argument to add_symtab_fns is a struct sym_fns which contains the name (or name prefix) of the symbol format, the length of the prefix, and pointers to four functions. These functions are called at various times to process symbolfiles whose identification matches the specified prefix.

The functions supplied by each module are:

xyz_symfile_init(struct sym_fns *sf)

Called from symbol_file_add when we are about to read a new symbol file. This function should clean up any internal state (possibly resulting from halfread previous files, for example) and prepare to read a new symbol file. Note that the symbol file which we are reading might be a new "main" symbol file, or might be a secondary symbol file whose symbols are being added to the existing symbol table. The argument to xyz_symfile_init is a newly allocated struct sym_fns whose bfd field contains the BFD for the new symbol file being read. Its private field has been zeroed, and can be modified as desired. Typically, a struct of private information will be malloc’d, and a pointer to it will be placed in the private field. There is no result from xyz_symfile_init, but it can call error if it detects an unavoidable problem.

xyz_new_init()

Called from symbol_file_add when discarding existing symbols. This function need only handle the symbol-reading module’s internal state; the symbol table data structures visible to the rest of GDB will be discarded by symbol_file_ add. It has no arguments and no result. It may be called after xyz_symfile_ init, if a new symbol table is being read, or may be called alone if all symbols are simply being discarded.

xyz_symfile_read(struct sym_fns *sf, CORE_ADDR addr, int mainline)

Called from symbol_file_add to actually read the symbols from a symbol-file into a set of psymtabs or symtabs. sf points to the struct sym fns originally passed to xyz_sym_init for possible initialization. addr is the offset between the file’s specified start address and its true address in memory. mainline is 1 if this is the main symbol table being read, and 0 if a secondary symbol file (e.g. shared library or dynamically loaded file) is being read.

In addition, if a symbol-reading module creates psymtabs when xyz symfile read is called, these psymtabs will contain a pointer to a function xyz_psymtab_to_symtab, which can be called from any point in the GDB symbol-handling code.

xyz_psymtab_to_symtab (struct partial_symtab *pst)

Called from psymtab_to_symtab (or the PSYMTAB TO SYMTAB macro) if the psymtab has not already been read in and had its pst->symtab pointer set. The argument is the psymtab to be fleshed-out into a symtab. Upon return, pst->readin should have been set to 1, and pst->symtab should contain a pointer to the new corresponding symtab, or zero if there were no symbols in that part of the symbol file.

5.2 Partial Symbol Tables

GDB has three types of symbol tables.

• full symbol tables (symtabs). These contain the main information about symbols and addresses.

• partial symbol tables (psymtabs). These contain enough information to know when to read the corresponding part of the full symbol table.

• minimal symbol tables (msymtabs). These contain information gleaned from nondebugging symbols.

This section describes partial symbol tables. A psymtab is constructed by doing a very quick pass over an executable file’s debugging

information. Small amounts of information are extracted – enough to identify which parts of the symbol table will need to be re-read and fully digested later, when the user needs the information. The speed of this pass causes GDB to start up very quickly. Later, as the detailed rereading occurs, it occurs in small pieces, at various times, and the delay therefrom is mostly invisible to the user.

The symbols that show up in a file’s psymtab should be, roughly, those visible to the debugger’s user when the program is not running code from that file. These include external symbols and types, static symbols and types, and enum values declared at file scope.

The psymtab also contains the range of instruction addresses that the full symbol table would represent.

The idea is that there are only two ways for the user (or much of the code in the debugger) to reference a symbol:

• by its address (e.g. execution stops at some address which is inside a function in this file). The address will be noticed to be in the range of this psymtab, and the full

    symtab will be read in. find_pc_function, find_pc_line, and other find_pc_...

functions handle this.

• by its name (e.g. the user asks to print a variable, or set a breakpoint on a function). Global names and file-scope names will be found in the psymtab, which will cause the symtab to be pulled in. Local names will have to be qualified by a global name, or a file-scope name, in which case we will have already read in the symtab as we evaluated the qualifier. Or, a local symbol can be referenced when we are "in" a local scope, in which case the first case applies. lookup_symbol does most of the work here.

The only reason that psymtabs exist is to cause a symtab to be read in at the right moment. Any symbol that can be elided from a psymtab, while still causing that to happen, should not appear in it. Since psymtabs don’t have the idea of scope, you can’t put local symbols in them anyway. Psymtabs don’t have the idea of the type of a symbol, either, so types need not appear, unless they will be referenced by name.

It is a bug for GDB to behave one way when only a psymtab has been read, and another way if the corresponding symtab has been read in. Such bugs are typically caused by a psymtab that does not contain all the visible symbols, or which has the wrong instruction address ranges.

The psymtab for a particular section of a symbol-file (objfile) could be thrown away after the symtab has been read in. The symtab should always be searched before the psymtab, so the psymtab will never be used (in a bug-free environment). Currently, psymtabs are allocated on an obstack, and all the psymbols themselves are allocated in a pair of large arrays on an obstack, so there is little to be gained by trying to free them unless you want to do a lot more work.

5.3 Types

Fundamental Types (e.g., FT VOID, FT BOOLEAN).

These are the fundamental types that GDB uses internally. Fundamental types from the various debugging formats (stabs, ELF, etc) are mapped into one of these. They are basically a union of all fundamental types that gdb knows about for all the languages that GDB knows about.

Type Codes (e.g., TYPE CODE PTR, TYPE CODE ARRAY).

Each time GDB builds an internal type, it marks it with one of these types. The type may be a fundamental type, such as TYPE CODE INT, or a derived type, such as TYPE CODE PTR which is a pointer to another type. Typically, several FT * types map to one TYPE CODE * type, and are distinguished by other members of the type struct, such as whether the type is signed or unsigned, and how many bits it uses.

Builtin Types (e.g., builtin type void, builtin type char).

These are instances of type structs that roughly correspond to fundamental types and are created as global types for GDB to use for various ugly historical reasons. We eventually want to eliminate these. Note for example that builtin type int initialized in gdbtypes.c is basically the same as a TYPE CODE INT type that is initialized in c-lang.c for an FT INTEGER fundamental type. The difference is that the builtin type is not associated with any particular objfile, and only one instance exists, while c-lang.c builds as many TYPE CODE INT types as needed, with each one associated with some particular objfile.

5.4 Object File Formats

5.4.1 a.out

The ‘a.out’ format is the original file format for Unix. It consists of three sections: text, data, and bss, which are for program code, initialized data, and uninitialized data, respectively.

The ‘a.out’ format is so simple that it doesn’t have any reserved place for debugging information. (Hey, the original Unix hackers used ‘adb’, which is a machine-language debugger.) The only debugging format for ‘a.out’ is stabs, which is encoded as a set of normal symbols with distinctive attributes.

The basic ‘a.out’ reader is in ‘dbxread.c’.

5.4.2 COFF

The COFF format was introduced with System V Release 3 (SVR3) Unix. COFF files may have multiple sections, each prefixed by a header. The number of sections is limited.

The COFF specification includes support for debugging. Although this was a step forward, the debugging information was woefully limited. For instance, it was not possible to represent code that came from an included file.

The COFF reader is in ‘coffread.c’.

5.4.3 ECOFF

ECOFF is an extended COFF originally introduced for Mips and Alpha workstations. The basic ECOFF reader is in ‘mipsread.c’.

5.4.4 XCOFF

The IBM RS/6000 running AIX uses an object file format called XCOFF. The COFF sections, symbols, and line numbers are used, but debugging symbols are dbx-style stabs whose strings are located in the ‘.debug’ section (rather than the string table). For more information, see See section “Top” in The Stabs Debugging Format.

The shared library scheme has a clean interface for figuring out what shared libraries are in use, but the catch is that everything which refers to addresses (symbol tables and breakpoints at least) needs to be relocated for both shared libraries and the main executable. At least using the standard mechanism this can only be done once the program has been run (or the core file has been read).

5.4.5 PE

Windows 95 and NT use the PE (Portable Executable) format for their executables. PE is basically COFF with additional headers.

While BFD includes special PE support, GDB needs only the basic COFF reader.

5.4.6 ELF

The ELF format came with System V Release 4 (SVR4) Unix. ELF is similar to COFF in being organized into a number of sections, but it removes many of COFF’s limitations.

The basic ELF reader is in ‘elfread.c’.

5.4.7 SOM

SOM is HP’s object file and debug format (not to be confused with IBM’s SOM, which is a cross-language ABI).

The SOM reader is in ‘hpread.c’.

5.4.8 Other File Formats

Other file formats that have been supported by GDB include Netware Loadable Modules

(‘nlmread.c’.

5.5 Debugging File Formats

This section describes characteristics of debugging information that are independent of the object file format.

5.5.1 stabs

stabs started out as special symbols within the a.out format. Since then, it has been encapsulated into other file formats, such as COFF and ELF.

While ‘dbxread.c’ does some of the basic stab processing, including for encapsulated versions, ‘stabsread.c’ does the real work.

5.5.2 COFF

The basic COFF definition includes debugging information. The level of support is minimal and non-extensible, and is not often used.

5.5.3 Mips debug (Third Eye)

ECOFF includes a definition of a special debug format. The file ‘mdebugread.c’ implements reading for this format.

5.5.4 DWARF 1

DWARF 1 is a debugging format that was originally designed to be used with ELF in SVR4 systems.

The DWARF 1 reader is in ‘dwarfread.c’.

5.5.5 DWARF 2

DWARF 2 is an improved but incompatible version of DWARF 1.

The DWARF 2 reader is in ‘dwarf2read.c’.

5.5.6 SOM

Like COFF, the SOM definition includes debugging information.

5.6 Adding a New Symbol Reader to GDB

If you are using an existing object file format (a.out, COFF, ELF, etc), there is probably little to be done.

If you need to add a new object file format, you must first add it to BFD. This is beyond the scope of this document.

You must then arrange for the BFD code to provide access to the debugging symbols. Generally GDB will have to call swapping routines from BFD and a few other BFD internal routines to locate the debugging information. As much as possible, GDB should not depend on the BFD internal data structures.

For some targets (e.g., COFF), there is a special transfer vector used to call swapping routines, since the external data structures on various platforms have different sizes and layouts. Specialized routines that will only ever be implemented by one object file format may be called directly. This interface should be described in a file ‘bfd/libxyz.h’, which is included by GDB.

6 Language Support

GDB’s language support is mainly driven by the symbol reader, although it is possible for the user to set the source language manually.

GDB chooses the source language by looking at the extension of the file recorded in the debug info; .c means C, .f means Fortran, etc. It may also use a special-purpose language identifier if the debug format supports it, such as DWARF.

6.1 Adding a Source Language to GDB

To add other languages to GDB’s expression parser, follow the following steps:

Create the expression parser.

This should reside in a file ‘lang-exp.y’. Routines for building parsed expressions into a ‘union exp_element’ list are in ‘parse.c’.

Since we can’t depend upon everyone having Bison, and YACC produces parsers that define a bunch of global names, the following lines must be included at the top of the YACC parser, to prevent the various parsers from defining the same global names:

     #define yyparse lang_parse
     #define yylex lang_lex
     #define yyerror lang_error
     #define yylval lang_lval
     #define yychar lang_char
     #define yydebug lang_debug
     #define yypact lang_pact
     #define yyr1 lang_r1
     #define yyr2 lang_r2
     #define yydef lang_def
     #define yychk lang_chk
     #define yypgo lang_pgo
     #define yyact lang_act
     #define yyexca lang_exca
     #define yyerrflag lang_errflag
     #define yynerrs lang_nerrs

At the bottom of your parser, define a struct language_defn and initialize it with the right values for your language. Define an initialize_lang routine and have it call ‘add_language(lang_language_defn)’ to tell the rest of GDB that your language exists. You’ll need some other supporting variables and functions, which will be used via pointers from your lang_language_defn. See the declaration of struct language_defn in ‘language.h’, and the other ‘*-exp.y’ files, for more information.

Add any evaluation routines, if necessary If you need new opcodes (that represent the operations of the language), add them to the enumerated type in ‘expression.h’. Add support code for these operations in eval.c:evaluate_subexp(). Add cases for new opcodes in two

functions from ‘parse.c’: prefixify_subexp() and length_of_subexp().

These compute the number of exp_elements that a given operation takes up.

Update some existing code Add an enumerated identifier for your language to the enumerated type enum

language in ‘defs.h’.

Update the routines in ‘language.c’ so your language is included. These routines include type predicates and such, which (in some cases) are language dependent. If your language does not appear in the switch statement, an error is reported. Also included in ‘language.c’ is the code that updates the variable current_ language, and the routines that translate the language_lang enumerated identifier into a printable string. Update the function _initialize_language to include your language. This function picks the default language upon startup, so is dependent upon which languages that GDB is built for.

Update allocate_symtab in ‘symfile.c’ and/or symbol-reading code so that the language of each symtab (source file) is set properly. This is used to determine the language to use at each stack frame level. Currently, the language is set based upon the extension of the source file. If the language can be better inferred from the symbol information, please set the language of the symtab in the symbol-reading code. Add helper code to expprint.c:print_subexp() to handle any new expression opcodes you have added to ‘expression.h’. Also, add the printed representations of your operators to op_print_tab.

Add a place of call

Add a call to lang_parse() and lang_error in parse.c:parse_exp_1().

Use macros to trim code The user has the option of building GDB for some or all of the languages. If the user decides to build GDB for the language lang, then every file dependent on ‘language.h’ will have the macro _LANG_lang defined in it. Use #ifdefs to leave out large routines that the user won’t need if he or she is not using your language. Note that you do not need to do this in your YACC parser, since if GDB is not build for lang, then ‘lang-exp.tab.o’ (the compiled form of your parser) is not linked into GDB at all. See the file ‘configure.in’ for how GDB is configured for different languages.

Edit ‘Makefile.in’

Add dependencies in ‘Makefile.in’. Make sure you update the macro variables such as HFILES and OBJS, otherwise your code may not get linked in, or, worse yet, it may not get tarred into the distribution!

7 Host Definition

With the advent of autoconf, it’s rarely necessary to have host definition machinery anymore.

7.1 Adding a New Host

Most of GDB’s host configuration support happens via autoconf. It should be rare to need new host-specific definitions. GDB still uses the host-specific definitions and files listed below, but these mostly exist for historical reasons, and should eventually disappear.

Several files control GDB’s configuration for host systems:

‘gdb/config/arch/xyz.mh’

Specifies Makefile fragments needed when hosting on machine xyz. In particular, this lists the required machine-dependent object files, by defining ‘XDEPFILES=...’. Also specifies the header file which describes host xyz, by

defining XM_FILE= xm-xyz.h. You can also define CC, SYSV_DEFINE, XM_CFLAGS,
XM_ADD_FILES, XM_CLIBS, XM_CDEPS, etc.; see ‘Makefile.in’.

‘gdb/config/arch/xm-xyz.h’

(‘xm.h’ is a link to this file, created by configure). Contains C macro definitions describing the host system environment, such as byte order, host C compiler and library.

‘gdb/xyz-xdep.c’

Contains any miscellaneous C code required for this machine as a host. On most machines it doesn’t exist at all. If it does exist, put ‘xyz-xdep.o’ into the

          XDEPFILES line in ‘gdb/config/arch/xyz.mh’.

Generic Host Support Files

There are some “generic” versions of routines that can be used by various systems. These can be customized in various ways by macros defined in your ‘xm-xyz.h’ file. If these routines work for the xyz host, you can just include the generic file’s name (with ‘.o’, not

‘.c’) in XDEPFILES.

Otherwise, if your machine needs custom support routines, you will need to write routines that perform the same functions as the generic file. Put them into xyz-xdep.c, and put

xyz-xdep.o into XDEPFILES.
‘ser-unix.c’

This contains serial line support for Unix systems. This is always included, via the makefile variable SER_HARDWIRE; override this variable in the ‘.mh’ file to avoid it.

‘ser-go32.c’

This contains serial line support for 32-bit programs running under DOS, using the GO32 execution environment.

‘ser-tcp.c’

This contains generic TCP support using sockets.

7.2 Host Conditionals

When GDB is configured and compiled, various macros are defined or left undefined, to control compilation based on the attributes of the host system. These macros and their meanings (or if the meaning is not documented here, then one of the source files where they are used is indicated) are:

GDBINIT_FILENAME

The default name of GDB’s initialization file (normally ‘.gdbinit’).

MEM_FNS_DECLARED

Your host config file defines this if it includes declarations of memcpy and memset. Define this to avoid conflicts between the native include files and the declarations in ‘defs.h’.

NO_STD_REGS

This macro is deprecated.

NO_SYS_FILE

Define this if your system does not have a <sys/file.h>.

SIGWINCH_HANDLER

If your host defines SIGWINCH, you can define this to be the name of a function to be called if SIGWINCH is received.

SIGWINCH_HANDLER_BODY

Define this to expand into code that will define the function named by the

expansion of SIGWINCH_HANDLER.
ALIGN_STACK_ON_STARTUP

Define this if your system is of a sort that will crash in tgetent if the stack happens not to be longword-aligned when main is called. This is a rare situation, but is known to occur on several different types of systems.

CRLF_SOURCE_FILES

Define this if host files use \r\n rather than \n as a line terminator. This will cause source file listings to omit \r characters when printing and it will allow \r\n line endings of files which are "sourced" by gdb. It must be possible to open files in binary mode using O_BINARY or, for fopen, "rb".

DEFAULT_PROMPT

The default value of the prompt string (normally "(gdb) ").

DEV_TTY The name of the generic TTY device, defaults to "/dev/tty".

FCLOSE_PROVIDED

Define this if the system declares fclose in the headers included in defs.h. This isn’t needed unless your compiler is unusually anal.

FOPEN_RB Define this if binary files are opened the same way as text files.

GETENV_PROVIDED

Define this if the system declares getenv in its headers included in defs.h. This isn’t needed unless your compiler is unusually anal.

HAVE_MMAP

In some cases, use the system call mmap for reading symbol tables. For some machines this allows for sharing and quick updates.

HAVE_SIGSETMASK

Define this if the host system has job control, but does not define sigsetmask(). Currently, this is only true of the RS/6000.

HAVE_TERMIO

Define this if the host system has termio.h.

HOST_BYTE_ORDER

The ordering of bytes in the host. This must be defined to be either BIG_ENDIAN

or LITTLE_ENDIAN.
INT_MAX
INT_MIN
LONG_MAX
UINT_MAX

ULONG_MAX

Values for host-side constants.

ISATTY Substitute for isatty, if not available.

LONGEST This is the longest integer type available on the host. If not defined, it will

default to long long or long, depending on CC_HAS_LONG_LONG.
CC_HAS_LONG_LONG

Define this if the host C compiler supports “long long”. This is set by the configure script.

PRINTF_HAS_LONG_LONG

Define this if the host can handle printing of long long integers via the printf format directive “ll”. This is set by the configure script.

HAVE_LONG_DOUBLE

Define this if the host C compiler supports “long double”. This is set by the configure script.

PRINTF_HAS_LONG_DOUBLE

Define this if the host can handle printing of long double float-point numbers via the printf format directive “Lg”. This is set by the configure script.

SCANF_HAS_LONG_DOUBLE

Define this if the host can handle the parsing of long double float-point numbers via the scanf format directive directive “Lg”. This is set by the configure script.

LSEEK_NOT_LINEAR

Define this if lseek (n) does not necessarily move to byte number n in the file. This is only used when reading source files. It is normally faster to define

CRLF_SOURCE_FILES when possible.

L_SET This macro is used as the argument to lseek (or, most commonly, bfd seek). FIXME, should be replaced by SEEK SET instead, which is the POSIX equivalent.

MALLOC_INCOMPATIBLE

Define this if the system’s prototype for malloc differs from the ANSI definition.

MMAP_BASE_ADDRESS

When using HAVE MMAP, the first mapping should go at this address.

MMAP_INCREMENT

when using HAVE MMAP, this is the increment between mappings.

NEED_POSIX_SETPGID

Define this to use the POSIX version of setpgid to determine whether job control is available.

NORETURN If defined, this should be one or more tokens, such as volatile, that can be used in both the declaration and definition of functions to indicate that they never return. The default is already set correctly if compiling with GCC. This will almost never need to be defined.

ATTR_NORETURN

If defined, this should be one or more tokens, such as __attribute__((noreturn)), that can be used in the declarations of functions to indicate that they never return. The default is already set correctly if compiling with GCC. This will almost never need to be defined.

USE_GENERIC_DUMMY_FRAMES

Define this to 1 if the target is using the generic inferior function call code. See blockframe.c for more information.

USE_MMALLOC

GDB will use the mmalloc library for memory allocation for symbol reading if this symbol is defined. Be careful defining it since there are systems on which mmalloc does not work for some reason. One example is the DECstation, where its RPC library can’t cope with our redefinition of malloc to call mmalloc. When defining USE_MMALLOC, you will also have to set MMALLOC in the Makefile, to point to the mmalloc library. This define is set when you configure with –with-mmalloc.

NO_MMCHECK

Define this if you are using mmalloc, but don’t want the overhead of checking the heap with mmcheck. Note that on some systems, the C runtime makes calls to malloc prior to calling main, and if free is ever called with these pointers after calling mmcheck to enable checking, a memory corruption abort is certain to occur. These systems can still use mmalloc, but must define NO MMCHECK.

MMCHECK_FORCE

Define this to 1 if the C runtime allocates memory prior to mmcheck being called, but that memory is never freed so we don’t have to worry about it triggering a memory corruption abort. The default is 0, which means that mmcheck will only install the heap checking functions if there has not yet been any memory allocation calls, and if it fails to install the functions, gdb will issue a warning. This is currently defined if you configure using –with-mmalloc.

NO_SIGINTERRUPT

Define this to indicate that siginterrupt() is not available.

R_OK Define if this is not in a system .h file.

SEEK_CUR

SEEK_SET Define these to appropriate value for the system lseek(), if not already defined.

STOP_SIGNAL

This is the signal for stopping GDB. Defaults to SIGTSTP. (Only redefined for the Convex.)

USE_O_NOCTTY

Define this if the interior’s tty should be opened with the O NOCTTY flag. (FIXME: This should be a native-only flag, but ‘inflow.c’ is always linked in.)

USG Means that System V (prior to SVR4) include files are in use. (FIXME: This

symbol is abused in ‘infrun.c’, ‘regex.c’, ‘remote-nindy.c’, and ‘utils.c’

for other things, at the moment.) lint Define this to help placate lint in some situations.

volatile Define this to override the defaults of __volatile__or /**/.

8 GDB Overview

8.1 Libraries used by GDB

GDB relies on a number of libraries:

‘config/’ Configuration options shared by GDB and all of the libraries it uses. GDB has it’s own much more extensive configuration in ‘gdb/config’.

‘readline/’

Contains the -lreadline and -lhistory libraries for command-line processing. The -lreadline library handles command-line editing, terminal interface, keymap interfaces, and file completion; the -lhistory library handles history processing and history substitution using csh-style syntax. For more informa-

tion, see ‘readline/doc/hist.texi’ and ‘readline/doc/rlman.texi’.

‘bfd/’ BFD is a package which allows applications to use the same routines to operate on object files whatever the object file format. A new object file format can be supported simply by creating a new BFD back end and adding it to the library. BFD is split into two parts: the front end, and the back ends (one for each object file format).

• The front end of BFD provides the interface to the user. It manages memory and various canonical data structures. The front end also decides which back end to use and when to call back end routines.

• The back ends provide BFD its view of the real world. Each back end provides a set of calls which the BFD front end can use to maintain its canonical form. The back ends also may keep around information for their own use, for greater efficiency.

For more information on BFD, see ‘bfd/doc/bfd.texi’. For more information on the use of BFD in GDB, see ⟨undefined⟩ [BFD support for GDB], page ⟨undefined⟩.

‘opcodes/’

A collection of routines to parse/print machine-language instructions and arguments for a number of processors.

‘mmalloc/’

The gnu malloc() library.

‘sim/’ Simulators for a number of common microprocessors. Allows GDB to debug machine code for architectures for which no CPU is readily available or which are not yet capable of interacting with GDB directly. Generally used to simulate and/or debug embedded systems.

‘texinfo/’

Texinfo is a documentation system that uses a single source file to produce both online information and printed output. Most GDB-related documentation is produced using this system. For more information, see ‘texinfo/doc/texinfo.texi’.

‘libiberty/’

Contains the -liberty library of free software. It is a collection of subroutines used by various GNU programs, typically functions that are included in GNU libc, but not in certain vendor versions of libc. Example functions provided by

-liberty:

getopt get options from command line

obstack stacks of arbitrarily-sized objects

strerror error message strings corresponding to errno

strtol string-to-long conversion

strtoul string-to-unsigned-long conversion

8.2 GDB Directory Structure

The sources to GDB itself are currently stored in four subdirectories, all of which are used to build the final executable:

‘gdb/’ The main GDB sources as provided by the FSF. Changes to this directory have been kept as small as possible to minimize the effort of merging them with the FSF sources (though many changes still have been made).

‘gdb-next/’

Apple-specific additions to the GDB source base. These will eventually be merged into the ‘gdb/’ directory, but are currently kept separate to make them easier to manage.

‘gdb-4.14/’

GDB sources as modified by NeXT for gdb-4.14. These are nearly obsolete, but are kept around until I have merged all of the original NeXT changes into the new GDB source base.

‘gdb-next-4.14/’

Apple-specific additions to the (nearly obsolete) GDB 4.14 source base. These are kept around solely for the purpose of building GDB 4.14 binaries when necessary.

Until recently, it was possible to build both GDB 4.17 and the GDB 4.14/4.17 hybrid that shipped with DR1 from the same source tree. GDB 4.17 was built in ‘gdb’ and used files from ‘gdb-next’ and ‘gdb/’, in that order, and GDB 4.14 was built in ‘gdb-4.14’ and used the files from ‘gdb-next-4.14/’, ‘gdb-next/’, ‘gdb-4.14/’, and ‘gdb/’, also in that order.

As of January 8, I have stopped building GDB 4.14 along with GDB 4.17 from the same sources. I suspect GDB 4.14 will no longer build from these sources without modification, although I suspect the necessary changes would be relatively minor.

GDB also uses the following subdirectories:

‘include/’

Header files shared by GDB and all of the libraries it uses. These files typically will typically apply to either:

General operating-system functions (such as symbol reading, IEEE floats, etc.) for which GDB and it’s libraries want to have a common interface. Interfaces between GDB and one of its libraries (such as ‘remote-sim.h’

    and ‘libiberty.h’).
‘gdb/config/’

Contains GDB-specific configuration files. For more information see ⟨undefined⟩ [Config], page ⟨undefined⟩.

8.3 Overview of Source Files

8.3.1 Top Level

‘main.c’ Contains the GDB top-level interpreter. Parses command-line arguments, performs GDB initialization, and passes control to command-line interpreter.

‘maint.c’ Collection of utility functions for viewing/debugging the internal state of GDB.

‘top.c’ Top-level routine used by GDB. Evaluates commands, and provides a number of top-level functions and mechanisms to set general purpose variables.

‘command.c’

parse and evaluate gdb commands and command documentation

‘printcmd.c’

user commands for printing expressions and displaying memory

8.3.2 GDB Targets / Program Control

‘target.c’

Defines the target abstraction, used to encapsulate all communications between GDB and a target. See the comments in ‘target.h’ for more information.

‘exec.c’ Interface between executable files and the ’target’ abstraction. Allows GDB to inspect/analyze executable images, without necessarily having a debuggable process available.

‘corefile.c’

Interface between core images and the ’target’ abstraction. Allows GDB to inspect/analyze memory and register dumps from corefile images. Not used in GDB 4.14.

‘thread.c’

Interface between GDB and multiply-threaded subprocesses. Not currently used by either GDB 4.14 or GDB 4.17.

‘fork-child.c’

Code to create an inferior process on UNIX systems.

‘infcmd.c’

User-level commands for inspecting/controlling the state of process execution. Commands such as ’step’, ’next’, ’finish’, and ’continue’ go here.

‘inflow.c’

Handles terminal modes and signal handling for UNIX child processes.

‘infrun.c’

Target-independent code to control the execution of an inferior process. Handles breakpoints, signal handling, shared library handling (on some systems), as well as far too many other things. Contains wait_for_inferior, probably the hairiest function in all of GDB.

‘inftarg.c’

Interface between the GDB ’target’ abstraction and UNIX child processes. Many of the functions in the file are overridden (via macros) in the nextstep-* functions.

8.3.3 Types, Values, and Expressions

A type is the fundamental data structure in GDB for representing type information. Each type structure is associated with a particular object file, with the exception of a few pre-created type structures used for backwards compatibility with other parts of GDB. GDB provides a number of “fundamental” data types; more complex data types can be represented by nesting type structures within each other. See Section 8.3.3 [Types], page 20 for more information.

A value is the GDB data structure for representing both R- and L-values of any type. A value contains a pointer to a GDB type structure, as well as a region of memory containing the value’s contents (for an R-value) or address (for an L-value).

A expression is the GDB data structure for all expressions in all programming languages. Expressions can be parsed and evaluated interactively according to the current language syntax, can be used by breakpoints and watchpoints to compute values, and can cause execution to take place within a target process (by evaluating function expressions). Expressions are parsed, evaluated, and printed using the language-dispatching mechanisms described in ‘language.c’ and Section 8.3.7 [Language-Specific Sources], page 23.

GDB source files to manipulate type structures:

‘typeprint.c’

Prints type information structures in readable form. Interfaces to the languagespecific type printing routines described in Section 8.3.7 [Language-Specific Sources], page 23.

‘gdbtypes.c’

Fundamental type definitions used by GDB.

GDB source files to manipulate expression structures:

‘parse.c’ Parse expressions typed at the command-line into expression. Interfaces to the language-specific expression parsing routines described in Section 8.3.7 [Language-Specific Sources], page 23.

‘eval.c’ Evaluates expression structures in the current execution context.

‘expprint.c’

Prints expression structures in readable (infix) form. Interfaces to the language-specific type printing routines described in Section 8.3.7 [Language-Specific Sources], page 23.

GDB source files to manipulate value structures:

‘valarith.c’

perform arithmetic operations on values

‘valops.c’

perform non-arithmetic operations on values

‘valprint.c’

print functions for values

‘values.c’

low-level packing/unpacking of value objects to/from raw format

‘findvar.c’

resolve variables to their value structures (as appropriate to the current stack environment).

8.3.4 Stack Analysis

‘blockframe.c’

machine-independent code to analyze stack frames

‘stack.c’ machine-independent stack frame analysis, user-level commands to manage and inspect the stack.

8.3.5 Breakpoints

‘breakpoint.c’

Machine-independent breakpoint code. Handles and dispatches all forms of breakpoints, including hardware breakpoints, software breakpoints, hardware and software watchpoints, and shared library breakpoints. Contains top-level commands to set, inspect, and manipulate breakpoints and watchpoints. Provides routine to read memory from inferior, replacing any memory that may have been overwritten by a breakpoint with its saved value.

‘mem-break.c’

Implements software breakpoints. To set a breakpoint at a given location, GDB saves the instruction at that location and inserts a software trap instruction in its place.

8.3.6 Symbol File Management

The following files allow GDB to parse and manage symbol information in a variety of formats. For an overview of GDB object file and symbol handling, see ⟨undefined⟩ [Symbol Tables], page ⟨undefined⟩.

‘objfiles.c’

create/destroy/manage objfile structures

‘symfile.c’

top-level commands to handle objfiles, also handles overlays (unused)

‘symmisc.c’

various objfile and (p)symtab utilities (mainly debugging)

‘symtab.c’

basic symbol table utilities (mainly lookup)

‘minsyms.c’

manage minimal symbol tables

‘buildsym.c’

build complete symbol data structures

The following source files provide symbol-reading interfaces for various file formats. Although all these files are compiled into GDB for Mac OS X, only the first three (‘stabsread’, ‘dbxread’, and ‘machoread’) are actively used by the rest of GDB.

‘stabsread.c’

common stabs parsing routines

‘dbxread.c’

read DBX (stabs) symbol files

‘machoread.c’

read Mach-O (stabs) symbol files

‘nlmread.c’

read Netware NLM symbol files (unused)

‘os9kread.c’

read OS9/OS9K symbol files (unused)

‘dwarf2read.c’

read DWARF2 symbol files (unused)

‘dwarfread.c’

read DWARF symbol files (unused)

‘elfread.c’

read ELF symbol files (unused)

‘coffread.c’

read COFF symbol files (unused)

‘mdebugread.c’

read ECOFF symbol files (unused)

‘mipsread.c’

read MIPS symbol files (unused)

8.3.7 Language-Specific Sources

The following files are used to provide language-specific expression evaluation and printing support. The file lang-exp handles expression parsing, lang-typeprint prints humanreadable versions of GDB ’type’ structures, and lang-valprint prints human-readable versions of GDB ’value’ structures, and lang-lang provides general language-specific support functions. For more information on language-specific support in GDB, see ⟨undefined⟩ [Languages], page ⟨undefined⟩.

C  ‘c-exp’, ‘c-lang’, ‘c-typeprint’, ‘c-valprint’
C++  ‘cp-valprint’
Objective-C
          ‘objc-exp’, ‘objc-lang’, ‘objc’
Chill  ‘ch-exp’, ‘ch-lang’, ‘ch-typeprint’, ‘ch-valprint’
Fortran ‘f-exp’, ‘f-lang’, ‘f-typeprint’, ‘f-valprint’
Java  ‘jv-exp’, ‘jv-lang’, ‘jv-typeprint’, ‘jv-valprint’
Modula II ‘m2-exp’, ‘m2-lang’, ‘m2-typeprint’, ‘m2-valprint’
Scheme  ‘scm-exp’, ‘scm-lang’, ‘scm-valprint’

8.3.8 Kernel Debugging

‘remote-kdp.c’

interface gdb ’target’ interface to a remote Mac OS X kernel via KDP

‘kdp-udp.c’

communications library for KDP

‘remote-utils.c’

more serial-line support (unused)

‘remote.c’

generic serial-line debugging (unused)

‘ser-tcp.c’

treat TCP connection as serial line (unused)

‘ser-unix.c’

interface to unix serial ports (unused)

‘serial.c’

implement generic serial routines (unused)

8.3.9 Sources Specific to Mac OS X

‘nextstep-nat-dyld.c’

handle dyld debugging messages and take action (mainly load symfiles)

‘nextstep-nat-inferior.c’

interface between GDB ’target’ abstraction and Mach process control functions

‘nextstep-nat-mutils.c’

mach functions to read/write memory, other manipulations

‘nextstep-nat-sigthread.c’

create/manage thread to detect signals sent to the inferior process

‘nextstep-nat-threads.c’

interface gdb ’target’ interface to a running Mac OS X process on the same machine

‘nextstep-tdep.c’

extra functions needed for all nextstep targets (empty)

‘nextstep-xdep.c’

extra functions needed for nextstep hosts (empty)

8.3.10 PowerPC-specific Sources

‘ppc-frameinfo.c’

determine/print PPC stack frame info (find prologue, etc)

‘ppc-frameops.c’

basic PPC stack frame operations (push dummy, push args, pop, find saved regs)

‘ppc-next-tcore.c’

fetch/store PPC registers to/from running Mach thread

‘ppc-next-tdep.c’

fetch/store PPC registers from Mach data structure

‘ppc-tdep.c’

PPC analysis functions used by rest of GDB

8.3.11 Miscellaneous

‘demangle’

Allow user to explicitly select function name demangling style (e.g. ’k+r’, ’lucid’, ’GNU’).

‘language’

Multiple-language support for GDB. Contains dispatch tables to the languagespecific routines, routines to detect the language of a source file / object module, and explicit commands to print/set the current language environment.

‘source’ View and navigate through source files. Also responsible for directory search path mechanisms.

8.3.12 Assorted Utilities

‘bcache’ efficiency additions for obstack

‘gnu-regex’

regular expression library ‘dcache’ caches inferior memory accesses (for remote targets)

‘complaints’

error-printing for symbol file reading allows error messages to be printed only once per operation, rather than repeated incessantly

‘copying’ prints license/warranty information

‘version’ automatically generated, contains version string

‘annotate’

convenience functions to print annotations for libgdb

‘environ’ utilities to read/modify the process environment

‘utils’ Utilities used by GDB. Provides routines to provide formatted output, memory management, and data conversion routines.

9 Target Architecture Definition

GDB’s target architecture defines what sort of machine-language programs GDB can work with, and how it works with them.

At present, the target architecture definition consists of a number of C macros.

9.1 Registers and Memory

GDB’s model of the target machine is rather simple. GDB assumes the machine includes a bank of registers and a block of memory. Each register may have a different size.

GDB does not have a magical way to match up with the compiler’s idea of which registers are which; however, it is critical that they do match up accurately. The only way to make this work is to get accurate information about the order that the compiler uses, and to reflect that in the REGISTER_NAME and related macros.

GDB can handle big-endian, little-endian, and bi-endian architectures.

9.2 Frame Interpretation

9.3 Inferior Call Setup

9.4 Compiler Characteristics 9.5 Target Conditionals

This section describes the macros that you can use to define the target machine.

ADDITIONAL_OPTIONS
ADDITIONAL_OPTION_CASES
ADDITIONAL_OPTION_HANDLER
ADDITIONAL_OPTION_HELP

These are a set of macros that allow the addition of additional command line options to GDB. They are currently used only for the unsupported i960 Nindy target, and should not be used in any other configuration.

ADDR_BITS_REMOVE (addr)

If a raw machine address includes any bits that are not really part of the address, then define this macro to expand into an expression that zeros those bits in addr. For example, the two low-order bits of a Motorola 88K address may be used by some kernels for their own purposes, since addresses must always be 4-byte aligned, and so are of no use for addressing. Those bits should be filtered out with an expression such as ((addr) & ~3).

BEFORE_MAIN_LOOP_HOOK

Define this to expand into any code that you want to execute before the main loop starts. Although this is not, strictly speaking, a target conditional, that is how it is currently being used. Note that if a configuration were to define it one way for a host and a different way for the target, GDB will probably not compile, let alone run correctly. This is currently used only for the unsupported i960 Nindy target, and should not be used in any other configuration.

BELIEVE_PCC_PROMOTION

Define if the compiler promotes a short or char parameter to an int, but still reports the parameter as its original type, rather than the promoted type.

BELIEVE_PCC_PROMOTION_TYPE

Define this if GDB should believe the type of a short argument when compiled by pcc, but look within a full int space to get its value. Only defined for Sun-3 at present.

BITS_BIG_ENDIAN

Define this if the numbering of bits in the targets does *not* match the endianness of the target byte order. A value of 1 means that the bits are numbered in a big-endian order, 0 means little-endian.

BREAKPOINT

This is the character array initializer for the bit pattern to put into memory where a breakpoint is set. Although it’s common to use a trap instruction for a breakpoint, it’s not required; for instance, the bit pattern could be an invalid instruction. The breakpoint must be no longer than the shortest instruction of the architecture. BREAKPOINT has been deprecated in favour of BREAKPOINT FROM PC.

BIG_BREAKPOINT

LITTLE_BREAKPOINT

Similar to BREAKPOINT, but used for bi-endian targets. BIG BREAKPOINT and LITTLE BREAKPOINT have been deprecated in favour of BREAKPOINT FROM PC.

REMOTE_BREAKPOINT
LITTLE_REMOTE_BREAKPOINT
BIG_REMOTE_BREAKPOINT

Similar to BREAKPOINT, but used for remote targets. BIG REMOTE BREAKPOINT and LITTLE REMOTE BREAKPOINT have been deprecated in favour of BREAKPOINT FROM PC.

BREAKPOINT_FROM_PC (pcptr, lenptr)

Use the program counter to determine the contents and size of a breakpoint instruction. It returns a pointer to a string of bytes that encode a breakpoint instruction, stores the length of the string to *lenptr, and adjusts pc (if necessary) to point to the actual memory location where the breakpoint should be inserted. Although it is common to use a trap instruction for a breakpoint, it’s not required; for instance, the bit pattern could be an invalid instruction. The breakpoint must be no longer than the shortest instruction of the architecture. Replaces all the other BREAKPOINT macros.

CALL_DUMMY_P

A C expresson that is non-zero when the target suports inferior function calls.

CALL_DUMMY_WORDS

Pointer to an array of LONGEST words of data containing host-byte-ordered REGISTER BYTES sized values that partially specify the sequence of instructions needed for an inferior function call. Should be deprecated in favour of a macro that uses target-byte-ordered data.

SIZEOF_CALL_DUMMY_WORDS

The size of CALL DUMMY WORDS. When CALL DUMMY P this must return a positive value. See also CALL DUMMY LENGTH.

CALL_DUMMY

A static initializer for CALL DUMMY WORDS. Deprecated.

CALL_DUMMY_LOCATION

inferior.h

CALL_DUMMY_STACK_ADJUST

Stack adjustment needed when performing an inferior function call. Should be deprecated in favor of something like STACK ALIGN.

CALL_DUMMY_STACK_ADJUST_P

Predicate for use of CALL DUMMY STACK ADJUST. Should be deprecated in favor of something like STACK ALIGN.

CANNOT_FETCH_REGISTER (regno)

A C expression that should be nonzero if regno cannot be fetched from an inferior process. This is only relevant if FETCH_INFERIOR_REGISTERS is not defined.

CANNOT_STORE_REGISTER (regno)

A C expression that should be nonzero if regno should not be written to the target. This is often the case for program counters, status words, and other special registers. If this is not defined, GDB will assume that all registers may be written.

DO_DEFERRED_STORES
CLEAR_DEFERRED_STORES

Define this to execute any deferred stores of registers into the inferior, and to cancel any deferred stores. Currently only implemented correctly for native Sparc configurations?

CPLUS_MARKER

Define this to expand into the character that G++ uses to distinguish compilergenerated identifiers from programmer-specified identifiers. By default, this expands into '$'. Most System V targets should define this to '.'.

DBX_PARM_SYMBOL_CLASS

Hook for the SYMBOL_CLASS of a parameter when decoding DBX symbol information. In the i960, parameters can be stored as locals or as args, depending on the type of the debug record.

DECR_PC_AFTER_BREAK

Define this to be the amount by which to decrement the PC after the program encounters a breakpoint. This is often the number of bytes in BREAKPOINT, though not always. For most targets this value will be 0.

DECR_PC_AFTER_HW_BREAK

Similarly, for hardware breakpoints.

DISABLE_UNSETTABLE_BREAK addr

If defined, this should evaluate to 1 if addr is in a shared library in which breakpoints cannot be set and so should be disabled.

DO_REGISTERS_INFO

If defined, use this to print the value of a register or all registers.

END_OF_TEXT_DEFAULT

This is an expression that should designate the end of the text section (? FIXME ?)

EXTRACT_RETURN_VALUE(type,regbuf,valbuf)

Define this to extract a function’s return value of type type from the raw register state regbuf and copy that, in virtual format, into valbuf.

EXTRACT_STRUCT_VALUE_ADDRESS(regbuf)

When EXTRACT STRUCT VALUE ADDRESS P this is used to to extract from an array regbuf (containing the raw register state) the address in which a function should return its structure value, as a CORE ADDR (or an expression that can be used as one).

EXTRACT_STRUCT_VALUE_ADDRESS_P

Predicate for EXTRACT STRUCT VALUE ADDRESS.

FLOAT_INFO

If defined, then the ‘info float’ command will print information about the processor’s floating point unit.

FP_REGNUM

If the virtual frame pointer is kept in a register, then define this macro to be the number (greater than or equal to zero) of that register. This should only need to be defined if TARGET_READ_FP and TARGET_WRITE_FP are not defined.

FRAMELESS_FUNCTION_INVOCATION(fi)

Define this to an expression that returns 1 if the function invocation represented by fi does not have a stack frame associated with it. Otherwise return 0.

FRAME_ARGS_ADDRESS_CORRECT

stack.c

FRAME_CHAIN(frame)

Given frame, return a pointer to the calling frame.

FRAME_CHAIN_COMBINE(chain,frame)

Define this to take the frame chain pointer and the frame’s nominal address and produce the nominal address of the caller’s frame. Presently only defined for HP PA.

FRAME_CHAIN_VALID(chain,thisframe)

Define this to be an expression that returns zero if the given frame is an outermost frame, with no caller, and nonzero otherwise. Three common definitions are available. default_frame_chain_valid (the default) is nonzero if the chain pointer is nonzero and given frame’s PC is not inside the startup file (such as

‘crt0.o’). alternate_frame_chain_valid is nonzero if the chain pointer is

nonzero and the given frame’s PC is not in main() or a known entry point function (such as _start()).

FRAME_INIT_SAVED_REGS(frame)

See ‘frame.h’. Determines the address of all registers in the current stack frame

storing each in frame->saved_regs. Space for frame->saved_regs shall be
allocated by FRAME_INIT_SAVED_REGS using either frame_saved_regs_zalloc
or frame_obstack_alloc.

FRAME FIND SAVED REGS and EXTRA FRAME INFO are deprecated.

FRAME_NUM_ARGS (fi)

For the frame described by fi return the number of arguments that are being passed. If the number of arguments is not known, return -1.

FRAME_SAVED_PC(frame)

Given frame, return the pc saved there. That is, the return address.

FUNCTION_EPILOGUE_SIZE

For some COFF targets, the x_sym.x_misc.x_fsize field of the function end symbol is 0. For such targets, you must define FUNCTION_EPILOGUE_SIZE to expand into the standard size of a function’s epilogue.

GCC_COMPILED_FLAG_SYMBOL
GCC2_COMPILED_FLAG_SYMBOL

If defined, these are the names of the symbols that GDB will look for to detect that GCC compiled the file. The default symbols are gcc_compiled. and gcc2_compiled., respectively. (Currently only defined for the Delta 68.)

GDB_MULTI_ARCH

If defined and non-zero, enables suport for multiple architectures within GDB. The support can be enabled at two levels. At level one, only definitions for previously undefined macros are provided; at level two, a multi-arch definition of all architecture dependant macros will be defined.

GDB_TARGET_IS_HPPA

This determines whether horrible kludge code in dbxread.c and partial-stab.h is used to mangle multiple-symbol-table files from HPPA’s. This should all be ripped out, and a scheme like elfread.c used.

GDB_TARGET_IS_MACH386
GDB_TARGET_IS_SUN3
GDB_TARGET_IS_SUN386

Kludges that should go away.

GET_LONGJMP_TARGET

For most machines, this is a target-dependent parameter. On the DECstation and the Iris, this is a native-dependent parameter, since <setjmp.h> is needed to define it. This macro determines the target PC address that longjmp() will jump to, assuming that we have just stopped at a longjmp breakpoint. It takes a CORE ADDR * as argument, and stores the target PC value through this pointer. It examines the current state of the machine as needed.

GET_SAVED_REGISTER

Define this if you need to supply your own definition for the function get_

saved_register.
HAVE_REGISTER_WINDOWS

Define this if the target has register windows.

REGISTER_IN_WINDOW_P (regnum)

Define this to be an expression that is 1 if the given register is in the window.

IBM6000_TARGET

Shows that we are configured for an IBM RS/6000 target. This conditional should be eliminated (FIXME) and replaced by feature-specific macros. It was introduced in haste and we are repenting at leisure.

IEEE_FLOAT

Define this if the target system uses IEEE-format floating point numbers.

INIT_EXTRA_FRAME_INFO (fromleaf, frame)

If additional information about the frame is required this should be stored in

frame->extra_info. Space for frame->extra_info is allocated using frame_
obstack_alloc.
INIT_FRAME_PC (fromleaf, prev)

This is a C statement that sets the pc of the frame pointed to by prev. [By default...]

INNER_THAN (lhs,rhs)

Returns non-zero if stack address lhs is inner than (nearer to the stack top) stack address rhs. Define this as lhs < rhs if the target’s stack grows downward in memory, or lhs > rsh if the stack grows upward.

IN_SIGTRAMP (pc, name)

Define this to return true if the given pc and/or name indicates that the current function is a sigtramp.

SIGTRAMP_START (pc)
SIGTRAMP_END (pc)

Define these to be the start and end address of the sigtramp for the given pc. On machines where the address is just a compile time constant, the macro expansion will typically just ignore the supplied pc.

IN_SOLIB_CALL_TRAMPOLINE pc name

Define this to evaluate to nonzero if the program is stopped in the trampoline that connects to a shared library.

IN_SOLIB_RETURN_TRAMPOLINE pc name

Define this to evaluate to nonzero if the program is stopped in the trampoline that returns from a shared library.

IS_TRAPPED_INTERNALVAR (name)

This is an ugly hook to allow the specification of special actions that should occur as a side-effect of setting the value of a variable internal to GDB. Currently only used by the h8500. Note that this could be either a host or target conditional.

NEED_TEXT_START_END

Define this if GDB should determine the start and end addresses of the text section. (Seems dubious.)

NO_HIF_SUPPORT

(Specific to the a29k.)

SOFTWARE_SINGLE_STEP_P

Define this as 1 if the target does not have a hardware single-step mechanism. The macro SOFTWARE_SINGLE_STEP must also be defined.

SOFTWARE_SINGLE_STEP(signal,insert_breapoints_p)

A function that inserts or removes (dependant on insert breapoints p) breakpoints at each possible destinations of the next instruction. See sparc-tdep.c and rs6000-tdep.c for examples.

PCC_SOL_BROKEN

(Used only in the Convex target.)

PC_IN_CALL_DUMMY

inferior.h

PC_LOAD_SEGMENT

If defined, print information about the load segment for the program counter. (Defined only for the RS/6000.)

PC_REGNUM

If the program counter is kept in a register, then define this macro to be the number (greater than or equal to zero) of that register. This should only need to be defined if TARGET_READ_PC and TARGET_WRITE_PC are not defined.

NPC_REGNUM

The number of the “next program counter” register, if defined.

NNPC_REGNUM

The number of the “next next program counter” register, if defined. Currently, this is only defined for the Motorola 88K.

PRINT_REGISTER_HOOK (regno)

If defined, this must be a function that prints the contents of the given register to standard output.

PRINT_TYPELESS_INTEGER

This is an obscure substitute for print_longest that seems to have been defined for the Convex target.

PROCESS_LINENUMBER_HOOK

A hook defined for XCOFF reading.

PROLOGUE_FIRSTLINE_OVERLAP

(Only used in unsupported Convex configuration.)

PS_REGNUM

If defined, this is the number of the processor status register. (This definition is only used in generic code when parsing "$ps".)

POP_FRAME

Used in ‘call_function_by_hand’ to remove an artificial stack frame.

PUSH_ARGUMENTS (nargs, args, sp, struct_return, struct_addr)

Define this to push arguments onto the stack for inferior function call. Return the updated stack pointer value.

PUSH_DUMMY_FRAME

Used in ‘call_function_by_hand’ to create an artificial stack frame.

REGISTER_BYTES

The total amount of space needed to store GDB’s copy of the machine’s register state.

REGISTER_NAME(i)

Return the name of register i as a string. May return NULL or NUL to indicate that register i is not valid.

REGISTER_NAMES

Deprecated in favor of REGISTER NAME.

REG_STRUCT_HAS_ADDR (gcc_p, type)

Define this to return 1 if the given type will be passed by pointer rather than directly.

SAVE_DUMMY_FRAME_TOS (sp)

Used in ‘call_function_by_hand’ to notify the target dependent code of the top-of-stack value that will be passed to the the inferior code. This is the value of the SP after both the dummy frame and space for parameters/results have been allocated on the stack.

SDB_REG_TO_REGNUM

Define this to convert sdb register numbers into GDB regnums. If not defined, no conversion will be done.

SHIFT_INST_REGS

(Only used for m88k targets.)

SKIP_PROLOGUE (pc)

A C expression that returns the address of the “real” code beyond the function entry prologue found at pc.

SKIP_PROLOGUE_FRAMELESS_P

A C expression that should behave similarly, but that can stop as soon as the function is known to have a frame. If not defined, SKIP_PROLOGUE will be used instead.

SKIP_TRAMPOLINE_CODE (pc)

If the target machine has trampoline code that sits between callers and the functions being called, then define this macro to return a new PC that is at the start of the real function.

SP_REGNUM

If the stack-pointer is kept in a register, then define this macro to be the number (greater than or equal to zero) of that register. This should only need to be defined if TARGET_WRITE_SP and TARGET_WRITE_SP are not defined.

STAB_REG_TO_REGNUM

Define this to convert stab register numbers (as gotten from ‘r’ declarations) into GDB regnums. If not defined, no conversion will be done.

STACK_ALIGN (addr)

Define this to adjust the address to the alignment required for the processor’s stack.

STEP_SKIPS_DELAY (addr)

Define this to return true if the address is of an instruction with a delay slot. If a breakpoint has been placed in the instruction’s delay slot, GDB will singlestep over that instruction before resuming normally. Currently only defined for the Mips.

STORE_RETURN_VALUE (type, valbuf)

A C expression that stores a function return value of type type, where valbuf is the address of the value to be stored.

SUN_FIXED_LBRAC_BUG

(Used only for Sun-3 and Sun-4 targets.)

SYMBOL_RELOADING_DEFAULT

The default value of the ‘symbol-reloading’ variable. (Never defined in current sources.)

TARGET_BYTE_ORDER_DEFAULT

The ordering of bytes in the target. This must be either BIG_ENDIAN or LITTLE_ ENDIAN. This macro replaces TARGET BYTE ORDER which is deprecated.

TARGET_BYTE_ORDER_SELECTABLE_P

Non-zero if the target has both BIG_ENDIAN and LITTLE_ENDIAN variants. This macro replaces TARGET BYTE ORDER SELECTABLE which is deprecated.

TARGET_CHAR_BIT

Number of bits in a char; defaults to 8.

TARGET_COMPLEX_BIT

Number of bits in a complex number; defaults to 2 * TARGET_FLOAT_BIT. At present this macro is not used.

TARGET_DOUBLE_BIT

Number of bits in a double float; defaults to 8 * TARGET_CHAR_BIT.

TARGET_DOUBLE_COMPLEX_BIT

Number of bits in a double complex; defaults to 2 * TARGET_DOUBLE_BIT. At present this macro is not used.

TARGET_FLOAT_BIT

Number of bits in a float; defaults to 4 * TARGET_CHAR_BIT.

TARGET_INT_BIT

Number of bits in an integer; defaults to 4 * TARGET_CHAR_BIT.

TARGET_LONG_BIT

Number of bits in a long integer; defaults to 4 * TARGET_CHAR_BIT.

TARGET_LONG_DOUBLE_BIT

Number of bits in a long double float; defaults to 2 * TARGET_DOUBLE_BIT.

TARGET_LONG_LONG_BIT

Number of bits in a long long integer; defaults to 2 * TARGET_LONG_BIT.

TARGET_PTR_BIT

Number of bits in a pointer; defaults to TARGET_INT_BIT.

TARGET_SHORT_BIT

Number of bits in a short integer; defaults to 2 * TARGET_CHAR_BIT.

TARGET_READ_PC
TARGET_WRITE_PC (val, pid)
TARGET_READ_SP
TARGET_WRITE_SP
TARGET_READ_FP
TARGET_WRITE_FP
These change the behavior of read_pc, write_pc, read_sp, write_sp, read_

fp and write_fp. For most targets, these may be left undefined. GDB will call the read and write register functions with the relevant _REGNUM argument. These macros are useful when a target keeps one of these registers in a hard to get at place; for example, part in a segment register and part in an ordinary register.

TARGET_VIRTUAL_FRAME_POINTER(pc,regp,offsetp)

Returns a (register, offset) pair representing the virtual frame pointer in use at the code address "pc". If virtual frame pointers are not used, a default definition simply returns FP_REGNUM, with an offset of zero.

USE_STRUCT_CONVENTION (gcc_p, type)

If defined, this must be an expression that is nonzero if a value of the given type being returned from a function must have space allocated for it on the stack. gcc p is true if the function being considered is known to have been compiled by GCC; this is helpful for systems where GCC is known to use different calling convention than other compilers.

VARIABLES_INSIDE_BLOCK (desc, gcc_p)

For dbx-style debugging information, if the compiler puts variable declarations inside LBRAC/RBRAC blocks, this should be defined to be nonzero. desc is the value of n_desc from the N_RBRAC symbol, and gcc p is true if GDB has no-

ticed the presence of either the GCC_COMPILED_SYMBOL or the GCC2_COMPILED_

SYMBOL. By default, this is 0.

OS9K_VARIABLES_INSIDE_BLOCK (desc, gcc_p)

Similarly, for OS/9000. Defaults to 1.

Motorola M68K target conditionals.

BPT_VECTOR

Define this to be the 4-bit location of the breakpoint trap vector. If not defined, it will default to 0xf.

REMOTE_BPT_VECTOR

Defaults to 1.

9.6 Adding a New Target

The following files define a target to GDB:

‘gdb/config/arch/ttt.mt’

Contains a Makefile fragment specific to this target. Specifies what object files are needed for target ttt, by defining ‘TDEPFILES=...’. Also specifies the header file which describes ttt, by defining ‘TM_FILE= tm-ttt.h’. You can also define ‘TM_CFLAGS’, ‘TM_CLIBS’, ‘TM_CDEPS’, but these are now deprecated and may go away in future versions of GDB.

‘gdb/config/arch/tm-ttt.h’

(‘tm.h’ is a link to this file, created by configure). Contains macro definitions about the target machine’s registers, stack frame format and instructions.

‘gdb/ttt-tdep.c’

Contains any miscellaneous code required for this target machine. On some machines it doesn’t exist at all. Sometimes the macros in ‘tm-ttt.h’ become very complicated, so they are implemented as functions here instead, and the macro is simply defined to call the function. This is vastly preferable, since it is easier to understand and debug.

‘gdb/config/arch/tm-arch.h’

This often exists to describe the basic layout of the target machine’s processor chip (registers, stack, etc). If used, it is included by ‘tm-ttt.h’. It can be shared among many targets that use the same processor.

‘gdb/arch-tdep.c’

Similarly, there are often common subroutines that are shared by all target machines that use this particular architecture.

If you are adding a new operating system for an existing CPU chip, add a ‘config/tm-os.h’ file that describes the operating system facilities that are unusual (extra symbol table info; the breakpoint instruction needed; etc). Then write a ‘arch/tm-os.h’ that just #includes

‘tm-arch.h’ and ‘config/tm-os.h’.

10 Target Vector Definition

The target vector defines the interface between GDB’s abstract handling of target systems, and the nitty-gritty code that actually exercises control over a process or a serial port. GDB includes some 30-40 different target vectors; however, each configuration of GDB includes only a few of them.

10.1 File Targets

Both executables and core files have target vectors.

10.2 Standard Protocol and Remote Stubs

GDB’s file ‘remote.c’ talks a serial protocol to code that runs in the target system. GDB provides several sample “stubs” that can be integrated into target programs or operating systems for this purpose; they are named ‘*-stub.c’.

The GDB user’s manual describes how to put such a stub into your target code. What follows is a discussion of integrating the SPARC stub into a complicated operating system (rather than a simple program), by Stu Grossman, the author of this stub.

The trap handling code in the stub assumes the following upon entry to trap low:

1. %l1 and %l2 contain pc and npc respectively at the time of the trap

2. traps are disabled

3. you are in the correct trap window

As long as your trap handler can guarantee those conditions, then there is no reason why you shouldn’t be able to ‘share’ traps with the stub. The stub has no requirement that it be jumped to directly from the hardware trap vector. That is why it calls exceptionHandler(), which is provided by the external environment. For instance, this could setup the hardware traps to actually execute code which calls the stub first, and then transfers to its own trap handler.

For the most point, there probably won’t be much of an issue with ‘sharing’ traps, as the traps we use are usually not used by the kernel, and often indicate unrecoverable error conditions. Anyway, this is all controlled by a table, and is trivial to modify. The most important trap for us is for ta 1. Without that, we can’t single step or do breakpoints. Everything else is unnecessary for the proper operation of the debugger/stub.

From reading the stub, it’s probably not obvious how breakpoints work. They are simply done by deposit/examine operations from GDB.

10.3 ROM Monitor Interface

10.4 Custom Protocols

10.5 Transport Layer

10.6 Builtin Simulator

11 Native Debugging

Several files control GDB’s configuration for native support:

‘gdb/config/arch/xyz.mh’

Specifies Makefile fragments needed when hosting or native on machine xyz. In particular, this lists the required native-dependent object files, by defining ‘NATDEPFILES=...’. Also specifies the header file which describes native support on xyz, by defining ‘NAT_FILE= nm-xyz.h’. You can also

          define ‘NAT_CFLAGS’, ‘NAT_ADD_FILES’, ‘NAT_CLIBS’, ‘NAT_CDEPS’, etc.; see
          ‘Makefile.in’.

‘gdb/config/arch/nm-xyz.h’

(‘nm.h’ is a link to this file, created by configure). Contains C macro definitions describing the native system environment, such as child process control and core file support.

‘gdb/xyz-nat.c’

Contains any miscellaneous C code required for this native support of this machine. On some machines it doesn’t exist at all.

There are some “generic” versions of routines that can be used by various systems. These can be customized in various ways by macros defined in your ‘nm-xyz.h’ file. If these routines work for the xyz host, you can just include the generic file’s name (with ‘.o’, not

‘.c’) in NATDEPFILES.

Otherwise, if your machine needs custom support routines, you will need to write routines that perform the same functions as the generic file. Put them into xyz-nat.c, and put xyz-

nat.o into NATDEPFILES.
‘inftarg.c’

This contains the target ops vector that supports Unix child processes on systems which use ptrace and wait to control the child.

‘procfs.c’

This contains the target ops vector that supports Unix child processes on systems which use /proc to control the child.

‘fork-child.c’

This does the low-level grunge that uses Unix system calls to do a "fork and exec" to start up a child process.

‘infptrace.c’

This is the low level interface to inferior processes for systems using the Unix ptrace call in a vanilla way.

11.1 Native core file Support

‘core-aout.c::fetch_core_registers()’

Support for reading registers out of a core file. This routine calls register_ addr(), see below. Now that BFD is used to read core files, virtually all machines should use core-aout.c, and should just provide fetch_core_

registers in xyz-nat.c (or REGISTER_U_ADDR in nm-xyz.h).
‘core-aout.c::register_addr()’
If your nm-xyz.h file defines the macro REGISTER_U_ADDR(addr, blockend,

regno), it should be defined to set addr to the offset within the ‘user’ struct of GDB register number regno. blockend is the offset within the “upage” of u.u_

ar0. If REGISTER_U_ADDR is defined, ‘core-aout.c’ will define the register_

addr() function and use the macro in it. If you do not define REGISTER_U_ ADDR, but you are using the standard fetch_core_registers(), you will need to define your own version of register_addr(), put it into your xyz-nat.c file, and be sure xyz-nat.o is in the NATDEPFILES list. If you have your own fetch_

core_registers(), you may not need a separate register_addr(). Many

custom fetch_core_registers() implementations simply locate the registers themselves.

When making GDB run native on a new operating system, to make it possible to debug core files, you will need to either write specific code for parsing your OS’s core files, or customize ‘bfd/trad-core.c’. First, use whatever #include files your machine uses to define the struct of registers that is accessible (possibly in the u-area) in a core file (rather than ‘machine/reg.h’), and an include file that defines whatever header exists on a core file (e.g. the u-area or a ‘struct core’). Then modify trad_unix_core_file_p() to use these values to set up the section information for the data segment, stack segment, any other segments in the core file (perhaps shared library contents or control information), “registers” segment, and if there are two discontiguous sets of registers (e.g. integer and float), the “reg2” segment. This section information basically delimits areas in the core file in a standard way, which the section-reading routines in BFD know how to seek around in.

Then back in GDB, you need a matching routine called fetch_core_registers(). If you can use the generic one, it’s in ‘core-aout.c’; if not, it’s in your ‘xyz-nat.c’ file. It will be passed a char pointer to the entire “registers” segment, its length, and a zero; or a char pointer to the entire “regs2” segment, its length, and a 2. The routine should suck out the supplied register values and install them into GDB’s “registers” array.

If your system uses ‘/proc’ to control processes, and uses ELF format core files, then you may be able to use the same routines for reading the registers out of processes and out of core files.

11.2 ptrace

11.3 /proc

11.4 win32

11.5 shared libraries

11.6 Native Conditionals

When GDB is configured and compiled, various macros are defined or left undefined, to control compilation when the host and target systems are the same. These macros should be defined (or left undefined) in ‘nm-system.h’.

ATTACH_DETACH

If defined, then GDB will include support for the attach and detach commands.

CHILD_PREPARE_TO_STORE

If the machine stores all registers at once in the child process, then define this to ensure that all values are correct. This usually entails a read from the child. [Note that this is incorrectly defined in ‘xm-system.h’ files currently.]

FETCH_INFERIOR_REGISTERS

Define this if the native-dependent code will provide its own routines fetch_

inferior_registers and store_inferior_registers in ‘HOST-nat.c’. If

this symbol is not defined, and ‘infptrace.c’ is included in this configuration, the default routines in ‘infptrace.c’ are used for these functions.

FILES_INFO_HOOK

(Only defined for Convex.)

FP0_REGNUM

This macro is normally defined to be the number of the first floating point register, if the machine has such registers. As such, it would appear only in target-specific code. However, /proc support uses this to decide whether floats are in use on this target.

GET_LONGJMP_TARGET

For most machines, this is a target-dependent parameter. On the DECstation and the Iris, this is a native-dependent parameter, since <setjmp.h> is needed to define it. This macro determines the target PC address that longjmp() will jump to, assuming that we have just stopped at a longjmp breakpoint. It takes a CORE ADDR * as argument, and stores the target PC value through this pointer. It examines the current state of the machine as needed.

KERNEL_U_ADDR

Define this to the address of the u structure (the “user struct”, also known as the “u-page”) in kernel virtual memory. GDB needs to know this so that it can subtract this address from absolute addresses in the upage, that are obtained via ptrace or from core files. On systems that don’t need this value, set it to zero.

KERNEL_U_ADDR_BSD

Define this to cause GDB to determine the address of u at runtime, by using Berkeley-style nlist on the kernel’s image in the root directory.

KERNEL_U_ADDR_HPUX

Define this to cause GDB to determine the address of u at runtime, by using HP-style nlist on the kernel’s image in the root directory.

ONE_PROCESS_WRITETEXT

Define this to be able to, when a breakpoint insertion fails, warn the user that another process may be running with the same executable.

PROC_NAME_FMT

Defines the format for the name of a ‘/proc’ device. Should be defined in ‘nm.h’ only in order to override the default definition in ‘procfs.c’.

PTRACE_FP_BUG

mach386-xdep.c

PTRACE_ARG3_TYPE

The type of the third argument to the ptrace system call, if it exists and is different from int.

REGISTER_U_ADDR

Defines the offset of the registers in the “u area”.

SHELL_COMMAND_CONCAT

If defined, is a string to prefix on the shell command used to start the inferior.

SHELL_FILE

If defined, this is the name of the shell to use to run the inferior. Defaults to

"/bin/sh".
SOLIB_ADD (filename, from_tty, targ)

Define this to expand into an expression that will cause the symbols in filename to be added to GDB’s symbol table.

SOLIB_CREATE_INFERIOR_HOOK

Define this to expand into any shared-library-relocation code that you want to be run just after the child process has been forked.

START_INFERIOR_TRAPS_EXPECTED

When starting an inferior, GDB normally expects to trap twice; once when the shell execs, and once when the program itself execs. If the actual number of traps is something other than 2, then define this macro to expand into the number expected.

SVR4_SHARED_LIBS

Define this to indicate that SVR4-style shared libraries are in use.

USE_PROC_FS

This determines whether small routines in ‘*-tdep.c’, which translate register values between GDB’s internal representation and the /proc representation, are compiled.

U_REGS_OFFSET

This is the offset of the registers in the upage. It need only be defined if the generic ptrace register access routines in ‘infptrace.c’ are being used

(that is, ‘infptrace.c’ is configured in, and FETCH_INFERIOR_REGISTERS is

not defined). If the default value from ‘infptrace.c’ is good enough, leave it undefined. The default value means that u.u ar0 points to the location of the registers. I’m guessing that #define U_REGS_OFFSET 0 means that u.u ar0 is the location of the registers.

CLEAR_SOLIB

objfiles.c

DEBUG_PTRACE

Define this to debug ptrace calls.

12 Support Libraries

12.1 BFD

BFD provides support for GDB in several ways:

identifying executable and core files BFD will identify a variety of file types, including a.out, coff, and several variants thereof, as well as several kinds of core files.

access to sections of files BFD parses the file headers to determine the names, virtual addresses, sizes, and file locations of all the various named sections in files (such as the text section or the data section). GDB simply calls BFD to read or write section X at byte offset Y for length Z.

specialized core file support BFD provides routines to determine the failing command name stored in a core file, the signal with which the program failed, and whether a core file matches (i.e. could be a core dump of) a particular executable file.

locating the symbol information GDB uses an internal interface of BFD to determine where to find the symbol information in an executable file or symbol-file. GDB itself handles the reading of symbols, since BFD does not “understand” debug symbols, but GDB uses BFD’s cached information to find the symbols, string table, etc.

12.2 opcodes

The opcodes library provides GDB’s disassembler. (It’s a separate library because it’s also used in binutils, for ‘objdump’).

12.3 readline

12.4 mmalloc

12.5 libiberty

12.6 gnu-regex

Regex conditionals.

C_ALLOCA
NFAILURES
RE_NREGS
SIGN_EXTEND_CHAR
SWITCH_ENUM_BUG
SYNTAX_TABLE

Sword
sparc

12.7 include

13 Coding

This chapter covers topics that are lower-level than the major algorithms of GDB.

13.1 Cleanups

Cleanups are a structured way to deal with things that need to be done later. When your code does something (like malloc some memory, or open a file) that needs to be undone later (e.g. free the memory or close the file), it can make a cleanup. The cleanup will be done at some future point: when the command is finished, when an error occurs, or when your code decides it’s time to do cleanups.

You can also discard cleanups, that is, throw them away without doing what they say. This is only done if you ask that it be done.

Syntax:

struct cleanup *old chain;

Declare a variable which will hold a cleanup chain handle.

old chain = make_cleanup (function, arg); Make a cleanup which will cause function to be called with arg (a char *) later. The result, old chain, is a handle that can be passed to do_cleanups or discard_cleanups later. Unless you are going to call do_cleanups or discard_cleanups yourself, you can ignore the result from make_cleanup.

do_cleanups (old chain);

Perform all cleanups done since make_cleanup returned old chain. E.g.:

               make_cleanup (a, 0);
               old = make_cleanup (b, 0);
               do_cleanups (old);

will call b() but will not call a(). The cleanup that calls a() will remain in the cleanup chain, and will be done later unless otherwise discarded.

discard_cleanups (old chain);

Same as do_cleanups except that it just removes the cleanups from the chain and does not call the specified functions.

Some functions, e.g. fputs_filtered() or error(), specify that they “should not be called when cleanups are not in place”. This means that any actions you need to reverse in the case of an error or interruption must be on the cleanup chain before you call these functions, since they might never return to your code (they ‘longjmp’ instead).

13.2 Wrapping Output Lines

   Output that goes through printf_filtered or fputs_filtered or fputs_demangled

needs only to have calls to wrap_here added in places that would be good breaking points. The utility routines will take care of actually wrapping if the line width is exceeded.

The argument to wrap_here is an indentation string which is printed only if the line breaks there. This argument is saved away and used later. It must remain valid until the next call to wrap_here or until a newline has been printed through the *_filtered functions. Don’t pass in a local variable and then return!

It is usually best to call wrap_here() after printing a comma or space. If you call it before printing a space, make sure that your indentation properly accounts for the leading space that will print if the line wraps there.

Any function or set of functions that produce filtered output must finish by printing a newline, to flush the wrap buffer, before switching to unfiltered (“printf”) output. Symbol reading routines that print warnings are a good example.

13.3 GDB Coding Standards

GDB follows the GNU coding standards, as described in ‘etc/standards.texi’. This file is also available for anonymous FTP from GNU archive sites. GDB takes a strict interpretation of the standard; in general, when the GNU standard recommends a practice but does not require it, GDB requires it.

GDB follows an additional set of coding standards specific to GDB, as described in the following sections.

You can configure with ‘--enable-build-warnings’ to get GCC to check on a number of these rules. GDB sources ought not to engender any complaints, unless they are caused by bogus host systems. (The exact set of enabled warnings is currently ‘-Wall

-Wpointer-arith -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations’.

13.3.1 Formatting

The standard GNU recommendations for formatting must be followed strictly.

Note that while in a definition, the function’s name must be in column zero; in a function declaration, the name must be on the same line as the return type.

In addition, there must be a space between a function or macro name and the opening parenthesis of its argument list (except for macro definitions, as required by C). There must not be a space after an open paren/bracket or before a close paren/bracket.

While additional whitespace is generally helpful for reading, do not use more than one blank line to separate blocks, and avoid adding whitespace after the end of a program line (as of 1/99, some 600 lines had whitespace after the semicolon). Excess whitespace causes difficulties for diff and patch.

13.3.2 Comments

The standard GNU requirements on comments must be followed strictly.

Block comments must appear in the following form, with no ‘/*’- or ’*/’-only lines, and no leading ‘*’:

     /* Wait for control to return from inferior to debugger.  If inferior
        gets a signal, we may decide to start it up again instead of
        returning.  That is why there is a loop in this function.  When
        this function actually returns it means the inferior should be left
        stopped and GDB should read more commands.  */

(Note that this format is encouraged by Emacs; tabbing for a multi-line comment works correctly, and M-Q fills the block consistently.)

Put a blank line between the block comments preceding function or variable definitions, and the definition itself.

In general, put function-body comments on lines by themselves, rather than trying to fit them into the 20 characters left at the end of a line, since either the comment or the code will inevitably get longer than will fit, and then somebody will have to move it anyhow.

13.3.3 C Usage

Code must not depend on the sizes of C data types, the format of the host’s floating point numbers, the alignment of anything, or the order of evaluation of expressions.

Use functions freely. There are only a handful of compute-bound areas in GDB that might be affected by the overhead of a function call, mainly in symbol reading. Most of GDB’s performance is limited by the target interface (whether serial line or system call).

However, use functions with moderation. A thousand one-line functions are just as hard to understand as a single thousand-line function.

13.3.4 Function Prototypes

Prototypes must be used to declare functions but never to define them. Prototypes for GDB functions must include both the argument type and name, with the name matching that used in the actual function definition.

For the sake of compatibility with pre-ANSI compilers, define prototypes with the PARAMS macro:

     extern int memory_remove_breakpoint PARAMS ((CORE_ADDR addr,
                                                  char *contents_cache));

Note the double parentheses around the parameter types. This allows an arbitrary number of parameters to be described, without freaking out the C preprocessor. When the function has no parameters, it should be described like:

     extern void noprocess PARAMS ((void));

The PARAMS macro expands to its argument in ANSI C, or to a simple () in traditional

All external functions should have a PARAMS declaration in a header file that callers include, except for _initialize_* functions, which must be external so that ‘init.c’ construction works, but shouldn’t be visible to random source files.

All static functions must be declared in a block near the top of the source file.

13.3.5 Clean Design

In addition to getting the syntax right, there’s the little question of semantics. Some things are done in certain ways in GDB because long experience has shown that the more obvious ways caused various kinds of trouble.

You can’t assume the byte order of anything that comes from a target (including values, object files, and instructions). Such things must be byte-swapped using SWAP_TARGET_AND_ HOST in GDB, or one of the swap routines defined in ‘bfd.h’, such as bfd_get_32.

You can’t assume that you know what interface is being used to talk to the target system. All references to the target must go through the current target_ops vector.

You can’t assume that the host and target machines are the same machine (except in the “native” support modules). In particular, you can’t assume that the target machine’s header files will be available on the host machine. Target code must bring along its own header files – written from scratch or explicitly donated by their owner, to avoid copyright problems.

Insertion of new #ifdef’s will be frowned upon. It’s much better to write the code portably than to conditionalize it for various systems.

New #ifdef’s which test for specific compilers or manufacturers or operating systems are unacceptable. All #ifdef’s should test for features. The information about which configurations contain which features should be segregated into the configuration files. Experience has proven far too often that a feature unique to one particular system often creeps into other systems; and that a conditional based on some predefined macro for your current system will become worthless over time, as new versions of your system come out that behave differently with regard to this feature.

Adding code that handles specific architectures, operating systems, target interfaces, or hosts, is not acceptable in generic code. If a hook is needed at that point, invent a generic hook and define it for your configuration, with something like:

     #ifdef WRANGLE_SIGNALS
        WRANGLE_SIGNALS (signo);
     #endif

In your host, target, or native configuration file, as appropriate, define WRANGLE_SIGNALS to do the machine-dependent thing. Take a bit of care in defining the hook, so that it can be used by other ports in the future, if they need a hook in the same place.

If the hook is not defined, the code should do whatever "most" machines want. Using #ifdef, as above, is the preferred way to do this, but sometimes that gets convoluted, in which case use

     #ifndef SPECIAL_FOO_HANDLING
     #define SPECIAL_FOO_HANDLING(pc, sp) (0)
     #endif

where the macro is used or in an appropriate header file.

Whether to include a small hook, a hook around the exact pieces of code which are system-dependent, or whether to replace a whole function with a hook depends on the case. A good example of this dilemma can be found in get_saved_register. All machines that GDB 2.8 ran on just needed the FRAME_FIND_SAVED_REGS hook to find the saved registers. Then the SPARC and Pyramid came along, and HAVE_REGISTER_WINDOWS and REGISTER_ IN_WINDOW_P were introduced. Then the 29k and 88k required the GET_SAVED_REGISTER hook. The first three are examples of small hooks; the latter replaces a whole function. In this specific case, it is useful to have both kinds; it would be a bad idea to replace all the uses of the small hooks with GET_SAVED_REGISTER, since that would result in much duplicated code. Other times, duplicating a few lines of code here or there is much cleaner than introducing a large number of small hooks.

Another way to generalize GDB along a particular interface is with an attribute struct. For example, GDB has been generalized to handle multiple kinds of remote interfaces – not by #ifdef’s everywhere, but by defining the "target ops" structure and having a current target (as well as a stack of targets below it, for memory references). Whenever something needs to be done that depends on which remote interface we are using, a flag in the current target ops structure is tested (e.g. ‘target has stack’), or a function is called through a pointer in the current target ops structure. In this way, when a new remote interface is added, only one module needs to be touched – the one that actually implements the new remote interface. Other examples of attribute-structs are BFD access to multiple kinds of object file formats, or GDB’s access to multiple source languages.

Please avoid duplicating code. For example, in GDB 3.x all the code interfacing between ptrace and the rest of GDB was duplicated in ‘*-dep.c’, and so changing something was very painful. In GDB 4.x, these have all been consolidated into ‘infptrace.c’. ‘infptrace.c’ can deal with variations between systems the same way any systemindependent file would (hooks, #if defined, etc.), and machines which are radically different don’t need to use infptrace.c at all.

Don’t put debugging printfs in the code.

14 Porting GDB

Most of the work in making GDB compile on a new machine is in specifying the configuration of the machine. This is done in a dizzying variety of header files and configuration scripts, which we hope to make more sensible soon. Let’s say your new host is called an xyz (e.g. ‘sun4’), and its full three-part configuration name is arch-xvend-xos (e.g. ‘sparc-sun-sunos4’). In particular:

In the top level directory, edit ‘config.sub’ and add arch, xvend, and xos to the lists of supported architectures, vendors, and operating systems near the bottom of the file. Also, add xyz as an alias that maps to arch-xvend-xos. You can test your changes by running

     ./config.sub xyz

and

     ./config.sub arch-xvend-xos

which should both respond with arch-xvend-xos and no error messages.

You need to port BFD, if that hasn’t been done already. Porting BFD is beyond the scope of this manual.

To configure GDB itself, edit ‘gdb/configure.host’ to recognize your system and set gdb_host to xyz, and (unless your desired target is already available) also edit ‘gdb/configure.tgt’, setting gdb_target to something appropriate (for instance, xyz).

Finally, you’ll need to specify and define GDB’s host-, native-, and target-dependent ‘.h’ and ‘.c’ files used for your configuration.

14.1 Configuring GDB for Release

From the top level directory (containing ‘gdb’, ‘bfd’, ‘libiberty’, and so on):

     make -f Makefile.in gdb.tar.gz

This will properly configure, clean, rebuild any files that are distributed pre-built (e.g. ‘c-exp.tab.c’ or ‘refcard.ps’), and will then make a tarfile. (If the top level directory has already been configured, you can just do make gdb.tar.gz instead.)

This procedure requires:

• symbolic links

• makeinfo (texinfo2 level)

• TEX

  • dvips
  • yacc or bison

… and the usual slew of utilities (sed, tar, etc.).

TEMPORARY RELEASE PROCEDURE FOR DOCUMENTATION

‘gdb.texinfo’ is currently marked up using the texinfo-2 macros, which are not yet a default for anything (but we have to start using them sometime).

For making paper, the only thing this implies is the right generation of ‘texinfo.tex’ needs to be included in the distribution.

For making info files, however, rather than duplicating the texinfo2 distribution, generate ‘gdb-all.texinfo’ locally, and include the files ‘gdb.info*’ in the distribution. Note the plural; makeinfo will split the document into one overall file and five or so included files.

15 Testsuite

The testsuite is an important component of the GDB package. While it is always worthwhile to encourage user testing, in practice this is rarely sufficient; users typically use only a small subset of the available commands, and it has proven all too common for a change to cause a significant regression that went unnoticed for some time.

The GDB testsuite uses the DejaGNU testing framework. DejaGNU is built using tcl and expect. The tests themselves are calls to various tcl procs; the framework runs all the procs and summarizes the passes and fails.

15.1 Using the Testsuite

To run the testsuite, simply go to the GDB object directory (or to the testsuite’s objdir) and type make check. This just sets up some environment variables and invokes DejaGNU’s runtest script. While the testsuite is running, you’ll get mentions of which test file is in use, and a mention of any unexpected passes or fails. When the testsuite is finished, you’ll get a summary that looks like this:

                     === gdb Summary ===
     # of expected passes            6016
     # of unexpected failures        58
     # of unexpected successes       5
     # of expected failures          183
     # of unresolved testcases       3
     # of untested testcases         5

The ideal test run consists of expected passes only; however, reality conspires to keep us from this ideal. Unexpected failures indicate real problems, whether in GDB or in the testsuite. Expected failures are still failures, but ones which have been decided are too hard to deal with at the time; for instance, a test case might work everywhere except on AIX, and there is no prospect of the AIX case being fixed in the near future. Expected failures should not be added lightly, since you may be masking serious bugs in GDB. Unexpected successes are expected fails that are passing for some reason, while unresolved and untested cases often indicate some minor catastrophe, such as the compiler being unable to deal with a test program.

When making any significant change to GDB, you should run the testsuite before and after the change, to confirm that there are no regressions. Note that truly complete testing would require that you run the testsuite with all supported configurations and a variety of compilers; however this is more than really necessary. In many cases testing with a single configuration is sufficient. Other useful options are to test one big-endian (Sparc) and one little-endian (x86) host, a cross config with a builtin simulator (powerpc-eabi, mips-elf), or a 64-bit host (Alpha).

If you add new functionality to GDB, please consider adding tests for it as well; this way future GDB hackers can detect and fix their changes that break the functionality you added. Similarly, if you fix a bug that was not previously reported as a test failure, please add a test case for it. Some cases are extremely difficult to test, such as code that handles host OS failures or bugs in particular versions of compilers, and it’s OK not to try to write tests for all of those.

15.2 Testsuite Organization

The testsuite is entirely contained in ‘gdb/testsuite’. While the testsuite includes some makefiles and configury, these are very minimal, and used for little besides cleaning up, since the tests themselves handle the compilation of the programs that GDB will run. The file ‘testsuite/lib/gdb.exp’ contains common utility procs useful for all GDB tests, while the directory ‘testsuite/config’ contains configuration-specific files, typically used for special-purpose definitions of procs like gdb_load and gdb_start.

The tests themselves are to be found in ‘testsuite/gdb.*’ and subdirectories of those. The names of the test files must always end with ‘.exp’. DejaGNU collects the test files by wildcarding in the test directories, so both subdirectories and individual files get chosen and run in alphabetical order.

The following table lists the main types of subdirectories and what they are for. Since DejaGNU finds test files no matter where they are located, and since each test file sets up its own compilation and execution environment, this organization is simply for convenience and intelligibility.

gdb.base

This is the base testsuite. The tests in it should apply to all configurations of GDB (but generic native-only tests may live here). The test programs should be in the subset of C that is valid K&R, ANSI/ISO, and C++ (ifdefs are allowed if necessary, for instance for prototypes).

gdb.lang

Language-specific tests for all languages besides C. Examples are ‘gdb.c++’ and

          ‘gdb.java’.

gdb.platform Non-portable tests. The tests are specific to a specific configuration (host or target), such as HP-UX or eCos. Example is ‘gdb.hp’, for HP-UX.

gdb.compiler Tests specific to a particular compiler. As of this writing (June 1999), there aren’t currently any groups of tests in this category that couldn’t just as sensibly be made platform-specific, but one could imagine a gdb.gcc, for tests of GDB’s handling of GCC extensions.

gdb.subsystem Tests that exercise a specific GDB subsystem in more depth. For instance, ‘gdb.disasm’ exercises various disassemblers, while ‘gdb.stabs’ tests pathways through the stabs symbol reader.

15.3 Writing Tests

In many areas, the GDB tests are already quite comprehensive; you should be able to copy existing tests to handle new cases.

You should try to use gdb_test whenever possible, since it includes cases to handle all the unexpected errors that might happen. However, it doesn’t cost anything to add new test procedures; for instance, ‘gdb.base/exprs.exp’ defines a test_expr that calls gdb_test multiple times.

Only use send_gdb and gdb_expect when absolutely necessary, such as when GDB has several valid responses to a command.

The source language programs do not need to be in a consistent style. Since GDB is used to debug programs written in many different styles, it’s worth having a mix of styles in the testsuite; for instance, some GDB bugs involving the display of source lines would never manifest themselves if the programs used GNU coding style uniformly.

16 Hints

Check the ‘README’ file, it often has useful information that does not appear anywhere else in the directory.

16.1 Getting Started

GDB is a large and complicated program, and if you first starting to work on it, it can be hard to know where to start. Fortunately, if you know how to go about it, there are ways to figure out what is going on.

This manual, the GDB Internals manual, has information which applies generally to many parts of GDB.

Information about particular functions or data structures are located in comments with those functions or data structures. If you run across a function or a global variable which does not have a comment correctly explaining what is does, this can be thought of as a bug in GDB; feel free to submit a bug report, with a suggested comment if you can figure out what the comment should say. If you find a comment which is actually wrong, be especially sure to report that.

Comments explaining the function of macros defined in host, target, or native dependent files can be in several places. Sometimes they are repeated every place the macro is defined. Sometimes they are where the macro is used. Sometimes there is a header file which supplies a default definition of the macro, and the comment is there. This manual also documents all the available macros.

Start with the header files. Once you some idea of how GDB’s internal symbol tables are stored (see ‘symtab.h’, ‘gdbtypes.h’), you will find it much easier to understand the code which uses and creates those symbol tables.

You may wish to process the information you are getting somehow, to enhance your understanding of it. Summarize it, translate it to another language, add some (perhaps trivial or non-useful) feature to GDB, use the code to predict what a test case would do and write the test case and verify your prediction, etc. If you are reading code and your eyes are starting to glaze over, this is a sign you need to use a more active approach.

Once you have a part of GDB to start with, you can find more specifically the part you are looking for by stepping through each function with the next command. Do not use step or you will quickly get distracted; when the function you are stepping through calls another function try only to get a big-picture understanding (perhaps using the comment at the beginning of the function being called) of what it does. This way you can identify which of the functions being called by the function you are stepping through is the one which you are interested in. You may need to examine the data structures generated at each stage, with reference to the comments in the header files explaining what the data structures are supposed to look like.

Of course, this same technique can be used if you are just reading the code, rather than actually stepping through it. The same general principle applies—when the code you are looking at calls something else, just try to understand generally what the code being called does, rather than worrying about all its details.

A good place to start when tracking down some particular area is with a command which invokes that feature. Suppose you want to know how single-stepping works. As a GDB user, you know that the step command invokes single-stepping. The command is invoked via command tables (see ‘command.h’); by convention the function which actually performs the command is formed by taking the name of the command and adding ‘_command’, or in the case of an info subcommand, ‘_info’. For example, the step command invokes the step_command function and the info display command invokes display_info. When this convention is not followed, you might have to use grep or M-x tags-search in emacs, or run GDB on itself and set a breakpoint in execute_command.

If all of the above fail, it may be appropriate to ask for information on bug-gdb. But never post a generic question like “I was wondering if anyone could give me some tips about understanding GDB”—if we had some magic secret we would put it in this manual. Suggestions for improving the manual are always welcome, of course.

16.2 Debugging GDB with itself

If GDB is limping on your machine, this is the preferred way to get it fully functional. Be warned that in some ancient Unix systems, like Ultrix 4.2, a program can’t be running in one process while it is being debugged in another. Rather than typing the command ./gdb ./gdb, which works on Suns and such, you can copy ‘gdb’ to ‘gdb2’ and then type

./gdb ./gdb2.

When you run GDB in the GDB source directory, it will read a ‘.gdbinit’ file that sets up some simple things to make debugging gdb easier. The info command, when executed without a subcommand in a GDB being debugged by gdb, will pop you back up to the top level gdb. See ‘.gdbinit’ for details.

If you use emacs, you will probably want to do a make TAGS after you configure your distribution; this will put the machine dependent routines for your local machine where they will be accessed first by M-.

Also, make sure that you’ve either compiled GDB with your local cc, or have run fixincludes if you are compiling with gcc.

16.3 Submitting Patches

Thanks for thinking of offering your changes back to the community of GDB users. In general we like to get well designed enhancements. Thanks also for checking in advance about the best way to transfer the changes.

The GDB maintainers will only install “cleanly designed” patches. This manual summarizes what we believe to be clean design for GDB.

If the maintainers don’t have time to put the patch in when it arrives, or if there is any question about a patch, it goes into a large queue with everyone else’s patches and bug reports.

The legal issue is that to incorporate substantial changes requires a copyright assignment from you and/or your employer, granting ownership of the changes to the Free Software Foundation. You can get the standard documents for doing this by sending mail to gnu@gnu.org and asking for it. We recommend that people write in "All programs owned by the Free Software Foundation" as "NAME OF PROGRAM", so that changes in many programs (not just GDB, but GAS, Emacs, GCC, etc) can be contributed with only one piece of legalese pushed through the bureacracy and filed with the FSF. We can’t start merging changes until this paperwork is received by the FSF (their rules, which we follow since we maintain it for them).

Technically, the easiest way to receive changes is to receive each feature as a small context diff or unidiff, suitable for "patch". Each message sent to me should include the changes to C code and header files for a single feature, plus ChangeLog entries for each directory where files were modified, and diffs for any changes needed to the manuals (gdb/doc/gdb.texinfo or gdb/doc/gdbint.texinfo). If there are a lot of changes for a single feature, they can be split down into multiple messages.

In this way, if we read and like the feature, we can add it to the sources with a single patch command, do some testing, and check it in. If you leave out the ChangeLog, we have to write one. If you leave out the doc, we have to puzzle out what needs documenting. Etc.

The reason to send each change in a separate message is that we will not install some of the changes. They’ll be returned to you with questions or comments. If we’re doing our job correctly, the message back to you will say what you have to fix in order to make the change acceptable. The reason to have separate messages for separate features is so that the acceptable changes can be installed while one or more changes are being reworked. If multiple features are sent in a single message, we tend to not put in the effort to sort out the acceptable changes from the unacceptable, so none of the features get installed until all are acceptable.

If this sounds painful or authoritarian, well, it is. But we get a lot of bug reports and a lot of patches, and many of them don’t get installed because we don’t have the time to finish the job that the bug reporter or the contributor could have done. Patches that arrive complete, working, and well designed, tend to get installed on the day they arrive. The others go into a queue and get installed as time permits, which, since the maintainers have many demands to meet, may not be for quite some time.

Please send patches directly to the GDB maintainers at gdb-patches@sourceware.cygnus.com.

16.4 Obsolete Conditionals

Fragments of old code in GDB sometimes reference or set the following configuration macros. They should not be used by new code, and old uses should be removed as those parts of the debugger are otherwise touched.

STACK_END_ADDR

This macro used to define where the end of the stack appeared, for use in interpreting core file formats that don’t record this address in the core file itself. This information is now configured in BFD, and GDB gets the info portably from there. The values in GDB’s configuration files should be moved into BFD configuration files (if needed there), and deleted from all of GDB’s config files. Any ‘foo-xdep.c’ file that references STACK END ADDR is so old that it has never been converted to use BFD. Now that’s old!

PYRAMID_CONTROL_FRAME_DEBUGGING

pyr-xdep.c

PYRAMID_CORE

pyr-xdep.c

PYRAMID_PTRACE

pyr-xdep.c

REG_STACK_SEGMENT

exec.c

Table of Contents

Scope of this Document . . . . . . . . . . . . . . . . . . . . . . . 1

1 Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Overall Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2.1 The Symbol Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2 The Target Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.3 Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3.1 Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2 Breakpoint Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.3 Single Stepping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.4 Signal Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.5 Thread Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.6 Inferior Function Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.7 Longjmp Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4 User Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4.1 Command Interpreter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.2 Console Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.3 TUI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.4 libgdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

5 Symbol Handling . . . . . . . . . . . . . . . . . . . . . . . . . . 5

5.1 Symbol Reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.2 Partial Symbol Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.3 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5.4 Object File Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5.4.1 a.out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.4.2 COFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.4.3 ECOFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.4.4 XCOFF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.4.5 PE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.4.6 ELF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.4.7 SOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.4.8 Other File Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.5 Debugging File Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.5.1 stabs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.5.2 COFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.5.3 Mips debug (Third Eye). . . . . . . . . . . . . . . . . . . . . . . . . 9 5.5.4 DWARF 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.5.5 DWARF 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.5.6 SOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.6 Adding a New Symbol Reader to GDB. . . . . . . . . . . . . . . . . . . 10

6 Language Support . . . . . . . . . . . . . . . . . . . . . . . . 10

6.1 Adding a Source Language to GDB . . . . . . . . . . . . . . . . . . . . . . 10

7 Host Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7.1 Adding a New Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.2 Host Conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8 GDB Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

8.1 Libraries used by GDB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 8.2 GDB Directory Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 8.3 Overview of Source Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

8.3.1 Top Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 8.3.2 GDB Targets / Program Control. . . . . . . . . . . . . . . . 19 8.3.3 Types, Values, and Expressions . . . . . . . . . . . . . . . . . 20 8.3.4 Stack Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 8.3.5 Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 8.3.6 Symbol File Management . . . . . . . . . . . . . . . . . . . . . . 21 8.3.7 Language-Specific Sources . . . . . . . . . . . . . . . . . . . . . . 23 8.3.8 Kernel Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 8.3.9 Sources Specific to Mac OS X . . . . . . . . . . . . . . . . . . 23 8.3.10 PowerPC-specific Sources . . . . . . . . . . . . . . . . . . . . . 24 8.3.11 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.3.12 Assorted Utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

9 Target Architecture Definition. . . . . . . . . . . . . 25

9.1 Registers and Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 9.2 Frame Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 9.3 Inferior Call Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 9.4 Compiler Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 9.5 Target Conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 9.6 Adding a New Target. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

10 Target Vector Definition . . . . . . . . . . . . . . . . . 36

10.1 File Targets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 10.2 Standard Protocol and Remote Stubs . . . . . . . . . . . . . . . . . . . 36 10.3 ROM Monitor Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 10.4 Custom Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 10.5 Transport Layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 10.6 Builtin Simulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

11 Native Debugging . . . . . . . . . . . . . . . . . . . . . . . 37

11.1 Native core file Support. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 11.2 ptrace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 11.3 /proc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 11.4 win32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 11.5 shared libraries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 11.6 Native Conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

12 Support Libraries . . . . . . . . . . . . . . . . . . . . . . . . 41

12.1 BFD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 12.2 opcodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 12.3 readline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 12.4 mmalloc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 12.5 libiberty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 12.6 gnu-regex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 12.7 include . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

13 Coding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

13.1 Cleanups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 13.2 Wrapping Output Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 13.3 GDB Coding Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

13.3.1 Formatting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 13.3.2 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 13.3.3 C Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 13.3.4 Function Prototypes . . . . . . . . . . . . . . . . . . . . . . . . . . 45 13.3.5 Clean Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

14 Porting GDB . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

14.1 Configuring GDB for Release. . . . . . . . . . . . . . . . . . . . . . . . . . . 48

15 Testsuite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

15.1 Using the Testsuite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 15.2 Testsuite Organization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 15.3 Writing Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

16 Hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

16.1 Getting Started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 16.2 Debugging GDB with itself . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 16.3 Submitting Patches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 16.4 Obsolete Conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53