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* Gdb-Internals: (gdbint).      The GNU debugger's internals.

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   This file documents the internals of the GNU debugger GDB.

Copyright 1990,1991,1992,1993,1994,1996,1998,1999,2000,2001    Free

Software Foundation, Inc.  Contributed by Cygnus Solutions.  Written by

John Gilmore.  Second Edition by Stan Shebs.

   Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.1 or

any later version published by the Free Software Foundation; with the

Invariant Sections being "Algorithms" and "Porting GDB", with the

Front-Cover texts being "A GNU Manual," and with the Back-Cover Texts

as in (a) below.

   (a) The FSF's Back-Cover Text is: "You have freedom to copy and

modify this GNU Manual, like GNU software.  Copies published by the Free

Software Foundation raise funds for GNU development."

File: gdbint.info,  Node: Symbol Handling,  Next: Language Support,  Prev: User Interface,  Up: Top

Symbol Handling

***************

   Symbols are a key part of GDB's operation.  Symbols include

variables, functions, and types.

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 (*note Support Libraries::).  BFD identifies the type of

the file by examining its header.  `find_sym_fns' 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

symbol files 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 half-read 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 needs 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.

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 non-debugging 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.

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.

Object File Formats

===================

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'.

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'.

ECOFF

-----

   ECOFF is an extended COFF originally introduced for Mips and Alpha

workstations.

   The basic ECOFF reader is in `mipsread.c'.

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

*Note Top: (stabs)Top.

   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).

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.

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'.

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'.

Other File Formats

------------------

   Other file formats that have been supported by GDB include Netware

Loadable Modules (`nlmread.c').

Debugging File Formats

======================

   This section describes characteristics of debugging information that

are independent of the object file format.

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.

COFF

----

   The basic COFF definition includes debugging information.  The level

of support is minimal and non-extensible, and is not often used.

Mips debug (Third Eye)

----------------------

   ECOFF includes a definition of a special debug format.

   The file `mdebugread.c' implements reading for this format.

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'.

DWARF 2

-------

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

   The DWARF 2 reader is in `dwarf2read.c'.

SOM

---

   Like COFF, the SOM definition includes debugging information.

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.

File: gdbint.info,  Node: Language Support,  Next: Host Definition,  Prev: Symbol Handling,  Up: Top

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, like with DWARF.

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 the `evaluate_subexp' function

     defined in the file `eval.c'.  Add cases for new opcodes in two

     functions from `parse.c': `prefixify_subexp' and

     `length_of_subexp'.  These compute the number of `exp_element's

     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 `print_subexp' (in `expprint.c') 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_exp_1'

     (defined in `parse.c').

_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 `#ifdef's 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 `tar'red into the

     distribution!

File: gdbint.info,  Node: Host Definition,  Next: Target Architecture Definition,  Prev: Language Support,  Up: Top

Host Definition

***************

   _Maintainer's note: In theory, new targets no longer need to use the

host framework described below.  Instead it should be possible to

handle everything using autoconf.  Patches eliminating this framework

welcome._

   With the advent of Autoconf, it's rarely necessary to have host

definition machinery anymore.

Adding a New Host

=================

   Most of GDB's host configuration support happens via Autoconf.  New

host-specific definitions should be rarely needed.  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 DJGPP (a.k.a. GO32) execution environment.

`ser-tcp.c'

     This contains generic TCP support using sockets.

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 `'.

`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 conversion specifier `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 conversion specifier

     `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 conversion specifier

     `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 header file (typically,

     `unistd.h').

`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 `/**/'.