     START-INFO-DIR-ENTRY
     * Gdb-Internals: (gdbint).	The GNU debugger internals.
     END-INFO-DIR-ENTRY

This file documents the internals of the GNU debugger GDB.

Copyright (C) 1990, 1991 Free Software Foundation, Inc. Contributed by Cygnus Support.  Written by John Gilmore.

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.

Permission is granted to copy or distribute modified versions of this manual under the terms of the GPL (for which purpose this text may be regarded as a program in the language TeX).

This file documents the internals of the GNU debugger GDB.  It is a collection of miscellaneous information with little form at this point. Mostly, it is a repository into which you can put information about GDB as you discover it (or as you design changes to GDB).

The README File
***************

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

Defining a New Host or Target Architecture
******************************************

When building support for a new host and/or target, much of the work you need to do is handled by specifying configuration files; see Adding a New Configuration.  Further work can be divided into ``host-dependent'' (see Adding a New Host) and ``target-dependent'' (see Adding a New Target).  The following discussion is meant to explain the difference between hosts and targets.

What is considered ``host-dependent'' versus ``target-dependent''?
==================================================================

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; unfortunately, that means you must add both host and target support for new machines in this category.

The config/mh-*, xm-*.h and *-xdep.c files are for host support.  Similarly, the config/mt-*, tm-*.h and *-tdep.c files are for target support.  The question is, what features or aspects of a debugging or cross-debugging environment are considered to be ``host'' support?

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.

Unix child process support is considered an aspect of the host.  Since when you fork on the host you are still on the host, the various macros needed for finding the registers in the upage, running ptrace, and such are all in the host-dependent files.

This is still somewhat of a grey area; I (John Gilmore) didn't do the xm-* and tm-* split for gdb (it was done by Jim Kingdon) so I have had to figure out the grounds on which it was split, and make my own choices as I evolve it.  I have moved many things out of the xdep files actually, partly as a result of BFD and partly by removing duplicated code.

Adding a New Configuration
**************************

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 XXX (e.g. sun4), and its full three-part configuration name is XARCH-XVEND-XOS (e.g.  sparc-sun-sunos4).  In particular:

In the top level directory, edit config.sub and add XARCH, XVEND, and XOS to the lists of supported architectures, vendors, and operating systems near the bottom of the file.  Also, add XXX as an alias that maps to XARCH-XVEND-XOS.  You can test your changes by running

     ./config.sub XXX

and

     ./config.sub XARCH-XVEND-XOS

which should both respond with XARCH-XVEND-XOS and no error messages.

Now, go to the bfd directory and create a new file bfd/hosts/h-XXX.h.  Examine the other h-*.h files as templates, and create one that brings in the right include files for your system, and defines any host-specific macros needed by GDB.

Then edit bfd/configure.in.  Add shell script code to recognize your XARCH-XVEND-XOS configuration, and set my_host to XXX when you recognize it.  This will cause your file h-XXX.h to be linked to sysdep.h at configuration time.

Also, if this host requires any changes to the Makefile, create a file bfd/config/mh-XXX, which includes the required lines.

(If you have the binary utilities and/or GNU ld in the same tree, you'll also have to edit binutils/configure.in or ld/configure.in to match what you've done in the bfd directory.)

It's possible that the libiberty and readline directories won't need any changes for your configuration, but if they do, you can change the configure.in file there to recognize your system and map to an mh-XXX file.  Then add mh-XXX to the config/ subdirectory, to set any makefile variables you need.  The only current options in there are things like -DSYSV.

Aha!  Now to configure GDB itself!  Edit gdb/configure.in to recognize your system and set gdb_host to XXX, and (unless your desired target is already available) also set gdb_target to something appropriate (for instance, XXX).  To handle new hosts, modify the segment after the comment # per-host; to handle new targets, modify after # per-target.

Finally, you'll need to specify and define GDB's host- and target-dependent .h and .c files used for your configuration; the next two chapters discuss those.

Adding a New Host
*****************

Once you have specified a new configuration for your host (see Adding a New Configuration), there are two remaining pieces to making GDB work on a new machine.  First, you have to make it host on the new machine (compile there, handle that machine's terminals properly, etc).  If you will be cross-debugging to some other kind of system that's already supported, you are done.

If you want to use GDB to debug programs that run on the new machine, you have to get it to understand the machine's object files, symbol files, and interfaces to processes.  see Adding a New Target

Several files control GDB's configuration for host systems:

gdb/config/mh-XXX
     Specifies Makefile fragments needed when hosting on machine XXX. In particular, this lists the required machine-dependent object files, by defining XDEPFILES=....  Also specifies the header file which describes host XXX, by defining XM_FILE= xm-XXX.h.  You can also define CC, REGEX and REGEX1, SYSV_DEFINE, XM_CFLAGS, XM_ADD_FILES, XM_CLIBS, XM_CDEPS, etc.; see Makefile.in.

gdb/xm-XXX.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, ptrace support, and core file structure.  Crib from existing xm-*.h files to create a new one.

gdb/XXX-xdep.c
     Contains any miscellaneous C code required for this machine as a host.  On some machines it doesn't exist at all.

There are some ``generic'' versions of routines that can be used by various host systems.  These can be customized in various ways by macros defined in your xm-XXX.h file.  If these routines work for the XXX 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 XXX-xdep.c, and put XXX-xdep.o into XDEPFILES.

Generic Host Support Files
--------------------------

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

coredep.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 coredep.c, and should just provide fetch_core_registers in XXX-xdep.c.

coredep.c::register_addr()
     If your xm-XXX.h file defines the macro REGISTER_U_ADDR(reg) to be the offset within the user struct of a register (represented as a GDB register number), coredep.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 XXX-xdep.c file, and be sure XXX-xdep.o is in the XDEPFILES 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.

Object files needed when the target system is an XXX are listed in the file config/mt-XXX, in the makefile macro TDEPFILES = ....  The header file that defines the target system should be called tm-XXX.h, and should be specified as the value of TM_FILE in config/mt-XXX.  You can also define TM_CFLAGS, TM_CLIBS, and TM_CDEPS in there; see Makefile.in.

Now, you are now ready to try configuring GDB to compile for your system. From the top level (above bfd, gdb, etc), do:

     ./configure XXX +target=vxworks960

This will configure your system to cross-compile for VxWorks on the Intel 960, which is probably not what you really want, but it's a test case that works at this stage.  (You haven't set up to be able to debug programs that run on XXX yet.)

If this succeeds, you can try building it all with:

     make

Good luck!  Comments and suggestions about this section are particularly welcome; send them to bug-gdb@prep.ai.mit.edu.

When hosting GDB 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-dep.c; if not, it's in your XXX-xdep.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. (see Defining a New Host or Target Architecture, for more info about this.)

Adding a New Target
*******************

For a new target called TTT, first specify the configuration as described in Adding a New Configuration.  If your new target is the same as your new host, you've probably already done that.

A variety of files specify attributes of the GDB target environment:

gdb/config/mt-TTT
     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, and other Makefile variables here; see Makefile.in.

gdb/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. Crib from existing tm-*.h files when building a new one.

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.

gdb/exec.c
     Defines functions for accessing files that are executable on the target system.  These functions open and examine an exec file, extract data from one, write data to one, print information about one, etc.  Now that executable files are handled with BFD, every target should be able to use the generic exec.c rather than its own custom code.

gdb/ARCH-pinsn.c
     Prints (disassembles) the target machine's instructions. This file is usually shared with other target machines which use the same processor, which is why it is ARCH-pinsn.c rather than TTT-pinsn.c.

gdb/ARCH-opcode.h
     Contains some large initialized data structures describing the target machine's instructions. This is a bit strange for a .h file, but it's OK since it is only included in one place.  ARCH-opcode.h is shared between the debugger and the assembler, if the GNU assembler has been ported to the target machine.

gdb/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-XXX.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.

When adding support for a new target machine, there are various areas of support that might need change, or might be OK.

If you are using an existing object file format (a.out or COFF), there is probably little to be done.  See bfd/doc/bfd.texinfo for more information on writing new a.out or COFF versions.

If you need to add a new object file format, you are beyond the scope of this document right now.  Look at the structure of the a.out and COFF support, build a transfer vector (xvec) for your new format, and start populating it with routines.  Add it to the list in bfd/targets.c.

If you are adding a new operating system for an existing CPU chip, add a tm-XOS.h file that describes the operating system facilities that are unusual (extra symbol table info; the breakpoint instruction needed; etc).  Then write a tm-XARCH-XOS.h that just #includes tm-XARCH.h and tm-XOS.h.  (Now that we have three-part configuration names, this will probably get revised to separate the XOS configuration from the XARCH configuration.)

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!

Configuring GDB for Release
***************************

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

     make gdb.tar.Z

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.

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.

Binary File Descriptor Library Support for GDB
**********************************************

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.

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 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 symbol-files whose identification matches the specified prefix.

The functions supplied by each module are:

XXX_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 XXX_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 XXX_symfile_init, but it can call error if it detects an unavoidable problem.

XXX_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 XXX_symfile_init, if a new symbol table is being read, or may be called alone if all symbols are simply being discarded.

XXX_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 XXX_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 XXX_symfile_read is called, these psymtabs will contain a pointer to a function XXX_psymtab_to_symtab, which can be called from any point in the GDB symbol-handling code.

XXX_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.

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:

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).

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.

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 machine-dependent 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.)

FRAME_CHAIN
     Given a GDB frame, 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.