Check the `README' file, it often has useful information that does not appear anywhere else in the directory.
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.
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.
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.
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
PYRAMID_CONTROL_FRAME_DEBUGGING
PYRAMID_CORE
PYRAMID_PTRACE
REG_STACK_SEGMENT
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