When you run a program under gdb, you must first generate
debugging information when you compile it.
You may start gdb with its arguments, if any, in an environment
of your choice. If you are doing native debugging, you may redirect
your program's input and output, debug an already running process, or
kill a child process.
6.1. Compiling for debugging
In order to debug a program effectively, you need to generate
debugging information when you compile it. This debugging information
is stored in the object file; it describes the data type of each
variable or function and the correspondence between source line numbers
and addresses in the executable code.
To request debugging information, specify the -g option when you run
Most compilers do not include information about preprocessor macros in
the debugging information if you specify the -g flag alone,
because this information is rather large. Version 3.1 of gcc,
the gnu C compiler, provides macro information if you specify the
options -gdwarf-2 and -g3; the former option requests
debugging information in the Dwarf 2 format, and the latter requests
"extra information". In the future, we hope to find more compact ways
to represent macro information, so that it can be included with
Many C compilers are unable to handle the -g and -O
options together. Using those compilers, you cannot generate optimized
executables containing debugging information.
gcc, the gnu C compiler, supports -g with or
without -O, making it possible to debug optimized code. We
recommend that you always use -g whenever you compile a
program. You may think your program is correct, but there is no sense
in pushing your luck.
When you debug a program compiled with -g -O, remember that the
optimizer is rearranging your code; the debugger shows you what is
really there. Do not be too surprised when the execution path does not
exactly match your source file! An extreme example: if you define a
variable, but never use it, gdb never sees that
variable--because the compiler optimizes it out of existence.
Some things do not work as well with -g -O as with just
-g, particularly on machines with instruction scheduling. If in
doubt, recompile with -g alone, and if this fixes the problem,
please report it to us as a bug (including a test case!).
Older versions of the gnu C compiler permitted a variant option
-gg for debugging information. gdb no longer supports this
format; if your gnu C compiler has this option, do not use it.