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
the compiler.
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
-g alone.
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.