To request debugging information, specify the -g option when you run the compiler.
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 variablebecause 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.