If you intend to run a program under the debugger, you must first generate debugging information when you compile it.
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.
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, the debugger never sees that variable — because the compiler optimizes it out of existence.
Some things do not work as well with -g -O as with -g -O0, particularly on machines with instruction scheduling. If in doubt, recompile with -g -O0, and if this fixes the problem, please report it to us as a bug (including a test case!).