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### 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 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 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.