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.
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