mingw/info/gdb/Compilation.html

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  18 <h3 class="section">Compiling for debugging</h3>
  19
  20    <p>In order to debug a program effectively, you need to generate
  21 debugging information when you compile it.  This debugging information
  22 is stored in the object file; it describes the data type of each
  23 variable or function and the correspondence between source line numbers
  24 and addresses in the executable code.
  25
  26    <p>To request debugging information, specify the <code>-g</code> option when you run
  27 the compiler.
  28
  29    <p>Many C compilers are unable to handle the <code>-g</code> and <code>-O</code>
  30 options together.  Using those compilers, you cannot generate optimized
  31 executables containing debugging information.
  32
  33    GCC, the <small>GNU</small> C compiler, supports <code>-g</code> with or
  34 without <code>-O</code>, making it possible to debug optimized code.  We
  35 recommend that you <em>always</em> use <code>-g</code> whenever you compile a
  36 program.  You may think your program is correct, but there is no sense
  37 in pushing your luck.
  38
  39    <p>When you debug a program compiled with <code>-g -O</code>, remember that the
  40 optimizer is rearranging your code; the debugger shows you what is
  41 really there.  Do not be too surprised when the execution path does not
  42 exactly match your source file!  An extreme example: if you define a
  43 variable, but never use it, GDB never sees that
  44 variable--because the compiler optimizes it out of existence.
  45
  46    <p>Some things do not work as well with <code>-g -O</code> as with just
  47 <code>-g</code>, particularly on machines with instruction scheduling.  If in
  48 doubt, recompile with <code>-g</code> alone, and if this fixes the problem,
  49 please report it to us as a bug (including a test case!).
  50
  51    <p>Older versions of the <small>GNU</small> C compiler permitted a variant option
  52 <code>-gg</code> for debugging information.  GDB no longer supports this
  53 format; if your <small>GNU</small> C compiler has this option, do not use it.
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