Compiler Options

While it is possible to run any executable under a debugger, it is much easier if you use some compiler options and don't use others. Usually, to make the executable smaller, much of the symbolic information (variable names, function names etc.) are thrown away by the compiler when it has finished with them. This means that the debugger can't tell what the value it's looking at is called, and makes the debugging session very cryptic.

The way around this is to use the '-g' compiler flag. This causes the compiler to insert lots of debugging information into the executable, so the debugger knows everything it needs to know. Using this flag will make the executable noticeably bigger, but won't affect the performance too much - it just appends tables of symbolic information to the executable file.

A lot of people are under the impression that the '-g' debugging flag makes a program run slower. The reason for this is that to use a debugger, you generally need to turn off any optimisation flags ('-O' or '-O1-9'). This can make a significant difference to the speed of the program, as the optimisers on modern compilers are pretty good.

The GNU compiler collection, gcc, is unusual in that it allows you to use '-g' alongside '-O' flags. While this seems like a good thing, there are hidden dangers - the optimiser may have rearranged the order of some bits of your code, and variables you used may not be changed when you think they should be, or they may have disappeared altogether!

In general, then, always use *either* '-g' *or* '-O', not both together.