What GCC options should be set to have GCC as strict as possible? (and I do mean as strict as possible) I'm writing in C89 and want my code to be ANSI/ISO compliant.
I'd recommend using:
-Wall -Wextra -std=c89 -pedantic -Wmissing-prototypes -Wstrict-prototypes \
-Wold-style-definition
You should compile with -O as well as -g as some warnings are only available when the optimizer is used (actually, I usually use -O3 for spotting the problems). You might prefer -std=gnu89 as that disables fewer extensions in the libraries. OTOH, if you're coding to strict ANSI C89, maybe you want them disabled. The -ansi option is equivalent to -std=c89 but not quite as explicit or flexible.
The missing prototypes warns you about functions which are used (or external functions defined) without a prototype in scope. The strict prototypes means you can't use 'empty parentheses' for function declarations or definitions (or function pointers); you either need (void) or the correct argument list. The old style definition spots K&R style function definitions, such as:
int old_style(a, b) int a; double b; { ... }
If you're lucky, you won't need to worry about that. I'm not so lucky at work, and I can't use strict prototypes, much to my chagrin, because there are too many sloppy function pointers around.
See also: What is the best command-line tool to clean up code