William Bateman wrote:

char numstr; 
unsigned long num = 500; 
sprintf( numstr, "%lu", num );

Check your documentation for sprintf; the standard prototype is:

int sprintf ( char *buffer, const char *fmtstr, argument… );

Note that the 1st parameter is a pointer to a char, but you have supplied a char value - this is an excellent way to really mess up any 'C' program, embedded or otherwise!! :-0