gcc does not include extern variables in symbol table when optimisation is used

Question

My program contains many externally defined variables. When I compile it with -O0 flag, I see them in the symbol table, but not when I use -O1 or -O2. How can I force the compil

Answer 1

If your foo.c only (essentially) has

extern const int my_symbol;

then compile it with -O1 or -O2, that symbol will be optimized out. However, if you use that symbol in foo.c, for example

extern const int my_symbol;
extern int my_flag;

void foo(void)
{
    if (my_symbol)
        my_flag = 1;
}

All of those symbols will exist in foo.o even if you compile it with -O1 or -O2.