How do you suppress GCC linker warnings?

Question

I've been on a crusade lately to eliminate warnings from our code and have become more familiar with GCC warning flags (such as -Wall, -Wno-<warning to disable>, -fdiagnostics-show-option, etc.). However I haven't been able to figure out how to disable (or even control) linker warnings. The most common linker warning that I was getting is of the following form:

ld: warning: <some symbol> has different visibility (default) in 
<path/to/library.a> and (hidden) in <path/to/my/class.o>

The reason I was getting this was because the library I was using was built using the default visibility while my application is built with hidden visibility. I've fixed this by rebuilding the library with hidden visibility.

My question though is: how would I suppress that warning if I wanted to? It's not something that I need to do now that I've figured out how to fix it but I'm still curious as to how you'd suppress that particular warning — or any linker warnings in general?

Using the -fdiagnostics-show-option for any of the C/C++/linker flags doesn't say where that warning comes from like with other compiler warnings.

Answer 1

Actually, you can't disable a GCC linker warning, as it's stored in a specific section of the binary library you're linking with. (The section is called .gnu.warning.symbol)

You can however mute it, like this (this is extracted from libc-symbols.h):

Without it:

#include <sys/stat.h>

int main()
{
    lchmod("/path/to/whatever", 0666);
    return 0;
}

Gives:

$ gcc a.c
/tmp/cc0TGjC8.o: in function « main »:
a.c:(.text+0xf): WARNING: lchmod is not implemented and will always fail

With disabling:

#include <sys/stat.h>

/* We want the .gnu.warning.SYMBOL section to be unallocated.  */
#define __make_section_unallocated(section_string)    \
  __asm__ (".section " section_string "\n\t.previous");

/* When a reference to SYMBOL is encountered, the linker will emit a
   warning message MSG.  */
#define silent_warning(symbol) \
  __make_section_unallocated (".gnu.warning." #symbol) 

silent_warning(lchmod)

int main()
{
    lchmod("/path/to/whatever", 0666);
    return 0;
}

gives:

$ gcc a.c
/tmp/cc195eKj.o: in function « main »:
a.c:(.text+0xf): WARNING:

With hiding:

#include <sys/stat.h>

#define __hide_section_warning(section_string)    \
    __asm__ (".section " section_string "\n.string \"\rHello world!                      \"\n\t.previous");

/* If you want to hide the linker's output */
#define hide_warning(symbol) \
  __hide_section_warning (".gnu.warning." #symbol) 


hide_warning(lchmod)

int main()
{
    lchmod("/path/to/whatever", 0666);
    return 0;
}

gives:

$ gcc a.c
/tmp/cc195eKj.o: in function « main »:
Hello world!

Obviously, in that case, replace Hello world! either by multiple space or some advertisement for your wonderful project.

Answer 2

Unfortunately ld does not appear to have any intrinsic way of suppressing specific options. One thing that I found useful was limiting the number of duplicate warnings by passing -Wl,--warn-once to g++ (or you can pass --warn-once directly to ld).