Behaviour of weak symbol without definition

Question

From what I searched, weak symbols are useful to provide overridable default implementations of library functions. However, it is also allowed that weak symbols be declared only, without definition. Quoting Wikipedia,

When linking a binary executable, a weakly declared symbol does not need a definition. In comparison, (by default) a declared strong symbol without a definition triggers an undefined symbol link error.

For example, the following program compiles fine to an executable, only to cause a segmentation fault at runtime:

void __attribute__((weak)) test(void);

int main(void) {
    test();
}

readelf and gdb confirms that the GOT entry of the function is 0x0.

• What would be a legitimate use of such weak symbols without (even an empty) implementation?
• Why does the linker use 0x0 as a placeholder, instead of issuing a diagnostic message?

Answer 1

What would be a legitimate use of such weak symbols without (even an empty) implementation?

You can call the function if it is defined, and not call it otherwise:

#include <stdio.h>
void __attribute__((weak)) test(void);

int main(void) {
    if (&test == NULL) {
      printf("test() is not defined\n");
    } else {
      printf("test() is defined, calling it now ...\n");
      test();
    }
    return 0;
}

Why does the linker use 0x0 as a placeholder, instead of issuing a diagnostic message?

The linker can't tell whether a definition will be available at runtime.

The entire reason you'd declare the function as weak external symbol is if you want to be able to handle that function's presence as optional.

If you want linker to issue diagnostics when test() is not defined, then don't declare it as a weak symbol.

Update:

What would be some other ways to use this without resorting to the LD_PRELOAD trick?

LD_PRELOAD has nothing to do with this.

Imagine your executable can work with old and new version of some_library.so. The new version defines somefunc(), and you want to call that function IFF it is present (e.g. because it is faster, or better in some other way than the "old" way of doing things).