How to make weak linking work with GCC?

问题:

There seem to be 3 ways of telling GCC to weak link a symbol:

• __attribute__((weak_import))
• __attribute__((weak))
• #pragma weak symbol_name

None of these work for me:

#pragma weak asdf
extern void asdf(void) __attribute__((weak_import, weak));
...
{
    if(asdf != NULL) asdf();
}

I always get a link error like this:

Undefined symbols:
  "_asdf", referenced from:
      _asdf$non_lazy_ptr in ccFA05kN.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

I am using GCC 4.0.1 on OS X 10.5.5. What am I doing wrong?

回答1:

I just looked into this and thought some others might be interested in my findings.

Weak linking with weak_import really only works well with dynamic libraries. You can get it to work with static linking (by specifying -undefined dynamic_lookup as suggested above) but this isn't such a hot idea. It means that no undefined symbols will be detected until runtime. This is something I would avoid in production code, personally.

Here is a Mac OS X Terminal session showing how to make it work:

Here is f.c

int f(int n)
{
    return n * 7;
}

Here is whatnof.c

#include <stdio.h>
#include <stdlib.h>

extern int f (int) __attribute__((weak_import));

int main() {
    if(f == NULL)
        printf("what, no f?\n");
    else
        printf("f(8) is %d\n", f(8));
    exit(0);
}

Make a dynamic library from f.c:

$ cc -dynamiclib -o f.dylib f.c

Compile and link against the dynamic lib, list dynamic libs.

$ cc -o whatnof whatnof.c f.dylib
$ otool -L whatnof
whatnof:
       f.dylib (compatibility version 0.0.0, current version 0.0.0)
       /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.0)

Run whatnof to see what happens:

$ whatnof
f(8) is 56

Now replace f.dylib with an empty library (no symbols):

$ mv f.dylib f.dylib.real
$ touch null.c
$ cc -dynamiclib -o f.dylib null.c

Run same whatnof to see what happens:

$ whatnof
what, no f?

The basic idea (or "use case") for weak_import is that it lets you link against a set of dynamic (shared) libraries, but then run the same code against earlier versions of the same libraries. You can check functions against NULL to see if they're supported in the particular dynamic library that the code is currently running against. This seems to be part of the basic development model supported by Xcode. I hope this example is useful; it has helped put my mind at ease about this part of the Xcode design.

回答2:

Add -Wl,-flat_namespace,-undefined,dynamic_lookup to the gcc compiler line that you use to do the final link.

回答3:

You need to set the MACOSX_DEPLOYMENT_TARGET variable to 10.2 or later. See Apple's documentation and their technote on weak linking.

回答4:

From the gcc doc manual:

weak

The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker.

which means that an object is legitimated to overwrite a weak symbol (defined in another object/library) without getting errors at link time. What is unclear is whether you are linking the library with the weak symbol or not. It's seems that both you have not defined the symbol and the library is not properly linked.