How to re-export a dylib symbol from a dependent bundle

dylib, linker, xcode

Solution

This might be what you're looking for, if I understood you correctly. I'll be using libpthread as the hypothetical dylib containing functions you want to re-export.

mybundle.c:

#include <pthread.h>
#include <stdio.h>
void *foo(void *ctx) {
    puts((char *)ctx);
    return 0;
}

mybundle.exp:

_foo
_pthread_create
_pthread_join

Compile the bundle, dynamically linking to libpthread.dylib:

josh$ gcc -bundle -lpthread -Wl,-exported_symbols_list,mybundle.exp -o mybundle.so mybundle.c

myloader.c:

#include <dlfcn.h>
#include <pthread.h>    // merely for type definitions
#include <assert.h>
#include <stdio.h>

int main() {
    void *(*foo)(void *ctx);
    /* the following cannot be declared globally without changing their names, 
       as they are already (and unnecessarily) declared in <pthread.h> */
    int (*pthread_create)(pthread_t *thrd, const pthread_attr_t *attr, void *(*proc)(void *), void *arg);
    int (*pthread_join)(pthread_t thrd, void **val);

    void *bundle;
    assert(bundle = dlopen("mybundle.so", RTLD_NOW));
    assert(foo = dlsym(bundle, "foo"));
    assert(pthread_create = dlsym(bundle, "pthread_create"));
    assert(pthread_join = dlsym(bundle, "pthread_join"));

    pthread_t myThrd;
    pthread_create(&myThrd, 0, foo, "in another thread");
    pthread_join(myThrd, 0);

    return 0;
}

Compile the loader:

josh$ gcc myloader.c -o myloader

Run:

josh$ ./myloader
in another thread

Observe that myloader is in no way linked to pthread, yet pthread functions are loaded and available at runtime through the bundle.

Problem

Working with Xcode, I'm looking to re-export a symbol (a function specifically) from a mach-o bundle binary, where the symbol is originally defined in a dylib. I've tried the -sub_library linker switch but that doesn't seem to re-export the dylib symbols, probably because I'm not building a dylib myself(?) And the reexport-l / reexport_library switches seem to be unsupported in Xcode's linker. Any ideas?

Original source

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