////////// bar.c
#include <stdio.h>

void bar ()
{
    printf("bar\n");
}

/////////// foo.c

#include <stdio.h>
#include <mach-o/dyld.h>
#include <mach-o/loader.h>


struct dyld_image_info {
    const struct mach_header*   imageLoadAddress;   /* base address image is mapped into */
    const char*                 imageFilePath;      /* path dyld used to load the image */
    uintptr_t                   imageFileModDate;   /* time_t of image file */
                                                    /* if stat().st_mtime of imageFilePath does not match imageFileModDate, */
                                                    /* then file has been modified since dyld loaded it */
};

enum dyld_image_states
{
    dyld_image_state_mapped                 = 10,       // No batch notification for this
    dyld_image_state_dependents_mapped      = 20,       // Only batch notification for this
    dyld_image_state_rebased                = 30,
    dyld_image_state_bound                  = 40,
    dyld_image_state_dependents_initialized = 45,       // Only single notification for this
    dyld_image_state_initialized            = 50,
    dyld_image_state_terminated             = 60        // Only single notification for this
};

typedef const char* (*dyld_image_state_change_handler)(enum dyld_image_states state, uint32_t infoCount, const struct dyld_image_info info[]);

void dyld_register_image_state_change_handler(enum dyld_image_states state, int batch, dyld_image_state_change_handler handler);


const char* onLoad (enum dyld_image_states state, uint32_t infoCount, const struct dyld_image_info info[])
{
    printf("loading image=%s\n", info[0].imageFilePath);
    return NULL;
}

const char* onUnload (enum dyld_image_states state, uint32_t infoCount, const struct dyld_image_info info[])
{
    printf("unloading image=%s\n", info[0].imageFilePath);
    return NULL;
}

void foo ()
{
    printf("foo\n");

    dyld_register_image_state_change_handler(dyld_image_state_initialized, 0, &onLoad);
    dyld_register_image_state_change_handler(dyld_image_state_terminated, 0, &onUnload);
}

//////// main.c

#include <stdio.h>
#include <dlfcn.h>

void loadBar ()
{
    void* handle = dlopen("bar.dylib", RTLD_LAZY);
    void (*bar)() = dlsym(handle, "bar");
    bar();
    int r = dlclose(handle);
}

void loadFoo ()
{
    void* handle = dlopen("foo.dylib", RTLD_LAZY);
    void (*foo)() = dlsym(handle, "foo");
    foo();
    int r = dlclose(handle);
}

int main (int argc, const char *argv[])
{
    loadFoo();
    loadBar();
    return 0;
}

$ gcc -o foo.dylib foo.c -dynamiclib
$ gcc -o bar.dylib bar.c -dynamiclib
$ gcc -o main main.c