foo.c

#include <assert.h>

void free(void *ptr) {
    assert(0);
}

void foo(void *ptr) {
    free(ptr);
}

main.c

#include <dlfcn.h>
#include <assert.h>
#include <stdlib.h>

int main() {
    void *lib = dlopen("libfoo." EXT, RTLD_NOW);
    assert(lib);

    void* ptr = malloc(8);
    void (*foo_fn)(void*) = dlsym(lib, "foo");
    assert(foo_fn);
    foo_fn(ptr);
}

Makefile

ifeq ($(shell uname),Darwin)
all:
	$(CC) -dynamiclib foo.c -o libfoo.dylib
	$(CC) main.c -o main -DEXT='"dylib"'
	./main
else
all:
	$(CC) -fPIC -shared foo.c -o libfoo.dylib
	$(CC) main.c -o main -ldl -DEXT='"so"'
	./main
endif

output-linux

$ make
cc -fPIC -shared foo.c -o libfoo.so
cc main.c -o main -ldl -DEXT='"so"'
./main

output-osx

$ make
cc -dynamiclib foo.c -o libfoo.dylib                                                                                
cc main.c -o main -DEXT='"dylib"'                                                                                   
./main                                                                                                              
Assertion failed: (0), function free, file foo.c, line 4.                                                           
make: *** [all] Abort trap: 6