Wrapping library functions using -Wl,--wrap=symbol

Makefile

CC := gcc
CFLAGS += -Wall -Wextra -std=c99
LDLIBS += -lpthread

all: test_wrap

test_wrap: LDFLAGS += -Wl,--wrap=pthread_create -Wl,--wrap=pthread_join
test_wrap: test_wrap.c wrap.c

clean:
	rm -rf test_wrap

.PHONY: all clean

test_wrap.c

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

unsigned int num_threads;

static void *thread_entry_fn(void *arg)
{
    (void)arg;
    return NULL;
}

int main(int argc, char *argv[])
{
    unsigned int n = argc > 1 && atoi(argv[1]) >= 0 ? atoi(argv[1]) : 4;
    pthread_t *threads = malloc(n * sizeof(pthread_t));
    if (!threads) return EXIT_FAILURE;

    for (unsigned int i = 0; i < n; i++) {
        pthread_create(&threads[i], NULL, thread_entry_fn, NULL);
    }

    assert(num_threads == n);

    for (unsigned int i = 0; i < n; i++) {
        pthread_join(threads[i], NULL);
    }

    assert(num_threads == 0);

    free(threads);

    return 0;
}

wrap.c

#include <pthread.h>

extern unsigned int num_threads;

// From the docs:
// Any undefined reference to `symbol` will be resolved to `__wrap_symbol`. Any
// undefined reference to `__real_symbol` will be resolved to `symbol`. This
// can be used to provide a wrapper for a system function. The wrapper function
// should be called `__wrap_symbol`. If it wishes to call the system function,
// it should call `__real_symbol`.

extern int __real_pthread_create(pthread_t *, const pthread_attr_t *, void *(*)(void *), void *);

extern int __real_pthread_join(pthread_t, void **);

int __wrap_pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void *), void *arg)
{
    num_threads++;
    return __real_pthread_create(thread, attr, start_routine, arg);
}

int __wrap_pthread_join(pthread_t thread, void **retval)
{
    num_threads--;
    return __real_pthread_join(thread, retval);
}