__attribute__((weak)) doesn't work

Question

I have wrote below code:

#include<stdio.h>
#include<pthread.h>

int pthread_create(
        pthread_t*,
        const pthread_attr_t *,
        void* (*)(void *),
        void*)  __attribute__ ((weak))  ;


int main(){
    if (pthread_create) {
        printf("multi-thread");
    } else {
        printf("single-thread");
    }
}

When compiling the code with different ways and running it, it shows the same consequence . The codes are undermentioned:

zoushengfu@zoushengfudeMacBook-Pro:~/Documents/code/c++$ gcc -o pt
pthread.cpp  
zoushengfu@zoushengfudeMacBook-Pro:~/Documents/code/c++$ ./pt 
multi-thread
zoushengfu@zoushengfudeMacBook-Pro:~/Documents/code/c++$ gcc -o pt pthread.cpp -lpthread
zoushengfu@zoushengfudeMacBook-Pro:~/Documents/code/c++$ ./pt
multi-thread

I want to know why the result is same wheather I use library pthread when compile it. And do I some error when using __attribute__((weak))