pthread mutex not working, getting a random value each time - c

Question

I'm learning POSIX pthreads and while using a mutex I started getting a weird output.
Every time I run the code it outputs a random number while I expect it to output 0. I checked the code and I could not figure out the reason why it happens so if can someone explain to me what is going wrong exactly it will be very appreciated.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int num = 0;
pthread_mutex_t mutex;
void* add(void* args)
{
int sign = *(int*) args;
for(int i = 0; i < 100; i++)
{
pthread_mutex_lock(&mutex);
num += sign;
pthread_mutex_unlock(&mutex);
}
pthread_exit(NULL);
return NULL;
}
int main()
{
pthread_t pool[1000];
int plus = +1;
int minus = -1;
pthread_mutex_init(&mutex, NULL);
for(int i = 0; i < 500; i++)
pthread_create(&pool[i], NULL, add, &plus);
for(int i = 500; i < 1000; i++)
pthread_create(&pool[i], NULL, add, &minus);
for(int i = 0; i < 1000; i++)
pthread_join(pool[i], NULL);
printf("%d", num);
return 0;
}

Answer 1

the following proposed code:
cleanly compiles
properly checks for errors
properly initializes the mutex
performs the desired functionality
results in an output of 0
Note: function: pthread_mutex_init() is a valid function, but a initial value of NULL is not valid
and now, the proposed code:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int num = 0;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void* add(void* args)
{
int sign = *(int*) args;
for(int i = 0; i < 100; i++)
{
pthread_mutex_lock(&mutex);
num += sign;
pthread_mutex_unlock(&mutex);
}
pthread_exit(NULL);
}
int main( void )
{
pthread_t pool[1000];
int plus = +1;
int minus = -1;
for(int i = 0; i < 500; i++)
if( pthread_create(&pool[i], NULL, add, &plus) != 0 )
{
perror( "pthread_create failed" );
pool[i] = 0;
}
for(int i = 500; i < 1000; i++)
{
if( pthread_create(&pool[i], NULL, add, &minus) != 0 )
{
perror( "pthread-create failed" );
pool[i] = 0;
}
}
for(int i = 0; i < 1000; i++)
{
if( pool[i] )
{
pthread_join(pool[i], NULL);
}
}
printf("%d", num);
return 0;
}
a run of the proposed code, on linux, results in:
0