POSIX C PROGRAM(MUTEX PROGRAM)

I am a beginner in C programming and I am trying to perform mutex on the program below, but I'm not getting the proper output.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define NUM_THREAD 4
void *func(void *p);
int counter=0,a=2;
pthread_mutex_t mutexsum = PTHREAD_MUTEX_INITIALIZER;

main()
{
    int i,rc;
    pthread_t threadid[NUM_THREAD];

    for(i = 0; i< NUM_THREAD; i++)
    {
        a = a + i;
    开发者_如何学编程    printf("Value of a is %d\n",a);
        rc = pthread_create(&threadid[i],NULL,func,(void *)a);
        if(rc)
        {
            printf("Error in thred creation thread[%d] %d",i,rc);
        }
    }

    for(i = 0; i< NUM_THREAD; i++)
    {
        pthread_join(threadid[i],NULL);
    }

    printf("Final value of counter is %d\n",counter);
    pthread_exit(NULL);
}

void *func(void *p)
{
    int i;
    i = (int) p;
    pthread_mutex_lock(&mutexsum);
    counter = counter+a;
    printf("%d\n",counter);
    pthread_mutex_unlock(&mutexsum);
    pthread_exit(NULL);
}

As per the above program and my understanding, the desired output will be 18, but it's giving 32.

func uses a to increment. I'm guessing you meant to increment by i. As it is, by the time each thread runs, a must be at its final value of 8, so you are adding 8 to counter four times.

You are not using i in your thread function, but a.