Thread: do I really have to do pthread_mutex_lock?

The code :

Code:

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

#define NUM_THREADS	8

typedef struct {
    int             id;
    int             *ptr_arr;
    pthread_t       thread;
    pthread_mutex_t mutex;
} some_struct;

void *some_func (void *arg)
{
    some_struct *w = (some_struct *)arg;

    /* do I really need to lock? */
    pthread_mutex_lock(&w->mutex);
    w->ptr_arr[w->id] = w->id * 4;
    pthread_mutex_unlock(&w->mutex);
}

int main (void)
{
    int         arr[NUM_THREADS];
    int         cnt;
    some_struct ss[NUM_THREADS];

    /* initialize */
    for (cnt = 0; cnt < NUM_THREADS; ++cnt) {
	arr[cnt]        = cnt;
	ss[cnt].id      = cnt;
	ss[cnt].ptr_arr = arr;
	pthread_mutex_init(&ss[cnt].mutex, NULL);
    }

    /* print array */
    for (cnt = 0; cnt < NUM_THREADS; ++cnt) {
	printf("%3d%8d\n", cnt, arr[cnt]);
    }

    /* do something */
    for (cnt = 0; cnt < NUM_THREADS; ++cnt) {
	pthread_create(&ss[cnt].thread, NULL, some_func, &ss[cnt]);
    }

    /* join the threads */
    for (cnt = 0; cnt < NUM_THREADS; ++cnt) {
	pthread_join(ss[cnt].thread, NULL);
    }

    printf("\n");
    /* print them again */
    for (cnt = 0; cnt < NUM_THREADS; ++cnt) {
	printf("%3d%8d\n", cnt, arr[cnt]);
    }

    return(0);
}

The explanation:
As you can see, there are 8 threads. Each of them is modifying a single area of an array according to the thread id.

The question:
Do I really need to lock the mutex?