Here is a simple program using pthreads and controlling access to a common global array (string).

Without the mutex, the threads would alter the string at the same time and create undesired results.

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;  // create a global initializer
char GlobalString[] = "Hello World";


void* ThreadFunc (void *num)
{
int j;
int i= *(int*)num;  // function accepts a pointer to any type 
                    //  but we need to cast it to an int
for (j = 1; j<i; j++)
{
sleep (rand() % 3); // wait 0 to 2 seconds
pthread_mutex_lock (&mutex); // lock the mutex
if (i == 10)
{
GlobalString[0]='a';  // change some letters in the string
//sleep(2);
GlobalString[1]='a';
}
else
{
GlobalString[0]='b'; // change letters to something else.
//sleep(1);
GlobalString[1]='b';
}
printf ("%s\n", GlobalString);
pthread_mutex_unlock (&mutex);  // release variable
}
}

int main ()
{
int i = 10;
pthread_t tid1, tid2;
pthread_mutex_init (&mutex, NULL); // use the global initializer to start to create a thread
pthread_create (&tid1, NULL, ThreadFunc, &i); // create a thread to run our thread function, pass it the value 10
//sleep(5);
int j = 13;
pthread_create (&tid2, NULL, ThreadFunc, &j); // create a nother thread with value 13

// if(tid1) pthread_join (tid1, NULL);  // wait for thread to finish before continuing, not nec.
// if(tid2) pthread_join (tid2, NULL);
pthread_exit(NULL);

return 0;
}

Second Example:
		 
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>

		 //A bad idea
		 //void *thread(void *vargp) {
		// printf("First Thread No' %ld\n",pthread_self());
		// int *ptr = (int*)vargp;
		 //int i= *ptr;
		 //printf("in the thread  %d\n",i );

		 //pthread_exit((void*)ptr);
		// }
		 

		 
void *thread(void *vargp) {
  printf("First Thread No' %ld\n",pthread_self());
  //  int *ptr = (int*)vargp;
  // int i= *ptr;
  //printf("in the thread  %d\n",i ); 

  pthread_exit((void*)vargp);
}
void *thread2(void *vargp) {
  printf("Second Thread No' %ld\n",pthread_self());
  vargp  = (int*) malloc(sizeof(int));
  *((int*)vargp) = 31;
  //printf("in the second thread  %d\n",*((int*)vargp) );
  pthread_exit(vargp);
}
int main() {
  int i = 42;
  int * m;
  int * n = &i;
  
  pthread_t tid, tid2;
  pthread_create(&tid, NULL, thread, n);
  pthread_create(&tid2, NULL, thread2, m);


  pthread_join(tid, (void**)&n);
  pthread_join(tid2, (void**)&m);
  printf("tid = %d\n",*n);
  printf("tid2 = %d\n",*m);

 }

		 Also bad

		 #include <stdlib.h>
		 #include <pthread.h>
		 void *thread(void *vargp) {
		 printf("First Thread No' %ld\n",pthread_self());
		 int *ptr = (int*)vargp;
		 pthread_exit((void*)*ptr);
		 }
		 void *thread2(void *vargp) {
		 printf("Second Thread No' %ld\n",pthread_self());
		 int *ptr = (int*)vargp;
		 *ptr = 0;
		 pthread_exit((void*)31);
		 }
		 int main() {
		 int j,i = 42;
		 pthread_t tid, tid2;
		 pthread_create(&tid, NULL, thread, (void*)&i);
		 pthread_create(&tid2, NULL, thread2, (void*)&i);
		 pthread_join(tid, (void**)&i);
		 pthread_join(tid2, (void**)&j);
		 printf("tid = %d\n",i);
		 printf("tid2 = %d\n",j);}