Answer by Paul Hankin for Simple multithreading in C

You're passing in the address of i to each of the threads, so they each read the current state of i (and without synchronization, so the results are unpredictable if not undefined).

Instead, you need to pass a pointer to thread-specific data (that has a lifetime at least as long as the thread). For example:

#include <stdio.h>#include <stdlib.h>#include <pthread.h>void *foo(void *arg){    int *myid = (int*) arg;    printf("Hello from thread %d\n",*myid);    return NULL;}int main(void){   pthread_t tid[4];   int ids[4];   int i;   for(i=0;i<4;i++){       ids[i] = i;       pthread_create(&tid[i],NULL,(void*) foo, &ids[i]);   }   for(i=0;i<4;i++){       pthread_join(tid[i],NULL);   }   return 0;}