Array iteration in multiple threads - c

Question

I've got the following example, let's say I want for each thread to count from 0 to 9.
void* iterate(void* arg) {
int i = 0;
while(i<10) {
i++;
}
pthread_exit(0);
}
int main() {
int j = 0;
pthread_t tid[100];
while(j<100) {
pthread_create(&tid[j],NULL,iterate,NULL);
pthread_join(tid[j],NULL);
}
}
variable i - is in a critical section, it will be overwritten multiple times and therefore threads will fail to count.
int* i=(int*)calloc(1,sizeof(int));
doesn't solve the problem either. I don't want to use mutex. What is the most common solution for this problem?

Answer 1

As other users are commenting, there are severals problems in your example:
Variable i is not shared (it should be a global variable, for instance), nor in a critical section (it is a local variable to each thread). To have a critical section you should use locks or transactional memory.
You don't need to create and destroy threads every iteration. Just create a number of threads at the beggining and wait for them to finish (join).
pthread_exit() is not necessary, just return from the thread function (with a value).
A counter is a bad example for threads. It requires atomic operations to avoid overwriting the value of other threads. Actually, a multithreaded counter is a typical example of why atomic accesses are necessary (see this tutorial, for example).
I recommend you to start with some tutorials, like this or this.
I also recommend frameworks like OpenMP, they simplify the semantics of multithreaded programs.
EDIT: example of a shared counter and 4 threads.
#include <stdio.h>
#include <pthread.h>
#define NUM_THREADS 4
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static int counter = 0;
void* iterate(void* arg) {
int i = 0;
while(i++ < 10) {
// enter critical section
pthread_mutex_lock(&mutex);
++counter;
pthread_mutex_unlock(&mutex);
}
return NULL;
}
int main() {
int j;
pthread_t tid[NUM_THREADS];
for(j = 0; j < NUM_THREADS; ++j)
pthread_create(&tid[j],NULL,iterate,NULL);
// let the threads do their magic
for(j = 0; j < NUM_THREADS; ++j)
pthread_join(tid[j],NULL);
printf("%d", counter);
return 0;
}