Question

I compiled this code and the 99th threads that it’s been created keeps creating more than one thread of number 99. Instead if i insert values from 1-10 or something small then the results are quite normal.

Here is the code.

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

pthread_mutex_t m=PTHREAD_MUTEX_INITIALIZER;
pthread_attr_t attr;

void* test(void *a)
{
    int i=*((int *)a);
    printf("The thread %d has started.\n",i);
    pthread_mutex_lock(&m);
    usleep(10000);
    printf("The thread %d has finished.\n",i);
    pthread_mutex_unlock(&m);
    pthread_exit(NULL);

}

int main()
{
    int i=0,j=0;
    pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_JOINABLE);
    pthread_t thread[100];

    for (i=0;i<100;i++)
    {
        j=i;
        pthread_create(&thread[i],&attr,test,&j);

    }

    for (i=0;i<100;i++)
        pthread_join(thread[i],NULL);
    return 0;
}

i get:

..../*Normal Results*/
The thread 99 has finished.
The thread 99 has finished.
The thread 99 has finished.
The thread 99 has finished.
The thread 99 has finished.
The thread 99 has finished.

Why is this happening?

Answer 1

You need to keep all theadIds

int indexes[PTHREAD_COUNT];

for (i=0;i<100;i++) {
    indexes[i] = i;
    pthread_create(&thread[i], &attr, test, &indexes[i]);
}