Question

#include <pthread.h>
#define NUM_THREADS 4

void *work(void *i){
    printf("Hello, world from %i\n", pthread_self());
    pthread_exit(NULL); 
}

int main(int argc, char **argv){
    int i;
    pthread_t id[NUM_THREADS];
    for(i = 0; i < NUM_THREADS; ++i){
       if(pthread_create(&id[i], NULL, work, NULL)){
          printf("Error creating the thread\n"); exit(19);
       } 
    }
    printf("After creating the thread. My id is: %i\n",
    pthread_self());
    return 0;
}

i know the output is :

Hello, world from 2
Hello, world from 3
After creating the thread. My id is: 1
Hello, world from …

First of all this is not a homework.
POSIX is not my field so I just want an explanation of the output (no need to explain functions used since I know what they do) some quick answers of:

1. ARE the ids for pthreads (2 , 3 , 1) specified by the system??
2. the ++i used … did it affect the output somehow?
3. why is it that there are only 4 threads in the end (3 + main) why not 5?!
4. why is it that after main printed After creating …. another thread executed??!! how come??!!

Answer 1

1. I’m not sure whether it’s specified, but I believe (nearly?) all thread ids are started at 1 (the main) and incremented from there.
2. No, ++i has nothing to do with the output.
3. Since you didn’t pthread_join the threads, the main function (and therefore the program) exited before the last thread executed.
4. I’m not sure precisely what you’re trying to ask here, but the answer is concurrency. There’s no guarantee of when or in what order the threads execute.