Question

I’m trying to make a simple program in C that will use main thread to print an result, but when I check thread ID when I create thread and when I print result its 2 different IDs. Here is my code:
Cx

#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdbool.h>
#include <time.h>
#include <sys/time.h>

void *Utskrift(void *simpleInt)
{
  int simple;

simple = (int)simpleInt;
/*Printing my result and thread id*/
printf(";Hello From Thread ! I got fed with
an int %d! AND it is THREAD    ::%d\n",simple,pthread_self());

 }


 main(){

pthread_t thread_id;
int test=2;
/*Using main thread to print test from method Utskrift*/
pthread_create (&thread_id, NULL,&Utskrift,(void *) test);
/*Taking look at my thread id*/
printf(" (pthread id %d) has started\n", pthread_self());
pthread_join(thread_id,NULL);


}

I’m new to Thread Programming and C as well. So I may have misunderstood pthread_create (&thread_id, NULL,&Utskrift,(void *) test);. Does it use my main thread to call method Utskrift and print the result, or does it create a new thread “child” to my main thread and then the child prints the result? If so, can you please explain for me how to use the main thread to print my “test”.

Output:

(pthread id -1215916352) has started ;Hello From Thread ! I got fed with an int 2! AND it is THREAD ::-1215919248

Answer 1

The pthread_create function(spec) creates a new thread which will execute the function you pass to it (in this case Utskrift). The value value passed in pthread_create‘s last parameter is passed to the function.

If you simply wanted to call the Utskrift function in your main thread, you could do it the normal way:

Utskrift((void *)test));

If you want to pass data back from your completed thread to another thread, you can use pthread_join which will return either the value returned by your thread’s start routine, or the value that thread passes to pthread_exit:

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

void *checker(void *arg) {
    int number = (int) arg;

    if (0 == number)
        return "number was zero";
    else
        return "number was not zero";
}

int main(void) {
    int test = 0;
    pthread_t thread_id;
    char *s;

    pthread_create (&thread_id, NULL, checker,(void *) test);
    pthread_join(thread_id, &s);
    printf("%s\n", s);

    test = 1;

    pthread_create (&thread_id, NULL, checker,(void *) test);
    pthread_join(thread_id, &s);
    printf("%s\n", s);

    return 0;
}