Question

1. (12) Explain what will be output of the following program?

#include <stdio.h>

#include <stdlib.h>

#include <pthread.h>

#define NUM_THREADS 3

/* create thread argument struct for thr_func() */

typedef struct _thread_data_t {

  int tid;

  double stuff;

} thread_data_t;

/* thread function */

void *thr_func(void *arg) {

  thread_data_t *data = (thread_data_t *)arg;

  printf("hello from thr_func, thread id: %d\n", data->tid);

  pthread_exit(NULL);

}

int main(int argc, char **argv) {

  pthread_t thr[NUM_THREADS];

  int i, rc;

  thread_data_t thr_data[NUM_THREADS];

  /* create threads */

  for (i = 0; i < NUM_THREADS; ++i) {

    thr_data[i].tid = i;

    pthread_create(&thr[i], NULL, thr_func, &thr_data[i])

  }

  /* block until all threads complete */

  for (i = 0; i < NUM_THREADS; ++i) {

    pthread_join(thr[i], NULL);

  }

   return EXIT_SUCCESS;

}

Answer 1

A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.

Many aspects of C-language threading (and indeed threading in other languages) are non-deterministic. That is, they cannot be predicted, and vary from one run depending on factors that you have no control over.

C does not contain any built-in support for multithreaded applications. Instead, it relies entirely upon the operating system to provide this feature.

The above program Gives output for the first Compile & Run ->

hello from thre_func, thread id : 0

hello from thre_func, thread id: 1

hello from thre_func, thread id: 2

-------------------------------------------------------------------

, for second-time compile and run->

hello from thre_func, thread id: 2

hello from thre_func, thread id: 1

hello from thre_func, thread id: 0

-------------------------------------------------------------------

and for the third time Runs gives->

hello from thre_func, thread id : 1

hello from thre_func, thread id: 2

hello from thre_func, thread id: 0

-> thread_data_t thr_data[NUM_THREADS]; is called a function that passes the argument "NUM_THREADS" defines in Macro and assign 3 to it.

pthread_create(&thr[i], NULL, thr_func, &thr_data[i]);

here above function creates a new thread and makes it executable. This routine can be called any number of times from anywhere within your code but in the above program, it is called 3 times with the help of for loop