Spawning Arbitrary Number of Threads

Question

I'm trying to write a program which will spawn an arbitrary number of threads, similar to the code I have in Convert a process based program into a thread based version? , which uses processes to do what I'm trying to accomplish, so far I have the following code, I'm getting a lot of warnings currently, but I'm really wondering if I'm approaching what I'm trying to do somewhat correctly.

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

void *runner(void *param); //the thread

int main(int argc, char *argv[]) {
  pthread_t tid = gettid();
  pthread_attr_t attr;

  if (argc != 2){
    fprintf(stderr, "Usage: a.out <integer value>\n");
    return -1;
  }

  if (atoi(argv[1]) < 0) {
    fprintf(stderr, "Argument %d must be non negative\n", atoi(argv[1]));
    return -1;
  }

  printf("My thread identifier is: %d\n", tid);

  // default attributes
  pthread_attr_init(&attr);

  // create the thread
  pthread_create(&tid, &attr, runner, argv[1]);

  // wait for the thread to exit
  pthread_join(tid, NULL);

}

void *runner(void *param){
  //int i, upper = atoi(param);
  int i;
  srand48(gettid());
  int max = nrand()%100;

  if (max > 0){
    for (i=1; i<=max; i++){
      printf("Child %d executes iteration\n", param, i);
    }
  }
  pthread_exit(0);
}

Appreciate any guidance I can get with this!

Answer 1

If I understand your objective, you want to create the number of threads as the command line parameter indicates.

(remembering that any specific OS can only support a fixed number of threads, which varies depending on the OS, so I will not validate the magnitude that number here.)

the following proposed code:

1. cleanly compiles
2. performs the desired functionality
3. documents why each header file is included
4. checks for error indications returned from C library functions, like pthread_create()

and now the proposed code:

#include <stdio.h>   // printf(), perror(), NULL
#include <pthread.h> // pthread_create(), pthread_join(), pthread_t
#include <stdlib.h>  // exit(), EXIT_FAILURE, atof()

void *runner(void *param); //the thread

int main(int argc, char *argv[]) 
{
    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s <integer value>\n", argv[0]);
        exit( EXIT_FAILURE );
    }

    // might want to use: `strtol()` rather than `atoi()` 
    // so can check for errors
    size_t maxThreads = (size_t)atoi(argv[1]);

    pthread_t tid[ maxThreads ];   
    for( size_t i=0; i<maxThreads; i++ )
    {
        tid[i] = 0;
    }

    // create the threads
    for( size_t i=0; i<maxThreads; i++ )
    {
        if( pthread_create( &tid[i], NULL, runner, (void *)i ) )
        { 
            perror( "pthread_create failed" );
        }
    }

    // wait for each thread to exit
    for( size_t i = 0; i<maxThreads; i++ )
    {
        // if thread was created, then wait for it to exit
        if( tid[i] != 0 )
        {
            pthread_join( tid[i], NULL );
        }
    }
}


void *runner(void *arg)
{
    size_t threadNum = (size_t)arg;
    printf( "in thread: %zu\n", threadNum );

    pthread_exit( NULL );
}

a run with no command line parameter results in: (where the executable is named: untitled

Usage: ./untitled <integer value>

a run with a command line parameter of 10 results in:

in thread: 0
in thread: 4
in thread: 2
in thread: 6
in thread: 1
in thread: 5
in thread: 7
in thread: 8
in thread: 9
in thread: 3

which makes it clear that threads are run in no particular order

Answer 2

1: I see no function called gettid()

pthread_t tid = gettid();
srand48(gettid());

2: You cannot print pthread_t as an integer, it's a structure

printf("My thread identifier is: %d\n", tid);

3: it's rand(), I have not seen nrand() before.

int max = nrand()%100;

Fix these and edit the question if required.