CODEWITHSUNDEEP
cpthreads
TMOTTM
TMOTTM

Reputation: 3381

How to pass an argument to a pthread thread function in C?

Is it possible to pass an argument to the thread function in pthreads? Say I create a thread:

int main() {
  pthread_t t1;
  pthread_create(&t1, NULL, callback, 10);
  pthread_join(t1, NULL);
  return 0;
}

And in callback I would like to just print out the 10. My problem now is that as per the C pthread API, the pthread_create() function signature is:

int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg);

Can I pass a pointer to an integer to the thread function and how?

Upvotes: 0

Views: 1770

Answers (2)

John Bollinger
John Bollinger

Reputation: 180201

For a single value of integer type not wider than intptr_t, it possible and fairly common to cast it to a pointer in the pthread_create call and cast it back in the thread function:

int main() {
    pthread_t t1;
    pthread_create(&t1, NULL, callback, (void *) 10);
    pthread_join(t1, NULL);
    return 0;
}


void *t1(void *arg) {
    int arg_as_int = (int) arg;
    // ...
}

Such casting is definitely allowed, and again, is pretty conventional.

I have recently started seeing objections to this approach here on SO, however, on the basis that the C standard allows the first of those casts to produce a trap representation, and that it does not require the round-trip of int to void * to int to be value-preserving. That argument characterizes the C standard correctly, but it disregards the facts that as a practical matter, the casting approach works on substantially every C implementation that supports pthreads in the first place, and that it is common enough that an implementation on which it did not work reliably would have difficulty being accepted.

Upvotes: 0

P.P
P.P

Reputation: 121387

You could use a variable, like:

  int val = 10;
  pthread_create(&t1, NULL, callback, &val);
  pthread_join(t1, NULL);
  ...

And in the callback:

void* callback(void *arg)
{
    int i = *(int *)arg;
    ....

    return NULL;
}

Note that this works because the lifetime of val is valid until the thread (callback) completes its execution as you wait with pthread_join(). Otherwise, you may want to allocate memory dynamically (e.g. malloc) and pass that object to the thread function.

Upvotes: 3

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