How do I call a function that returns a void pointer?

Question

I have this piece of code shown below.

Where nrthr stands for number of threads and is entered by the user. I always want my "main program" to call testFunc once, so if the user enter nrthr to be number 3 I then want to create 2 new threads. So my question is... how can I just call testFunc before the while loop?

int threadCount = 0;
...

// Call testFunc here

while(threadCount < nrthr - 1) {
        fprintf(stderr, "Created thread: %d\n", threadCount);
        if(pthread_create(&(tid[threadCount++]), NULL, testFunc, args) != 0)
            fprintf(stderr, "Can't create thread\n");
}

void *testFunc(void *arg)
{
    ...
}

What is wrong with something like `void* pin, *pout; pout = testFunc(pin);` ? — GMichael, Jun 30 '16 at 12:42
what prevents you from calling it as a normal function void *ret = testFunc(somedata/NULL)? — Sam Daniel, Jun 30 '16 at 12:43
I guess that's how I should do it, I just thought I could call it without creating a variable since the function doesn't return anything. — Fjodor, Jun 30 '16 at 12:46
@Fjodor, sure, if you don't care about the return value, you don't need to store it in a variable, and you can just call your function: `testFunc(NULL)`, or whatever value you want to pass to the function — nos, Jun 30 '16 at 12:49

score 2 · Answer 1 · answered Jun 30 '16 at 12:44

You can call testFunc like this:

void *result = testFunc(args);

Beware, however, if testFunc calls any pthread related functions. Since in this case the function is not running in a separate thread, calling functions like pthread_exit will not work as you might expect.

score 2 · Accepted Answer · edited May 23 '17 at 11:58

If testFunc is supposed to run on a separate thread, then it may be the case that it doesn't simply do something and return.

If that assumption is true, you can't simply call it before your loop, otherwise your main thread won't reach the point of creating the other threads to run simultaneously.

If that assumption is false, then you can simply call it like any other function, testFunc(args), and ignore the return value if you don't care about it. Another thing to note is the behaviour of pthread_exit when it's called from the main thread - See Is it OK to call pthread_exit from main?.

How do I call a function that returns a void pointer?

2 Answers2