Threads creation in C

Question

void print_hello_world() {
    pid_t pid = getpid();
    printf("Hello world %d\n", pid);
    pthread_exit(0);
}

void main() {
    pthread_t thread;
    pthread_create(&thread, NULL, (void *) &print_hello_world, NULL);
    print_hello_world();
}

I really couldn't understand what is the need of (void *) in pthread_create. And do we need "&print_hello_world" in the same or could drop "&" as I have read somewhere that while passing function pointers we don't need to place "&" before the function name.

score 3 · Accepted Answer · edited May 23 '17 at 10:26

3

Yes, there's no need for cast or & operator there:

pthread_create(&thread, NULL, print_hello_world, NULL);

should suffice as a function name gets converted into a function pointer when passing as argument to function(s).

Note that the function pointer you pass to pthread_create() takes a void* as argument and returns a void *. So your function should be:

void* print_hello_world(void *unused) {
...
}

It's the C way of implementing "generic" data type. For example, you can pass a int* or struct args* to the thread function and retrieve it back. E.g.

 int i=5;
 pthread_create(&thread, NULL, print_hello_world, &i);

and in the function print_hello_world(), you would do:

void *print_hello_world(void *value) {
  int i = *(int*)value;
  ... 
}

Basically void* allows you to pass any data pointer to the thread function here. If pthread_create()'s thread function were to take int*, you wouldn't be able to pass a struct args* to it, for example.

I suggest you read the following posts for more information on void pointer and its usage in C:

Concept of void pointer in C programming and What does void* mean and how to use it?

edited May 23 '17 at 10:26

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answered Aug 04 '15 at 13:10

P.P

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What could void* possibly mean? As int* mean the return value is an integer pointer. void pointer makes no sense. :/ – UtkarshPramodGupta Aug 04 '15 at 13:23
Could you pls elaborate? I couldn't understand – UtkarshPramodGupta Aug 04 '15 at 13:31
@UtkarshGupta The comment was getting too big. Please see the updated the answer. – P.P Aug 04 '15 at 13:37
Couldn't we have casted the passed argument from (int*) to (struct*) or vice-versa, had that been the case? – UtkarshPramodGupta Aug 04 '15 at 14:39
@UtkarshGupta Casting `TYPE1 *` to `TYPE2 *` is not guaranteed to work for all types (except casting to a character type: `char *`, `unsigned char*` or `signed char*`). Using `void *` is the only portable way without worrying about aliasing. – P.P Aug 04 '15 at 14:42
Are you sure about that? – UtkarshPramodGupta Aug 04 '15 at 14:43
@Olaf I never said conversion between different pointer types is valid. OP asked "caste[d] the passed argument from from (int*) to (struct*) or vice-versa" and I replied to that. I don't get what you are trying to say. – P.P Aug 04 '15 at 18:04
@Olaf It's not about the the function pointer but the argument that the thread function takes and value it returns. I added one example with the argument it takes. The same is applicable to the return value too -- both are data pointers. There was no reference to function pointers in my comment. – P.P Aug 04 '15 at 18:20
Fair point! Sorry, I apparently missed the initial comment of Utkarsh. – too honest for this site Aug 04 '15 at 18:32

too honest for this site · Answer 2 · 2015-08-04T17:48:27.650

2

Casting a function pointer to/from void * is actually undefined behaviour. See 6.3.2.3, especially p1 and p8. Note that functions are no objects in C.

So the cast is wrong in fact and the addressof-operator & is unnecessary. You can cast, however, one function pointer to another (see §8 of the reference). But here, you shoud definitively have a proper signature for your function, as there is a reason it takes a pointer and returns one, too. So: do not cast, but get the signature (and semantics) correct.

Note: An empty identifier-list in a function declaration which is part of the definition is an obsolescent feature. Using the prototype-style (void) for an empty argument list is recommended. Also the minimal required signature for main is int main(void) (with respect to the above said).

edited Aug 04 '15 at 17:48

answered Aug 04 '15 at 13:15

too honest for this site

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Could you give a citation for the claim that a function with no arguments must be declared `[type] foo(void)`? Also, the real problem is that `pthread_create()` expects a pointer to a function with of type `void *(*)(void *)`. – EOF Aug 04 '15 at 16:37
@EOF: For `()` vs. `(void)` I was a bit overhasty; that is just to become obsolete in a future version (hopefully); i edited my text accordingly.Regarding the function pointer: [6.3.2.3§8](http://port70.net/~nsz/c/c11/n1570.html#6.3.2.3p8) actually covers that already. Oh, wait, I just noticed what you mean. Edited. – too honest for this site Aug 04 '15 at 17:46

Threads creation in C

2 Answers2