call_once, once_flag, ONCE_FLAG_INIT - cppreference.com (original) (raw)

Defined in header <threads.h>
void call_once( once_flag* flag, void (*func)(void) ); (1) (since C11)
typedef /* unspecified */ once_flag (2) (since C11)
#define ONCE_FLAG_INIT /* unspecified */ (3) (since C11)

  1. Calls function func exactly once, even if invoked from several threads. The completion of the function func synchronizes with all previous or subsequent calls to call_once with the same flag variable.

  2. Complete object type capable of holding a flag used by call_once.

  3. Expands to a value that can be used to initialize an object of type once_flag.

[edit] Parameters

flag - pointer to an object of type call_once that is used to ensure func is called only once
func - the function to execute only once

[edit] Return value

(none)

[edit] Notes

The POSIX equivalent of this function is pthread_once.

[edit] Example

#include <stdio.h> #include <threads.h>   void do_once(void) { puts("called once"); }   static once_flag flag = ONCE_FLAG_INIT; int func(void* data) { call_once(&flag, do_once); }   int main(void) { thrd_t t1, t2, t3, t4; thrd_create(&t1, func, NULL); thrd_create(&t2, func, NULL); thrd_create(&t3, func, NULL); thrd_create(&t4, func, NULL);   thrd_join(t1, NULL); thrd_join(t2, NULL); thrd_join(t3, NULL); thrd_join(t4, NULL); }

Output:

[edit] References

[edit] See also