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call_once, once_flag, ONCE_FLAG_INIT

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.

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

Return value

(none).

Notes

The POSIX equivalent of this function is pthread_once.

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:

called once

References

C17 standard (ISO/IEC 9899:2018):

7.26.2.1 The call_once function (p: 275)
7.26.1/3 ONCE_FLAG_INIT (p: 274)

C11 standard (ISO/IEC 9899:2011):