call_once, once_flag, ONCE_FLAG_INIT

Header: <threads.h>

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.

# Declarations

void call_once( once_flag* flag, void (*func)(void) );

(since C11)

typedef /* unspecified */ once_flag

(since C11)

#define ONCE_FLAG_INIT /* unspecified */

(since C11)

# 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);
}

# See also

C++ documentation