call_once, once_flag, ONCE_FLAG_INIT

 

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Concurrency support library

Threads thrd_create thrd_equal thrd_current    thrd_sleep thrd_yield thrd_exit thrd_detach thrd_join thrd_successthrd_timedoutthrd_busythrd_nomemthrd_error    Atomic operations atomic_init ATOMIC_VAR_INIT (until C23) ATOMIC_***_LOCK_FREE atomic_is_lock_free atomic_store atomic_load atomic_exchange atomic_compare_exchange atomic_fetch_add atomic_fetch_sub atomic_fetch_or atomic_fetch_xor atomic_fetch_and

Atomic flags atomic_flag ATOMIC_FLAG_INIT atomic_flag_test_and_set atomic_flag_clear Memory ordering memory_order kill_dependency atomic_thread_fence atomic_signal_fence Mutual exclusion mtx_init mtx_lock mtx_timedlock mtx_trylock call_once mtx_unlock mtx_destroy mtx_plainmtx_recursivemtx_timed Condition variables cnd_init cnd_signal cnd_broadcast cnd_wait cnd_timedwait cnd_destroy Thread-local storage thread_local TSS_DTOR_ITERATIONS    tss_create tss_get tss_set tss_delete

 

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

Run this code

#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):

• 7.26.2.1 The call_once function (p: 378)

• 7.26.1/3 ONCE_FLAG_INIT (p: 376)

See also

C++ documentation for call_once