| Defined in header <threads.h> | ||
|---|---|---|
void call_once( once_flag* flag, void (*func)(void) ); | (1) | (since C11) |
typedef once_flag /* unspecified */ | (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
See also
C++ documentation for call_once |
References
- 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)
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