call_once, once_flag, ONCE_FLAG_INIT
From cppreference.com
| Defined in header <threads.h>
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| 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_once3) 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
- 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
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