call_once, once_flag, ONCE_FLAG_INIT
From cppreference.com
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Defined in header
<threads.h>
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||
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void
call_once
(
once_flag
*
flag,
void
(
*
func
)
(
void
)
)
;
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(1) | (since C11) |
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typedef
/* unspecified */
once_flag
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(2) | (since C11) |
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#define ONCE_FLAG_INIT /* unspecified */
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(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):
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- 7.26.2.1 The call_once function (p: 275)
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- 7.26.1/3 ONCE_FLAG_INIT (p: 274)
- C11 standard (ISO/IEC 9899:2011):
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- 7.26.2.1 The call_once function (p: 378)
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- 7.26.1/3 ONCE_FLAG_INIT (p: 376)
See also
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C++ documentation
for
call_once
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