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.Dd October 16, 2016 .Dt CALL_ONCE 3 .Os .Sh NAME .Nm call_once .Nd calls function exactly once .Sh LIBRARY .Lb libpthread .Sh SYNOPSIS n threads.h .Ft void .Fn call_once "once_flag *flag" "void (*func)(void)" .Vt #define ONCE_FLAG_INIT /* implementation specified */ .Sh DESCRIPTION The .Nm function uses the .Fa flag parameter to ensure that .Fa func is called exactly once, even if called from several threads.
p The .Dv ONCE_FLAG_INIT definition expands to a value that can be used to initialize an object of type .Dv once_flag .
p This portable interface is implemented on top of the .Xr pthread_once 3 functionality. .Sh RETURN VALUES None. .Sh EXAMPLES The following calls .Nm from two threads using the portable .Xr thrd 3 interface. d -literal -offset indent #include <stdio.h> #include <threads.h> static once_flag oflag = ONCE_FLAG_INIT; void called_once(void) { printf("called once\n"); } int tfun(void *ptr) { call_once(&oflag, called_once); } int main(int argc, char **argv) { thrd_t th1, th2; thrd_create(&th1, tfun, NULL); thrd_create(&th2, tfun, NULL); thrd_join(th1, NULL); thrd_join(th2, NULL); return 0; } .Ed .Sh SEE ALSO .Xr pthread_once 3 , .Xr threads 3 .Sh STANDARDS The .Nm function conforms to .St -isoC-2011 . .Sh HISTORY This interface first appeared in .Nx 9 . .Sh AUTHORS .An Kamil Rytarowski Aq Mt kamil@NetBSD.org