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.\" Copyright 2016 Joyent, Inc.
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.Dd "February 17, 2023"
.Dt CALL_ONCE 3C
.Os
.Sh NAME
.Nm call_once
.Nd ensure function is only called once
.Sh SYNOPSIS
.In treads.h
.Vt "once_flag once = ONCE_FLAG_INIT;"
.Ft void
.Fo call_once
.Fa "once_flag *once"
.Fa "void (*func)(void)"
.Fc
.Sh DESCRIPTION
The
.Fn call_once
function is used to ensure that an operation occurs only once, even
across multiple threads.
Each instance of a properly initialized
.Ft once_flag
can be passed to the
.Ft call_once
function; however, only a single caller will successfully execute the
specified function,
.Fa func .
This ensures that the argument
.Fa func
is called only once.
Note, the argument
.Fa once
is the only thing used as a point of synchronization.
If multiple callers use the same pointer for
.Fa once ,
but use different values for
.Fa func ,
then only one of the functions will be successfully called.
.Pp
The argument
.Fn once
should always be initialized to the symbol
.Sy ONCE_FLAG_INIT
before calling
.Fn call_once .
Failure to do so will result in undefined behavior.
.Pp
Like
.Xr pthread_once 3C ,
the
.Fn call_once
function is not itself a cancellation point; however, if the thread
calling
.Fn func
encounters a cancellation point and is cancelled, then the value pointed
to by
.Fa once
will be as though
.Fn call_once
had not been called, as
.Fn func
had not completed successfully.
.Sh RETURN VALUES
The
.Fn call_once
function does not return any values.
Upon its completion, it is guaranteed that
.Fa func
will have been called at most once across the liftime of the
.Fa once
argument .
.Sh INTERFACE STABILITY
.Sy Standard
.Sh MT-LEVEL
.Sy MT-Safe
.Sh SEE ALSO
.Xr pthread_once 3C ,
.Xr threads.h 3HEAD ,
.Xr attributes 7 ,
.Xr threads 7