xref: /illumos-gate/usr/src/man/man3c/cnd.3c (revision bbf215553c7233fbab8a0afdf1fac74c44781867)

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.Dd "Jan 11, 2015"
.Dt CND 3C
.Os
.Sh NAME
.Nm cnd ,
.Nm cnd_broadcast ,
.Nm cnd_destroy ,
.Nm cnd_init ,
.Nm cnd_signal ,
.Nm cnd_timedwait ,
.Nm cnd_wait
.Nd C11 condition variable functions
.Sh SYNOPSIS
.In threads.h
.Ft int
.Fo cnd_init
.Fa "cnd_t *cnd"
.Fc
.Ft void
.Fo cnd_destroy
.Fa "cnd_t *cnd"
.Fc
.Ft int
.Fo cnd_broadcast
.Fa "cnd_t *cnd"
.Fc
.Ft int
.Fo cnd_signal
.Fa "cnd_t *cnd"
.Fc
.Ft int
.Fo cnd_timedwait
.Fa "cnd_t *restrict cnd"
.Fa "mtx_t *restrict mtx"
.Fa "const struct timespec *abstime"
.Fc
.Ft int
.Fo cnd_wait
.Fa "cnd_t *restrict cnd"
.Fa "mtx_t *restrict mtx"
.Fc
.Sh DESCRIPTION
The
.Sy cnd
family of functions implement condition variables which allow threads
within a process to wait until a condition occurs and be signaled when
it does.
These functions behave similar to both the POSIX threads and illumos threads;
however, they have slightly different call signatures and return values.
For more information, see
.Xr threads 7 .
Importantly, they do not allow for inter-process synchronization.
.Ss Creating and Destroy Condition Variables
The function
.Fn cnd_init
initializes the condition variable referred to by
.Fa cnd .
The condition variable is suitable for intra-process use.
Initializing an already initialized condition variable results in undefined
behavior.
.Pp
The function
.Fn cnd_destroy
destroys an initialized condition variable at which point it is illegal
to use it, though it may be initialized again.
.Ss Condition Waiting
The function
.Fn cond_wait
can be used to wait on a condition variable.
A thread that waits on a condition variable blocks until another thread signals
that the condition has changed, generally after making the condition that was
false, true.
.Pp
The function
.Fn cond_wait
atomically release the mutex pointed to by
.Fa mtx
and blocks on the condition variable
.Fa cond .
When the thread returns, it will once again be holding
.Fa mtx
and must check the current state of the condition.
There is no guarantee that another thread has not gotten in and changed the
value before being woken.
In addition, a thread blocking on a condition variable, may be woken spuriously,
such as when a signal is received or
.Fn fork
is called .
.Pp
The function
.Fn cond_timedwait
allows a thread to block in a similar fashion to
.Fn cond_wait ,
except that when the absolute time specified in seconds since the epoch
(based on
.Sy CLOCK_REALTIME )
in UTC, expires, then the thread will be woken up.
The timeout is specified in
.Fa abstime .
.Ss Conditional Signaling
The
.Fn cnd_signal
and
.Fn cnd_broadcast
functions can be used to signal threads waiting on the condition variable
.Fa cnd
that they should be woken up and check the variable again.
The
.Fn cnd_signal
function will only wake a single thread that is blocked on the
condition variable
.Fa cnd ;
while
.Fn cnd_broadcast
will wake up every thread waiting on the condition variable
.Fa cnd .
.Pp
A thread calling either
.Fn cnd_signal
or
.Fn cnd_broadcast
is not required to hold any of the mutexes that are associated with the
condition variable.
.Pp
If there are no threads currently blocked in the condition variable
.Fa cnd
then neither function has an effect.
.Sh RETURN VALUES
Upon successful completion, the
.Fn cond_init
function returns
.Sy thrd_success.
If insufficient memory was available, then
.Sy thrd_nomem
is returned; otherwise, if any other error occurred,
.Sy thrd_error
is returned.
.Pp
Upon successful completion, the
.Fn cond_broadcast ,
.Fn cond_signal ,
and
.Fn cond_wait
functions return
.Sy thrd_success .
Otherwise, they return
.Sy thrd_error
to indicate that an error occurred and they were unable to complete.
.Pp
Upon successful completion, the
.Fn cond_timedwait
function returns
.Sy thrd_success .
If
.Fa abstime
expires without being signaled, it instead returns
.Sy thrd_timedout .
Otherwise,
.Sy thrd_error
is returned to indicate an error.
.Sh INTERFACE STABILITY
.Sy Standard
.Sh MT-LEVEL
.Sy MT-Safe
.Sh SEE ALSO
.Xr cond_broadcast 3C ,
.Xr cond_destroy 3C ,
.Xr cond_init 3C ,
.Xr cond_signal 3C ,
.Xr cond_timedwait 3C ,
.Xr cond_wait 3C ,
.Xr threads.h 3HEAD ,
.Xr attributes 7 ,
.Xr threads 7