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+.\"	$NetBSD: semop.2,v 1.19 2026/01/02 22:22:22 nia Exp $
+.\"
+.\" Copyright (c) 1995 Frank van der Linden
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. The name of the author may not be used to endorse or promote products
+.\"    derived from this software without specific prior written permission
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd October 3, 2024
+.Dt SEMOP 2
+.Os
+.Sh NAME
+.Nm semop, semtimedop
+.Nd semaphore operations
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In sys/sem.h
+.Ft int
+.Fn semop "int semid" "struct sembuf *sops" "size_t nsops"
+.Ft int
+.Fn semtimedop "int semid" "struct sembuf *sops" "size_t nsops" "struct timespec *timeout"
+.Sh DESCRIPTION
+.Fn semop
+provides a number of atomic operations on a set of semaphores.
+The semaphore set is specified by
+.Fa semid ,
+.Fa sops
+is an array of semaphore operations, and
+.Fa nsops
+is the number of operations in this array.
+The
+.Va sembuf
+structures in the array contain the following members:
+.Bd -literal
+    unsigned short sem_num; /* semaphore # */
+    short          sem_op;  /* semaphore operation */
+    short          sem_flg; /* operation flags */
+.Ed
+.Pp
+Each operation (specified in
+.Va sem_op )
+is applied to semaphore number
+.Va sem_num
+in the set of semaphores specified by
+.Fa semid .
+The value of
+.Va sem_op
+determines the action taken in the following way:
+.Bl -bullet
+.It
+.Va sem_op
+is less than 0.
+The current process is blocked until the value of the
+semaphore is greater than or equal to the absolute value of
+.Va sem_op .
+The absolute value of
+.Va sem_op
+is then subtracted from the value of the semaphore, and the calling
+process continues.
+Negative values of
+.Va sem_op
+are thus used to enter critical regions.
+.It
+.Va sem_op
+is greater than 0.
+Its value is added to the value of the specified semaphore.
+This is used to leave critical regions.
+.It
+.Va sem_op
+is equal to 0.
+The calling process is blocked until the value of the
+specified semaphore reaches 0.
+.El
+.Pp
+The behaviour of each operation is influenced by the flags set in
+.Va sem_flg
+in the following way:
+.Bl -tag -width IPC_NOWAITX
+.It Dv IPC_NOWAIT
+In the case where the calling process would normally block, waiting
+for a semaphore to reach a certain value,
+.Dv IPC_NOWAIT
+makes the
+call return immediately, returning a value of \-1 and setting
+.Va errno
+to
+.Er EAGAIN .
+.It SEM_UNDO
+Keep track of the changes that this call makes to the value of a semaphore,
+so that they can be undone when the calling process terminates.
+This is useful to prevent other processes waiting on a semaphore to block
+forever, should the process that has the semaphore locked terminate in a
+critical section.
+.El
+.Pp
+.Fn semtimedop
+is similar to
+.Fn semop ,
+but it also allows specifying a timeout.
+When the semaphore is not available,
+the thread typically sleeps until the semaphore is available.
+.Fn semtimedop
+allows specifying a maximum amount of time in
+.Fa timeout
+argument that a thread should sleep while waiting for the semaphore to be available.
+If the specified time limit has been reached,
+.Fn semtimedop
+fails with
+.Er EAGAIN
+(and none of the operations in
+.Fa sops
+are performed).
+If
+.Fa timeout
+is
+.Dv NULL ,
+.Fn semtimedop
+behaves exactly like
+.Fn semop .
+.Sh RETURN VALUES
+Upon successful completion both
+.Fn semop
+and
+.Fn semtimedop
+return a value of 0. Otherwise, \-1 is returned and the global variable
+.Va errno
+is set to indicate the error.
+.Sh ERRORS
+.Fn semop
+will fail if:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+There is no semaphore associated with
+.Fa semid .
+.It Bq Er EIDRM
+The semaphore set was removed while the process was waiting for one of
+its semaphores to reach a certain value.
+.It Bq Er EACCES
+The calling process has no permission to access the specified semaphore set.
+.It Bq Er E2BIG
+The value of
+.Fa nsops
+is too big.
+The maximum is defined as
+.Dv MAX_SOPS
+in
+.In sys/sem.h .
+.It Bq Er EFBIG
+.Va sem_num
+in one of the sem_buf structures is less than 0, or greater than the actual
+number of semaphores in the set specified by
+.Fa semid .
+.It Bq Er ENOSPC
+.Dv SEM_UNDO
+was requested, and there is not enough space left in the kernel to
+store the undo information.
+.It Bq Er EAGAIN
+The requested operation can not immediately be performed, and
+.Dv IPC_NOWAIT
+was set in
+.Va sem_flg .
+.It Bq Er EFAULT
+.Fa sops
+points to an illegal address.
+.It Bq Er EINTR
+While blocked in this system call, the thread caught a signal.
+.El
+.Pp
+In addition,
+.Fn semtimedop
+will fail if:
+.Bl -tag -width Er
+.It Bq Er EAGAIN
+An operation could not proceed immediately and either
+.Dv IPC_NOWAIT
+was specified in
+.Va sem_flg
+or the time limit specified in
+.Fa timeout
+expired.
+.It Bq Er EFAULT
+An address specified in the
+.Fa timeout
+argument isn't accessible.
+.El
+.Sh EXAMPLES
+The following example shows how to perform a semaphore operation with a timeout:
+.Bd -literal -offset indent
+
+/* Performs a semaphore operation with a 5 sec timeout*/
+
+struct sembuf sops[1];        /* Semaphore operation structure */
+struct timespec timeout;      /* Timeout structure */
+
+/* Create semaphore set with 1 semaphore */
+int semid = semget(key, 1, 0666 | IPC_CREAT);
+
+/* Initialize semaphore to 0 */
+if (semctl(semid, 0, SETVAL, 0) == -1) {
+    warn("semctl SETVAL");
+    exit(EXIT_FAILURE);
+}
+
+sops[0].sem_num = 0;          /* Operation on semaphore 0 */
+sops[0].sem_op = -1;          /* Decrement semaphore by 1 */
+sops[0].sem_flg = 0;          /* No flags */
+
+timeout.tv_sec = 5;           /* 5 seconds */
+timeout.tv_nsec = 0;          /* 0 nanoseconds */
+
+if (semtimedop(semid, sops, 1, &timeout) == -1) {
+    warn("semtimedop");       /* Print error message */
+}
+.Ed
+.Sh SEE ALSO
+.Xr semctl 2 ,
+.Xr semget 2
+.Sh STANDARDS
+The
+.Nm
+system call conforms to
+.St -xsh5 .
+.Sh HISTORY
+Semaphores appeared in the first release of
+.At V .