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semop.2 revision 1.6 
 $NetBSD: semop.2,v 1.6 1999/03/22 19:45:08 garbled Exp $ 

 Copyright (c) 1995 Frank van der Linden
 All rights reserved.

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 This product includes software developed for the NetBSD Project
 by Frank van der Linden
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.Dd August 17, 1995
.Dt SEMOP 2
.Os
.Sh NAME
.Nm semop
.Nd semaphore operations
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ipc.h>
.Fd #include <sys/sem.h>
.Ft int
.Fn semop "int semid" "struct sembuf *sops" "size_t nsops"
.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:
d -literal  u_short sem_num; /* semaphore # */
 short sem_op; /* semaphore operation */
 short sem_flg; /* operation flags */
.Ed

p
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:
l -bullet t .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.
t .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.
t .Va sem_op
is equal to 0. The calling process is blocked until the value of the
specified semaphore reaches 0.
.El

p
The behaviour of each operation is influenced by the flags set in
.Va sem_flg
in the following way:
l -tag -width IPC_NOWAITX t 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 .
t 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
.Sh RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn semop
will fail if:
l -tag -width Er t Bq Er EINVAL There is no semaphore associated with
.Fa semid .

p
The semaphore set was removed while the process was waiting for one of
its semaphores to reach a certain value.
t Bq Er EACCES The calling process has no permission to access the specified semaphore set.
t Bq Er E2BIG The value of
.Fa nsops
is too big. The maximum is defined as
.Dv MAX_SOPS
in
.Aq Pa sys/sem.h .
t 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 .
t Bq Er ENOSPC .Dv SEM_UNDO
was requested, and there is not enough space left in the kernel to
store the unfo information.
t Bq Er EAGAIN The requested operation can not immediately be performed, and
.Dv IPC_NOWAIT
was set in
.Va sem_flg .
t Bq Er EFAULT .Fa sops
points to an illegal address.
.El
.Sh SEE ALSO
.Xr semctl 2 ,
.Xr semget 2
.Sh BUGS
In case of a removed semaphore identifier,
.Va errno
should be set to
.Er EIDRM ,
but
.Nx
does not define that error.