.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\"
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" Modified 1996-10-22, Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2002-01-08, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 2003-04-28, Ernie Petrides <petrides@redhat.com>
.\" Modified 2004-05-27, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Language and formatting clean-ups
.\"	Added notes on /proc files
.\" 2005-04-08, mtk, Noted kernel version numbers for semtimedop()
.\" 2007-07-09, mtk, Added an EXAMPLE code segment.
.\"
.TH SEMOP 2 2008-10-04 "Linux" "Linux Programmer's Manual"
.SH NAME
semop, semtimedop \- semaphore operations
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/ipc.h>
.B #include <sys/sem.h>
.sp
.BI "int semop(int " semid ", struct sembuf *" sops ", unsigned " nsops );
.sp
.BI "int semtimedop(int " semid ", struct sembuf *" sops ", unsigned " nsops ,
.BI "               struct timespec *" timeout );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR semtimedop ():
_GNU_SOURCE
.SH DESCRIPTION
Each semaphore in a semaphore set has the following associated values:
.sp
.in +4n
.nf
unsigned short  semval;   /* semaphore value */
unsigned short  semzcnt;  /* # waiting for zero */
unsigned short  semncnt;  /* # waiting for increase */
pid_t           sempid;   /* process that did last op */
.sp
.in -4n
.fi
.BR semop ()
performs operations on selected semaphores in the set indicated by
.IR semid .
Each of the
.I nsops
elements in the array pointed to by
.I sops
specifies an operation to be performed on a single semaphore.
The elements of this structure are of type
.IR "struct sembuf" ,
containing the following members:
.sp
.in +4n
.nf
unsigned short sem_num;  /* semaphore number */
short          sem_op;   /* semaphore operation */
short          sem_flg;  /* operation flags */
.sp
.in -4n
.fi
Flags recognized in
.I sem_flg
are
.B IPC_NOWAIT
and
.BR SEM_UNDO .
If an operation specifies
.BR SEM_UNDO ,
it will be automatically undone when the process terminates.
.PP
The set of operations contained in
.I sops
is performed in
.IR "array order" ,
and
.IR atomically ,
that is, the operations are performed either as a complete unit,
or not at all.
The behavior of the system call if not all operations can be
performed immediately depends on the presence of the
.B IPC_NOWAIT
flag in the individual
.I sem_flg
fields, as noted below.

Each operation is performed on the
.IR sem_num \-th
semaphore of the semaphore set, where the first semaphore of the set
is numbered 0.
There are three types of operation, distinguished by the value of
.IR sem_op .
.PP
If
.I sem_op
is a positive integer, the operation adds this value to
the semaphore value
.RI  ( semval ).
Furthermore, if
.B SEM_UNDO
is specified for this operation, the system updates the process undo count
.RI ( semadj )
for this semaphore.
This operation can always proceed \(em it never forces a process to wait.
The calling process must have alter permission on the semaphore set.
.PP
If
.I sem_op
is zero, the process must have read permission on the semaphore
set.
This is a "wait-for-zero" operation: if
.I semval
is zero, the operation can immediately proceed.
Otherwise, if
.B IPC_NOWAIT
is specified in
.IR sem_flg ,
.BR semop ()
fails with
.I errno
set to
.B EAGAIN
(and none of the operations in
.I sops
is performed).
Otherwise
.I semzcnt
(the count of processes waiting until this semaphore's value becomes zero)
is incremented by one and the process sleeps until
one of the following occurs:
.IP \(bu 3
.I semval
becomes 0, at which time the value of
.I semzcnt
is decremented.
.IP \(bu
The semaphore set
is removed:
.BR semop ()
fails, with
.I errno
set to
.BR EIDRM .
.IP \(bu
The calling process catches a signal:
the value of
.I semzcnt
is decremented and
.BR semop ()
fails, with
.I errno
set to
.BR EINTR .
.IP \(bu
The time limit specified by
.I timeout
in a
.BR semtimedop ()
call expires:
.BR semop ()
fails, with
.I errno
set to
.BR EAGAIN .
.PP
If
.I sem_op
is less than zero, the process must have alter permission on the
semaphore set.
If
.I semval
is greater than or equal to the absolute value of
.IR sem_op ,
the operation can proceed immediately:
the absolute value of
.I sem_op
is subtracted from
.IR semval ,
and, if
.B SEM_UNDO
is specified for this operation, the system updates the process undo count
.RI ( semadj )
for this semaphore.
If the absolute value of
.I sem_op
is greater than
.IR semval ,
and
.B IPC_NOWAIT
is specified in
.IR sem_flg ,
.BR semop ()
fails, with
.I errno
set to
.B EAGAIN
(and none of the operations in
.I sops
is performed).
Otherwise
.I semncnt
(the counter of processes waiting for this semaphore's value to increase)
is incremented by one and the process sleeps until
one of the following occurs:
.IP \(bu 3
.I semval
becomes greater than or equal to the absolute value of
.IR sem_op ,
at which time the value of
.I semncnt
is decremented, the absolute value of
.I sem_op
is subtracted from
.I semval
and, if
.B SEM_UNDO
is specified for this operation, the system updates the process undo count
.RI ( semadj )
for this semaphore.
.IP \(bu
The semaphore set is removed from the system:
.BR semop ()
fails, with
.I errno
set to
.BR EIDRM .
.IP \(bu
The calling process catches a signal:
the value of
.I semncnt
is decremented and
.BR semop ()
fails, with
.I errno
set to
.BR EINTR .
.IP \(bu
The time limit specified by
.I timeout
in a
.BR semtimedop ()
call expires: the system call fails, with
.I errno
set to
.BR EAGAIN .
.PP
On successful completion, the
.I sempid
value for each semaphore specified in the array pointed to by
.I sops
is set to the process ID of the calling process.
In addition, the
.I sem_otime
.\" and
.\" .I sem_ctime
is set to the current time.
.PP
.BR semtimedop ()
behaves identically to
.BR semop ()
except that in those cases were the calling process would sleep,
the duration of that sleep is limited by the amount of elapsed
time specified by the
.I timespec
structure whose address is passed in the
.I timeout
argument.
If the specified time limit has been reached,
.BR semtimedop ()
fails with
.I errno
set to
.B EAGAIN
(and none of the operations in
.I sops
is performed).
If the
.I timeout
argument is NULL,
then
.BR semtimedop ()
behaves exactly like
.BR semop ().
.SH "RETURN VALUE"
If successful
.BR semop ()
and
.BR semtimedop ()
return 0;
otherwise they return \-1
with
.I errno
indicating the error.
.SH ERRORS
On failure,
.I errno
is set to one of the following:
.TP
.B E2BIG
The argument
.I nsops
is greater than
.BR SEMOPM ,
the maximum number of operations allowed per system
call.
.TP
.B EACCES
The calling process does not have the permissions required
to perform the specified semaphore operations,
and does not have the
.B CAP_IPC_OWNER
capability.
.TP
.B EAGAIN
An operation could not proceed immediately and either
.B IPC_NOWAIT
was specified in
.I sem_flg
or the time limit specified in
.I timeout
expired.
.TP
.B EFAULT
An address specified in either the
.I sops
or the
.I timeout
argument isn't accessible.
.TP
.B EFBIG
For some operation the value of
.I sem_num
is less than 0 or greater than or equal to the number
of semaphores in the set.
.TP
.B EIDRM
The semaphore set was removed.
.TP
.B EINTR
While blocked in this system call, the process caught a signal; see
.BR signal (7).
.TP
.B EINVAL
The semaphore set doesn't exist, or
.I semid
is less than zero, or
.I nsops
has a nonpositive value.
.TP
.B ENOMEM
The
.I sem_flg
of some operation specified
.B SEM_UNDO
and the system does not have enough memory to allocate the undo
structure.
.TP
.B ERANGE
For some operation
.I sem_op+semval
is greater than
.BR SEMVMX ,
the implementation dependent maximum value for
.IR semval .
.SH VERSIONS
.BR semtimedop ()
first appeared in Linux 2.5.52,
and was subsequently backported into kernel 2.4.22.
Glibc support for
.BR semtimedop ()
first appeared in version 2.3.3.
.SH "CONFORMING TO"
SVr4, POSIX.1-2001.
.\" SVr4 documents additional error conditions EINVAL, EFBIG, ENOSPC.
.SH NOTES
The
.I sem_undo
structures of a process aren't inherited by the child produced by
.BR fork (2),
but they are inherited across an
.BR execve (2)
system call.
.PP
.BR semop ()
is never automatically restarted after being interrupted by a signal handler,
regardless of the setting of the
.B SA_RESTART
flag when establishing a signal handler.
.PP
.I semadj
is a per-process integer which is simply the (negative) count
of all semaphore operations performed specifying the
.B SEM_UNDO
flag.
When a semaphore's value is directly set using the
.B SETVAL
or
.B SETALL
request to
.BR semctl (2),
the corresponding
.I semadj
values in all processes are cleared.
.PP
The \fIsemval\fP, \fIsempid\fP, \fIsemzcnt\fP, and \fIsemnct\fP values
for a semaphore can all be retrieved using appropriate
.BR semctl (2)
calls.
.PP
The following limits on semaphore set resources affect the
.BR semop ()
call:
.TP
.B SEMOPM
Maximum number of operations allowed for one
.BR semop ()
call (32)
(on Linux, this limit can be read and modified via the third field of
.IR /proc/sys/kernel/sem ).
.\" This /proc file is not available in Linux 2.2 and earlier -- MTK
.TP
.B SEMVMX
Maximum allowable value for
.IR semval :
implementation dependent (32767).
.PP
The implementation has no intrinsic limits for
the adjust on exit maximum value
.RB ( SEMAEM ),
the system wide maximum number of undo structures
.RB ( SEMMNU )
and the per-process maximum number of undo entries system parameters.
.SH BUGS
When a process terminates, its set of associated
.I semadj
structures is used to undo the effect of all of the
semaphore operations it performed with the
.B SEM_UNDO
flag.
This raises a difficulty: if one (or more) of these semaphore adjustments
would result in an attempt to decrease a semaphore's value below zero,
what should an implementation do?
One possible approach would be to block until all the semaphore
adjustments could be performed.
This is however undesirable since it could force process termination to
block for arbitrarily long periods.
Another possibility is that such semaphore adjustments could be ignored
altogether (somewhat analogously to failing when
.B IPC_NOWAIT
is specified for a semaphore operation).
Linux adopts a third approach: decreasing the semaphore value
as far as possible (i.e., to zero) and allowing process
termination to proceed immediately.

In kernels 2.6.x, x <= 10, there is a bug that in some circumstances
prevents a process that is waiting for a semaphore value to become
zero from being woken up when the value does actually become zero.
This bug is fixed in kernel 2.6.11.
.\" The bug report:
.\" http://marc.theaimsgroup.com/?l=linux-kernel&m=110260821123863&w=2
.\" the fix:
.\" http://marc.theaimsgroup.com/?l=linux-kernel&m=110261701025794&w=2
.SH EXAMPLE
The following code segment uses
.BR semop ()
to atomically wait for the value of semaphore 0 to become zero,
and then increment the semaphore value by one.
.nf

    struct sembuf sops[2];
    int semid;

    /* Code to set \fIsemid\fP omitted */

    sops[0].sem_num = 0;        /* Operate on semaphore 0 */
    sops[0].sem_op = 0;         /* Wait for value to equal 0 */
    sops[0].sem_flg = 0;

    sops[1].sem_num = 0;        /* Operate on semaphore 0 */
    sops[1].sem_op = 1;         /* Increment value by one */
    sops[1].sem_flg = 0;

    if (semop(semid, sops, 2) == \-1) {
        perror("semop");
        exit(EXIT_FAILURE);
    }
.fi
.SH "SEE ALSO"
.BR semctl (2),
.BR semget (2),
.BR sigaction (2),
.BR capabilities (7),
.BR sem_overview (7),
.BR svipc (7),
.BR time (7)
.SH COLOPHON
This page is part of release 3.27 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.