Section (2) semop
Name
semop, semtimedop — System V semaphore operations
Synopsis
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h>
int
semop( |
int semid, |
struct sembuf *sops, | |
size_t nsops) ; |
int
semtimedop( |
int semid, |
struct sembuf *sops, | |
size_t nsops, | |
const struct timespec *timeout) ; |
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Note | ||
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DESCRIPTION
Each semaphore in a System V semaphore set has the following associated values:
unsigned short semval; /* semaphore value */ unsigned short semzcnt; /* # waiting for zero */ unsigned short semncnt; /* # waiting for increase */ pid_t sempid; /* PID of process that last
semop
() performs operations
on selected semaphores in the set indicated by semid
. Each of the nsops
elements in the array
pointed to by sops
is
a structure that specifies an operation to be performed on a
single semaphore. The elements of this structure are of type
struct sembuf,
containing the following members:
unsigned short sem_num; /* semaphore number */ short sem_op; /* semaphore operation */ short sem_flg; /* operation flags */
Flags recognized in sem_flg
are IPC_NOWAIT
and SEM_UNDO
. If an operation specifies
SEM_UNDO
, it will be
automatically undone when the process terminates.
The set of operations contained in sops
is performed in
array order, and
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 IPC_NOWAIT
flag
in the individual sem_flg
fields, as noted
below.
Each operation is performed on the 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 sem_op
.
If sem_op
is a
positive integer, the operation adds this value to the
semaphore value (semval
). Furthermore, if
SEM_UNDO
is specified for this
operation, the system subtracts the value sem_op
from the semaphore
adjustment (semadj
)
value for this semaphore. This operation can always
proceed—it never forces a thread to wait. The calling
process must have alter permission on the semaphore set.
If sem_op
is
zero, the process must have read permission on the semaphore
set. This is a wait-for-zero operation: if semval
is zero, the operation
can immediately proceed. Otherwise, if IPC_NOWAIT
is specified in sem_flg
, semop
() fails with errno
set to EAGAIN (and none of the operations in
sops
is performed).
Otherwise, semzcnt
(the count of threads waiting until this semaphore_zsingle_quotesz_s value
becomes zero) is incremented by one and the thread sleeps
until one of the following occurs:
-
semval
becomes 0, at which time the value ofsemzcnt
is decremented. -
The semaphore set is removed:
semop
() fails, witherrno
set to EIDRM. -
The calling thread catches a signal: the value of
semzcnt
is decremented andsemop
() fails, witherrno
set to EINTR.
If sem_op
is
less than zero, the process must have alter permission on the
semaphore set. If semval
is greater than or
equal to the absolute value of sem_op
, the operation can
proceed immediately: the absolute value of sem_op
is subtracted from
semval
, and, if
SEM_UNDO
is specified for this
operation, the system adds the absolute value of sem_op
to the semaphore
adjustment (semadj
)
value for this semaphore. If the absolute value of sem_op
is greater than
semval
, and
IPC_NOWAIT
is specified in
sem_flg
,
semop
() fails, with
errno
set to EAGAIN (and none of the operations in
sops
is performed).
Otherwise, semncnt
(the counter of threads waiting for this semaphore_zsingle_quotesz_s value to
increase) is incremented by one and the thread sleeps until
one of the following occurs:
-
semval
becomes greater than or equal to the absolute value ofsem_op
: the operation now proceeds, as described above. -
The semaphore set is removed from the system:
semop
() fails, witherrno
set to EIDRM. -
The calling thread catches a signal: the value of
semncnt
is decremented andsemop
() fails, witherrno
set to EINTR.
On successful completion, the sempid
value for each
semaphore specified in the array pointed to by sops
is set to the caller_zsingle_quotesz_s
process ID. In addition, the sem_otime
is set to the
current time.
semtimedop()
semtimedop
() behaves
identically to semop
() except
that in those cases where the calling thread would sleep,
the duration of that sleep is limited by the amount of
elapsed time specified by the timespec structure whose address is
passed in the timeout
argument. (This sleep
interval will be rounded up to the system clock
granularity, and kernel scheduling delays mean that the
interval may overrun by a small amount.) If the specified
time limit has been reached, semtimedop
() fails with errno
set to EAGAIN (and none of the operations in
sops
is performed).
If the timeout
argument is NULL, then semtimedop
() behaves exactly like
semop
().
Note that if semtimedop
()
is interrupted by a signal, causing the call to fail with
the error EINTR, the
contents of timeout
are left unchanged.
RETURN VALUE
If successful, semop
() and
semtimedop
() return 0;
otherwise they return −1 with errno
indicating the error.
ERRORS
On failure, errno
is set to
one of the following:
- E2BIG
-
The argument
nsops
is greater thanSEMOPM
, the maximum number of operations allowed per system call. - EACCES
-
The calling process does not have the permissions required to perform the specified semaphore operations, and does not have the
CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace. - EAGAIN
-
An operation could not proceed immediately and either
IPC_NOWAIT
was specified insem_flg
or the time limit specified intimeout
expired. - EFAULT
-
An address specified in either the
sops
or thetimeout
argument isn_zsingle_quotesz_t accessible. - EFBIG
-
For some operation the value of
sem_num
is less than 0 or greater than or equal to the number of semaphores in the set. - EIDRM
-
The semaphore set was removed.
- EINTR
-
While blocked in this system call, the thread caught a signal; see signal(7).
- EINVAL
-
The semaphore set doesn_zsingle_quotesz_t exist, or
semid
is less than zero, ornsops
has a nonpositive value. - ENOMEM
-
The
sem_flg
of some operation specifiedSEM_UNDO
and the system does not have enough memory to allocate the undo structure. - ERANGE
-
For some operation
sem_op+semval
is greater thanSEMVMX
, the implementation dependent maximum value forsemval
.
VERSIONS
semtimedop
() first appeared
in Linux 2.5.52, and was subsequently backported into kernel
2.4.22. Glibc support for semtimedop
() first appeared in version
2.3.3.
NOTES
The inclusion of <
sys/types.h
>
and <
sys/ipc.h
>
isn_zsingle_quotesz_t required on Linux or by any version of POSIX. However,
some old implementations required the inclusion of these
header files, and the SVID also documented their inclusion.
Applications intended to be portable to such old systems may
need to include these header files.
The sem_undo
structures of a process aren_zsingle_quotesz_t inherited by the child
produced by fork(2), but they are
inherited across an execve(2) system call.
semop
() is never
automatically restarted after being interrupted by a signal
handler, regardless of the setting of the SA_RESTART
flag when establishing a signal
handler.
A semaphore adjustment (semadj
) value is a
per-process, per-semaphore integer that is the negated sum of
all operations performed on a semaphore specifying the
SEM_UNDO
flag. Each process has
a list of semadj
values—one value for each semaphore on which it has
operated using SEM_UNDO
. When a
process terminates, each of its per-semaphore semadj
values is added to the
corresponding semaphore, thus undoing the effect of that
process_zsingle_quotesz_s operations on the semaphore (but see BUGS below).
When a semaphore_zsingle_quotesz_s value is directly set using the
SETVAL
or SETALL
request to semctl(2), the
corresponding semadj
values in all
processes are cleared. The clone(2) CLONE_SYSVSEM
flag allows more than one
process to share a semadj
list; see clone(2) for details.
The semval
,
sempid
, semzcnt
, and semnct
values for a semaphore
can all be retrieved using appropriate semctl(2) calls.
Semaphore limits
The following limits on semaphore set resources affect
the semop
() call:
SEMOPM
-
Maximum number of operations allowed for one
semop
() call. Before Linux 3.19, the default value for this limit was 32. Since Linux 3.19, the default value is 500. On Linux, this limit can be read and modified via the third field of/proc/sys/kernel/sem
.Note
: this limit should not be raised above 1000, because of the risk of thatsemop
() fails due to kernel memory fragmentation when allocating memory to copy thesops
array. SEMVMX
-
Maximum allowable value for
semval
: implementation dependent (32767).
The implementation has no intrinsic limits for the
adjust on exit maximum value (SEMAEM
), the system wide maximum number
of undo structures (SEMMNU
)
and the per-process maximum number of undo entries system
parameters.
BUGS
When a process terminates, its set of associated
semadj
structures
is used to undo the effect of all of the semaphore operations
it performed with the SEM_UNDO
flag. This raises a difficulty: if one (or more) of these
semaphore adjustments would result in an attempt to decrease
a semaphore_zsingle_quotesz_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 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 thread 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.
EXAMPLE
The following code segment uses semop
() to atomically wait for the value of
semaphore 0 to become zero, and then increment the semaphore
value by one.
struct sembuf sops[2]; int semid; /* Code to setsemid
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); }
SEE ALSO
clone(2), semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7), sysvipc(7), time(7)
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified 1996-10-22, Eric S. Raymond <esrthyrsus.com> Modified 2002-01-08, Michael Kerrisk <mtk.manpagesgmail.com> Modified 2003-04-28, Ernie Petrides <petridesredhat.com> Modified 2004-05-27, Michael Kerrisk <mtk.manpagesgmail.com> Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.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. |