SEMOP(2) Linux Programmers Manual SEMOP(2)
NAME
semop, semtimedop - semaphore operations
SYNOPSIS
#include
#include
#include
int semop(int semid, struct sembuf *sops, unsigned nsops);
int semtimedop(int semid, struct sembuf *sops, unsigned nsops,
struct timespec *timeout);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
semtimedop(): _GNU_SOURCE
DESCRIPTION
Each semaphore in a 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; /* process that did last op */
semop() performs operations on selected semaphores in the set indicated
by semid. Each of the nsops elements in the array pointed to by sops
specifies an operation to be performed on a single semaphore. The ele
ments of this structure are of type struct sembuf, containing the fol
lowing 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 opera
tion specifies SEM_UNDO, it will be automatically undone when the pro
cess terminates.
The set of operations contained in sops is performed in array order,
and atomically, that is, the operations are performed either as a com
plete 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 updates the process undo count (semadj) for
this semaphore. This operation can always proceed it never forces a
process 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 processes waiting until this semaphores value becomes zero) is
incremented by one and the process sleeps until one of the following
occurs:
semval becomes 0, at which time the value of semzcnt is decremented.
The semaphore set is removed: semop() fails, with errno set to
EIDRM.
The calling process catches a signal: the value of semzcnt is decre
mented and semop() fails, with errno set to EINTR.
The time limit specified by timeout in a semtimedop() call expires:
semop() fails, with errno set to EAGAIN.
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 speci
fied for this operation, the system updates the process undo count
(semadj) 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 processes waiting for
this semaphores value to increase) is incremented by one and the pro
cess sleeps until one of the following occurs:
semval becomes greater than or equal to the absolute value of
sem_op, at which time the value of semncnt is decremented, the abso
lute value of sem_op is subtracted from semval and, if SEM_UNDO is
specified for this operation, the system updates the process undo
count (semadj) for this semaphore.
The semaphore set is removed from the system: semop() fails, with
errno set to EIDRM.
The calling process catches a signal: the value of semncnt is decre
mented and semop() fails, with errno set to EINTR.
The time limit specified by timeout in a semtimedop() call expires:
the system call fails, with errno set to EAGAIN.
On successful completion, the sempid value for each semaphore specified
in the array pointed to by sops is set to the process ID of the calling
process. In addition, the sem_otime is set to the current time.
semtimedop() behaves identically to 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 timespec struc
ture whose address is passed in the timeout argument. 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 time
out argument is NULL, then semtimedop() behaves exactly like semop().
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 than SEMOPM, 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.
EAGAIN An operation could not proceed immediately and either IPC_NOWAIT
was specified in sem_flg or the time limit specified in timeout
expired.
EFAULT An address specified in either the sops or the timeout argument
isnt 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 process caught a signal;
see signal(7).
EINVAL The semaphore set doesnt exist, or semid is less than zero, or
nsops has a non-positive value.
ENOMEM The sem_flg of some operation specified SEM_UNDO and the system
does not have enough memory to allocate the undo structure.
ERANGE For some operation sem_op+semval is greater than SEMVMX, the
implementation dependent maximum value for semval.
VERSIONS
semtimedop() first appeared in Linux 2.5.52, and was subsequently back
ported into kernel 2.4.22. Glibc support for semtimedop() first
appeared in version 2.3.3.
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
The sem_undo structures of a process arent inherited across a fork(2)
system call, 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.
semadj is a per-process integer which is simply the (negative) count of
all semaphore operations performed specifying the SEM_UNDO flag. When
a semaphores value is directly set using the SETVAL or SETALL request
to semctl(2), the corresponding semadj values in all processes are
cleared.
The semval, sempid, semzcnt, and semnct values for a semaphore can all
be retrieved using appropriate semctl(2) calls.
The following limits on semaphore set resources affect the semop()
call:
SEMOPM Maximum number of operations allowed for one semop() call (32)
(on Linux, this limit can be read and modified via the third
field of /proc/sys/kernel/sem).
SEMVMX Maximum allowable value for semval: implementation dependent
(32767).
The implementation has no intrinsic limits for the adjust on exit maxi
mum 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
semaphores 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 alto
gether (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 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.
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 set semid 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, &sop, 2) == -1) {
perror("semop");
exit(EXIT_FAILURE);
}
SEE ALSO
semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7),
svipc(7), time(7)
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2007-07-26 SEMOP(2)
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