semop
SEMOP(2) Linux Programmer’s Manual SEMOP(2)
NAME
semop, semtimedop - semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop(int semid, struct sembuf *sops, unsigned nsops);
int semtimedop(int semid, struct sembuf *sops, unsigned nsops, struct
timespec *timeout);
DESCRIPTION
A semaphore is represented by an anonymous structure including the
following members:
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 */
The function semop performs operations on selected members of the
semaphore set indicated by semid. Each of the nsops elements in the
array pointed to by sops specifies an operation to be performed on a
semaphore by a struct sembuf including 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 opera-
tion asserts SEM_UNDO, it will be undone when the process exits.
The set of operations contained in sops is performed atomically, that
is, the operations are performed at the same time, and only if they
can all be simultaneously performed. The behaviour 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 semaphore 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 asserted 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 access 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 asserted in sem_flg, the system call fails with errno set to EAGAIN
(and none of the operations in sops is performed). Otherwise 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:
· semval becomes 0, at which time the value of semzcnt is decre-
mented.
· The semaphore set is removed: the system call fails, with errno
set to EIDRM.
· The calling process catches a signal: the value of semzcnt is
decremented and the system call 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.
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
asserted 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 asserted in sem_flg, the system
call fails, with errno set to EAGAIN (and none of the operations in
sops is performed). Otherwise semncnt (the counter of processes wait-
ing for this semaphore’s value to increase) is incremented by one and
the process 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
absolute value of sem_op is subtracted from semval and, if
SEM_UNDO is asserted for this operation, the system updates the
process undo count (semadj) for this semaphore.
· The semaphore set is removed from the system: the system call
fails with errno set to EIDRM.
· The calling process catches a signal: the value of semncnt is
decremented and the system call 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 speci-
fied 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.
The function semtimedop behaves identically to the function 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 speci-
fied by the timespec structure whose address is passed in the timeout
parameter. If the specified time limit has been reached, the system
call fails with errno set to EAGAIN (and none of the operations in
sops is performed). If the timeout parameter is NULL, then semtimedop
behaves exactly like semop.
RETURN VALUE
If successful the system call returns 0, otherwise it returns -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 has no access permissions on the semaphore
set as required by one of the specified operations.
EAGAIN An operation could not proceed immediately and either
IPC_NOWAIT was asserted in its sem_flg or the time limit speci-
fied in timeout expired.
EFAULT An address specified in either the sops or timeout parameters
isn’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 process caught a signal.
EINVAL The semaphore set doesn’t exist, or semid is less than zero, or
nsops has a non-positive value.
ENOMEM The sem_flg of some operation asserted 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.
NOTES
The sem_undo structures of a process aren’t inherited across a fork(2)
system call, but they are inherited across a execve(2) system call.
semop is never automatically restarted after being interrupted by a
signal handler, regardless of the setting of the SA_RESTART flags 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 semaphore’s 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 followings are limits on semaphore set resources affecting a semop
call:
SEMOPM Maximum number of operations allowed for one semop call (32).
SEMVMX Maximum allowable value for semval: implementation dependent
(32767).
The implementation has no intrinsic limits for the adjust on exit max-
imum 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 per-
formed 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’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.
CONFORMING TO
SVr4, SVID. SVr4 documents additional error conditions EINVAL, EFBIG,
ENOSPC.
SEE ALSO
ipc(5), semctl(2), semget(2), sigaction(2)
Linux 2.4 2003-04-28 SEMOP(2)
