semop - semaphore operations
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/sem.h>
int semop ( int semid, struct sembuf *sops, unsigned nsops )
The function performs operations on selected members of the semaphore
set indicated by semid. Each of the nsops elements in the array
pointed to by sops specify an operation to be performed on a semaphore
by a struct sembuf including the following members:
short sem_num; /* semaphore number: 0 = first */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
Flags recognized in sem_flg are IPC_NOWAIT and SEM_UNDO. If an operation
asserts SEM_UNDO, it will be undone when the process exits.
The system call semantic assures that the operations will be performed
if and only if all of them will succeed. Each operation is performed
on the sem_num-th semaphore of the semaphore set - where the first semaphore
of the set is semaphore 0 - and is one among the following
three.
If sem_op is a positive integer, the operation adds this value to sem-
val. Furthermore, if SEM_UNDO is asserted for this operation, the system
updates the process undo count for this semaphore. The operation
always goes through, so no process sleeping can happen. The calling
process must have alter permissions on the semaphore set.
If sem_op is zero, the process must have read access permissions on the
semaphore set. If semval is zero, the operation goes through. Otherwise,
if IPC_NOWAIT is asserted in sem_flg, the system call fails
(undoing all previous actions performed) with errno set to EAGAIN.
Otherwise semzcnt is incremented by one and the process sleeps until
one of the following occurs:
o semval becomes 0, at which time the value of semzcnt is
decremented.
o The semaphore set is removed: the system call fails with
errno set to EIDRM.
o The calling process receives a signal that has to be
caught: the value of semzcnt is decremented and the system
call fails with errno set to EINTR.
If sem_op is less than zero, the process must have alter permissions on
the semaphore set. If semval is greater than or equal to the absolute
value of sem_op, the absolute value of sem_op is subtracted by semval.
Furthermore, if SEM_UNDO is asserted for this operation, the system
updates the process undo count for this semaphore. Then the operation
goes through. Otherwise, if IPC_NOWAIT is asserted in sem_flg, the
system call fails (undoing all previous actions performed) with errno
set to EAGAIN. Otherwise semncnt is incremented by one and the process
sleeps until one of the following occurs:
o semval becomes greater 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 for this semaphore.
o The semaphore set is removed from the system: the system
call fails with errno set to EIDRM.
o The calling process receives a signal that has to be
caught: the value of semncnt is decremented and the system
call fails with errno set to EINTR.
In case of success, the sempid member of the structure sem for each
semaphore specified in the array pointed to by sops is set to the
process-ID of the calling process. Furthermore both sem_otime and
sem_ctime are set to the current time.
If successful the system call returns 0, otherwise it returns -1 with
errno indicating the error.
For a failing return, errno will be set to one among the following values:
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 go through and IPC_NOWAIT was
asserted in its sem_flg.
EFAULT The address pointed to by sops 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 Sleeping on a wait queue, the process received a signal that
had to be caught.
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
has not enough memory to allocate the undo structure.
ERANGE For some operation semop+semval is greater than SEMVMX, the
implementation dependent maximum value for semval.
The sem_undo structures of a process aren't inherited by its child on
execution of a fork(2) system call. They are instead inherited by the
substituting process resulting by the execution of the execve(2) system
call.
The followings are limits on semaphore set resources affecting a semop
call:
SEMOPM Maximum number of operations allowed for one semop call:
policy dependent.
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.
The system maintains a per process sem_undo structure for each semaphore
altered by the process with undo requests. Those structures are
free at process exit. One major cause for unhappiness with the undo
mechanism is that it does not fit in with the notion of having an
atomic set of operations an array of semaphores. The undo requests for
an array and each semaphore therein may have been accumulated over many
semopt calls. Should the process sleep when exiting, or should all
undo operations be applied with the IPC_NOWAIT flag in effect? Currently
those undo operations which go through immediately are applied,
and those that require a wait are ignored silently. Thus harmless undo
usage is guaranteed with private semaphores only.
SVr4, SVID. SVr4 documents additional error conditions EINVAL, EFBIG,
ENOSPC.
ipc(5), semctl(2), semget(2)
Linux 0.99.13 1993-11-01 SEMOP(2)
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