SEMOP(2) Linux Programmer's Manual SEMOP(2)
semop - semaphore operations
int semop(int semid, struct sembuf *sops, unsigned nsops);
A semaphore is represented by an anonymous structure including the fol-
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
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
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
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
o semval becomes 0, at which time the value of semzcnt is decre-
o The semaphore set is removed: the system call fails, with errno
set to EIDRM.
o The calling process catches a signal: 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 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 per-
formed). Otherwise 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:
o 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.
o The semaphore set is removed from the system: the system call
fails with errno set to EIDRM.
o The calling process catches a signal: the value of semncnt is
decremented and the system call fails with errno set to EINTR.
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.
If successful the system call returns 0, otherwise it returns -1 with
errno indicating the error.
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 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 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.
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
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
SEMOPM Maximum number of operations allowed for one semop call (32).
SEMVMX Maximum allowable value for semval: implementation dependent
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
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'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 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.
SVr4, SVID. SVr4 documents additional error conditions EINVAL, EFBIG,
ipc(5), semctl(2), semget(2), sigaction(2)
Linux 2.4 2002-01-08 SEMOP(2)