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 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 opera-
tion 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 operation 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, distin-
guished 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
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 pro-
ceed. 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:
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 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 pro-
ceed 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 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
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
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
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-
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 depen-
dent 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 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 maximum value (SEMAEM),
the system wide maximum number of undo structures (SEMMNU) and the per-process maximum
number of undo entries system parameters.
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 per-
formed. 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.
SVr4, SVID. SVr4 documents additional error conditions EINVAL, EFBIG, ENOSPC.
ipc(5), semctl(2), semget(2), sigaction(2)
Linux 2.4 2002-01-08 SEMOP(2)