SEMOP(2) Linux Programmer's Manual SEMOP(2)
NAME
semop - semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop(int semid, struct sembuf *sops, unsigned nsops);
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 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, 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:
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 abso-
lute 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 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 abso-
lute 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.
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 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.
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 maximum 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 peri-
ods. 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 2002-01-08 SEMOP(2)