Manpage of SEMOP


Section: Linux Programmer's Manual (2)
Updated: 2016-10-08


semop, semtimedop - System V semaphore operations  


#include <sys/types.h>#include <sys/ipc.h>#include <sys/sem.h>int semop(int semid, struct sembuf *sops, size_t nsops);int semtimedop(int semid, struct sembuf *sops, size_t nsops,               const struct timespec *timeout);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

semtimedop(): _GNU_SOURCE  


Each semaphore in a System V semaphore set has the following associated values:

unsigned short  semval;   /* semaphore value */
unsigned short  semzcnt;  /* # waiting for zero */
unsigned short  semncnt;  /* # waiting for increase */
pid_t           sempid;   /* PID of process that last
                             modified semaphore value */

semop() performs operations on selected semaphores in the set indicated by semid. Each of the nsopselements in the array pointed to by sopsis a structure that specifies an operation to be performed on a single semaphore. The elements of this structure are of type struct sembuf, containing the following members:

unsigned short sem_num;  /* semaphore number */
short          sem_op;   /* semaphore operation */
short          sem_flg;  /* operation flags */

Flags recognized in sem_flgare IPC_NOWAITand SEM_UNDO. If an operation specifies SEM_UNDO, it will be automatically undone when the process terminates.

The set of operations contained in sopsis performed in array order, and atomically, that is, the operations are performed either as a complete unit, or not at all. The behavior of the system call if not all operations can be performed immediately depends on the presence of the IPC_NOWAITflag in the individual sem_flgfields, 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 numbered 0. There are three types of operation, distinguished by the value of sem_op.

If sem_opis a positive integer, the operation adds this value to the semaphore value (semval). Furthermore, if SEM_UNDOis specified for this operation, the system subtracts the value sem_opfrom the semaphore adjustment (semadj) value for this semaphore. This operation can always proceed---it never forces a thread to wait. The calling process must have alter permission on the semaphore set.

If sem_opis zero, the process must have read permission on the semaphore set. This is a "wait-for-zero" operation: if semvalis zero, the operation can immediately proceed. Otherwise, if IPC_NOWAITis specified in sem_flg, semop() fails with errnoset to EAGAIN(and none of the operations in sopsis performed). Otherwise, semzcnt(the count of threads waiting until this semaphore's value becomes zero) is incremented by one and the thread sleeps until one of the following occurs:

semvalbecomes 0, at which time the value of semzcntis decremented.
The semaphore set is removed: semop() fails, with errnoset to EIDRM.
The calling thread catches a signal: the value of semzcntis decremented and semop() fails, with errnoset to EINTR.

If sem_opis less than zero, the process must have alter permission on the semaphore set. If semvalis greater than or equal to the absolute value of sem_op, the operation can proceed immediately: the absolute value of sem_opis subtracted from semval, and, if SEM_UNDOis specified for this operation, the system adds the absolute value of sem_opto the semaphore adjustment (semadj) value for this semaphore. If the absolute value of sem_opis greater than semval, and IPC_NOWAITis specified in sem_flg, semop() fails, with errnoset to EAGAIN(and none of the operations in sopsis performed). Otherwise, semncnt(the counter of threads waiting for this semaphore's value to increase) is incremented by one and the thread sleeps until one of the following occurs:

semvalbecomes greater than or equal to the absolute value of sem_op: the operation now proceeds, as described above.
The semaphore set is removed from the system: semop() fails, with errnoset to EIDRM.
The calling thread catches a signal: the value of semncntis decremented and semop() fails, with errnoset to EINTR.

On successful completion, the sempidvalue for each semaphore specified in the array pointed to by sopsis set to the caller's process ID. In addition, the sem_otimeis set to the current time.  


semtimedop() behaves identically to semop() except that in those cases where the calling thread would sleep, the duration of that sleep is limited by the amount of elapsed time specified by the timespecstructure whose address is passed in the timeoutargument. (This sleep interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the interval may overrun by a small amount.) If the specified time limit has been reached, semtimedop() fails with errnoset to EAGAIN(and none of the operations in sopsis performed). If the timeoutargument is NULL, then semtimedop() behaves exactly like semop().

Note that if semtimedop() is interrupted by a signal, causing the call to fail with the error EINTR, the contents of timeoutare left unchanged.  


If successful, semop() and semtimedop() return 0; otherwise they return -1 with errnoindicating the error.  


On failure, errnois set to one of the following:
The argument nsopsis greater than SEMOPM, the maximum number of operations allowed per system call.
The calling process does not have the permissions required to perform the specified semaphore operations, and does not have the CAP_IPC_OWNERcapability in the user namespace that governs its IPC namespace.
An operation could not proceed immediately and either IPC_NOWAITwas specified in sem_flgor the time limit specified in timeoutexpired.
An address specified in either the sopsor the timeoutargument isn't accessible.
For some operation the value of sem_numis less than 0 or greater than or equal to the number of semaphores in the set.
The semaphore set was removed.
While blocked in this system call, the thread caught a signal; see signal(7).
The semaphore set doesn't exist, or semidis less than zero, or nsopshas a nonpositive value.
The sem_flgof some operation specified SEM_UNDOand the system does not have enough memory to allocate the undo structure.
For some operation sem_op+semvalis greater than SEMVMX, the implementation dependent maximum value for semval.


semtimedop() first appeared in Linux 2.5.52, and was subsequently backported into kernel 2.4.22. Glibc support for semtimedop() first appeared in version 2.3.3.  


POSIX.1-2001, POSIX.1-2008, SVr4.  


The inclusion of <sys/types.h>and <sys/ipc.h>isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files.

The sem_undostructures of a process aren't inherited by the child produced by fork(2), but they are inherited across an execve(2) system call.

semop() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTARTflag when establishing a signal handler.

A semaphore adjustment (semadj) value is a per-process, per-semaphore integer that is the negated sum of all operations performed on a semaphore specifying the SEM_UNDOflag. Each process has a list of semadjvalues---one value for each semaphore on which it has operated using SEM_UNDO. When a process terminates, each of its per-semaphore semadjvalues is added to the corresponding semaphore, thus undoing the effect of that process's operations on the semaphore (but see BUGS below). When a semaphore's value is directly set using the SETVALor SETALLrequest to semctl(2), the corresponding semadjvalues in all processes are cleared. The clone(2) CLONE_SYSVSEMflag allows more than one process to share a semadjlist; see clone(2) for details.

The semval, sempid, semzcnt, and semnct values for a semaphore can all be retrieved using appropriate semctl(2) calls.  

Semaphore limits

The following limits on semaphore set resources affect the semop() call:
Maximum number of operations allowed for one semop() call. Before Linux 3.19, the default value for this limit was 32. Since Linux 3.19, the default value is 500. On Linux, this limit can be read and modified via the third field of /proc/sys/kernel/sem. Note: this limit should not be raised above 1000, because of the risk of that semop() fails due to kernel memory fragmentation when allocating memory to copy the sopsarray.
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 semadjstructures is used to undo the effect of all of the semaphore operations it performed with the SEM_UNDOflag. 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_NOWAITis 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.

In kernels 2.6.x, x <= 10, there is a bug that in some circumstances prevents a thread that is waiting for a semaphore value to become zero from being woken up when the value does actually become zero. This bug is fixed in kernel 2.6.11.  


The following code segment uses semop() to atomically wait for the value of semaphore 0 to become zero, and then increment the semaphore value by one.
    struct sembuf sops[2];
    int semid;

    /* Code to set semid omitted */

    sops[0].sem_num = 0;        /* Operate on semaphore 0 */
    sops[0].sem_op = 0;         /* Wait for value to equal 0 */
    sops[0].sem_flg = 0;

    sops[1].sem_num = 0;        /* Operate on semaphore 0 */
    sops[1].sem_op = 1;         /* Increment value by one */
    sops[1].sem_flg = 0;

    if (semop(semid, sops, 2) == -1) {


clone(2), semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7), svipc(7), time(7)



Semaphore limits

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