semop(2)

NAME
semop, semtimedop - System V semaphore operations

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#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 *_Nullable timeout);

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

semtimedop():
    _GNU_SOURCE

DESCRIPTION
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 the semaphore value */

semop() performs operations on selected semaphores in the set indicated by semid. Each of the nsops elements in the array pointed to by sops is 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_flg are IPC_NOWAIT and SEM_UNDO. If an operation specifies SEM_UNDO, it will be automatically undone when the process terminates.

The set of operations contained in sops is 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_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 numbered 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 specified for this operation, the system subtracts the value sem_op from 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_op is zero, the process must have read 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 specified in sem_flg, semop() fails with errno set to EAGAIN (and none of the operations in sops is 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:

•   semval becomes 0, at which time the value of semzcnt is decremented.

•   The semaphore set is removed: semop() fails, with errno set to EIDRM.

•   The calling thread catches a signal: the value of semzcnt is decremented and semop() 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 specified for this operation, the system adds the absolute value of sem_op to the semaphore adjustment (semadj) value for this semaphore. If the absolute value of sem_op is greater than semval, and IPC_NOWAIT is specified in sem_flg, semop() fails, with errno set to EAGAIN (and none of the operations in sops is 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:

•   semval becomes 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 errno set to EIDRM.

•   The calling thread catches a signal: the value of semncnt is decremented and semop() 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 caller's process ID. In addition, the sem_otime is set to the current time.

  semtimedop()
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 timespec structure whose address is passed in the timeout argument. (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 errno set to EAGAIN (and none of the operations in sops is performed). If the timeout argument 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 timeout are left unchanged.

RETURN VALUE
On success, semop() and semtimedop() return 0. On failure, they return -1, and set errno to indicate the error.

ERRORS

E2BIG
    The argument nsops is greater than SEMOPM, the maximum number of operations allowed per system call.

EACCES
    The calling process does not have the permissions required to perform the specified semaphore operations, and does not have the CAP_IPC_OWNER capability in the user namespace that governs its IPC namespace.

EAGAIN
    An operation could not proceed immediately and either IPC_NOWAIT was specified in sem_flg or the time limit specified in timeout expired.

EFAULT
    An address specified in either the sops or the timeout argument 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 thread caught a signal; see signal(7).

EINVAL
    The semaphore set doesn't exist, or semid is less than zero, or nsops has a nonpositive value.

ENOMEM
    The sem_flg of some operation specified 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.

STANDARDS
POSIX.1-2024.

HISTORY

semop()
    SVr4, POSIX.1-2001.

semtimedop()
    SVr4, Linux 2.5.52 (backported into Linux 2.4.22), glibc 2.3.3.

NOTES
The sem_undo structures 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_RESTART flag 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_UNDO flag. Each process has a list of semadj values—one value for each semaphore on which it has operated using SEM_UNDO. When a process terminates, each of its per-semaphore semadj values 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 SETVAL or SETALL request to semctl(2), the corresponding semadj values in all processes are cleared. The clone(2) CLONE_SYSVSEM flag allows more than one process to share a semadj list; 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:

SEMOPM
    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 sops array.

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 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 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.

In Linux 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 Linux 2.6.11.

EXAMPLES
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) {
        perror("semop");
        exit(EXIT_FAILURE);
    }

A further example of the use of semop() can be found in shmop(2).

SEE ALSO
clone(2), semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7), sysvipc(7), time(7)