semop(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

SEMOP(2)                Linux Programmer's Manual               SEMOP(2)

NAME         top

       semop, semtimedop - System V semaphore operations

SYNOPSIS         top

       #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

DESCRIPTION         top

       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

       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         top

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

ERRORS         top

       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.

VERSIONS         top

       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.

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4.

NOTES         top

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

       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         top

       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 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.

EXAMPLES         top

       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         top

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

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                       SEMOP(2)

Pages that refer to this page: ipcs(1)lsipc(1)pcp-ipcs(1)clone(2)fork(2)ipc(2)semctl(2)semget(2)syscalls(2)unshare(2)proc(5)pthreads(7)sem_overview(7)signal(7)sysvipc(7)