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

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

NAME         top

       semop, semtimedop - System V semaphore operations

SYNOPSIS         top

       #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

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
                                        modified 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         top

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

ERRORS         top

       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 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/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_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.

EXAMPLE         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);
           }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.08 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                            2016-10-08                         SEMOP(2)