SEMOP(2)                  (2020-04-11)                   SEMOP(2)

     NAME
          semop, semtimedop - System V semaphore operations

     SYNOPSIS
          #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
          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 ele-
          ments of this structure are of type struct sembuf, contain-
          ing 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.

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          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, distin-
          guished 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 sub-
          tracts 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 call-
          ing 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.  Oth-
          erwise, 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 incre-
          mented by one and the thread sleeps until one of the follow-
          ing occurs:

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

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

          +o 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 sub-
          tracted 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 incre-
          mented by one and the thread sleeps until one of the follow-
          ing occurs:

          +o semval becomes greater than or equal to the absolute value
            of sem_op: the operation now proceeds, as described above.

          +o The semaphore set is removed from the system: semop()
            fails, with errno set to EIDRM.

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          +o 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 sema-
          phore 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 dura-
          tion 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, caus-
          ing the call to fail with the error EINTR, the contents of
          timeout are left unchanged.

     RETURN VALUE
          If successful, semop() and semtimedop() return 0; otherwise
          they return -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 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.

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

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

     NOTES
          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.  Appli-
          cations 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 inter-
          rupted 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

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          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 sema-
          phore 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 frag-
               mentation 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 num-
          ber 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 sema-
          phore 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

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          (somewhat analogously to failing when IPC_NOWAIT is speci-
          fied 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 cir-
          cumstances 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
          The following code segment uses semop() to atomically wait
          for the value of semaphore 0 to become zero, and then incre-
          ment 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)

     COLOPHON
          This page is part of release 5.10 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/.

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