semop

SEMOP(2)                    Linux Programmer's Manual                   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 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
       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.

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

COLOPHON
       This page is part of release 4.14 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                              2017-09-15                           SEMOP(2)