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NAME

       semop, semtimedop - semaphore operations

SYNOPSIS

       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>

       int semop(int semid, struct sembuf *sops, unsigned nsops);

       int semtimedop(int semid, struct sembuf *sops, unsigned nsops,
                      struct timespec *timeout);

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

       semtimedop(): _GNU_SOURCE

DESCRIPTION

       Each semaphore in a 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;   /* process that did last op */

       semop() performs operations on selected semaphores in the set indicated
       by semid.  Each of the nsops elements in the array pointed to  by  sops
       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 updates the process undo count (semadj)  for
       this semaphore.  This operation can always proceed -- it never forces a
       process 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 processes waiting until this  semaphore's  value  becomes  zero)  is
       incremented  by  one  and the process sleeps until one of the following
       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  process  catches  a  signal:  the  value of semzcnt is
          decremented and semop() fails, with errno set to EINTR.

       o  The time limit specified by timeout in a semtimedop() call  expires:
          semop() fails, with errno set to EAGAIN.

       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 updates the process undo count
       (semadj) 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 processes waiting for
       this semaphore's value to increase)  is  incremented  by  one  and  the
       process sleeps until one of the following occurs:

       o  semval  becomes  greater  than  or  equal  to  the absolute value of
          sem_op, at which time the  value  of  semncnt  is  decremented,  the
          absolute  value of sem_op is subtracted from semval and, if SEM_UNDO
          is specified for this operation, the system updates the process undo
          count (semadj) for this semaphore.

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

       o  The calling process catches  a  signal:  the  value  of  semncnt  is
          decremented and semop() fails, with errno set to EINTR.

       o  The  time limit specified by timeout in a semtimedop() call expires:
          the system call fails, with errno set to EAGAIN.

       On successful completion, the sempid value for each semaphore specified
       in the array pointed to by sops is set to the process ID of the calling
       process.  In addition, the sem_otime is set to the current time.

       semtimedop() behaves identically to semop() except that in those  cases
       were  the  calling  process  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.  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().

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.

       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 process 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

       SVr4, POSIX.1-2001.

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.

       semadj is a per-process integer which is simply the (negative) count of
       all semaphore operations performed specifying the SEM_UNDO flag.   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  semval, sempid, semzcnt, and semnct values for a semaphore can all
       be retrieved using appropriate semctl(2) calls.

       The following limits on semaphore  set  resources  affect  the  semop()
       call:

       SEMOPM Maximum  number  of operations allowed for one semop() call (32)
              (on Linux, this limit can be read and  modified  via  the  third
              field of /proc/sys/kernel/sem).

       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 process 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

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

COLOPHON

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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.