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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;   /* ID of 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  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  semtimeop()  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.

       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(2) 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

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