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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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