Provided by: manpages-dev_3.35-0.1ubuntu1_all bug

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:

       ·  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  process catches a signal: the value of semzcnt is decremented and semop()
          fails, with errno set to EINTR.

       ·  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:

       ·  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.

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

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

       ·  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

       This page is part of release 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.