unixdev.net


Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (Debian-5.0)
Page:
Section:
Apropos / Subsearch:
optional field

SEMOP(2)                   Linux Programmer's Manual                  SEMOP(2)



NAME
       semop, semtimedop - semaphore operations

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/sem.h>&gt;

       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 ele-
       ments of this structure are of type struct sembuf, containing the  fol-
       lowing 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 opera-
       tion 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  com-
       plete  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 sema-
       phore 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  sema-
       phore set.  This is a "wait-for-zero" operation: if semval is zero, the
       operation can immediately proceed.  Otherwise, if IPC_NOWAIT is  speci-
       fied  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 incre-
       mented 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 decre-
          mented 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  speci-
       fied  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 abso-
          lute  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 decre-
          mented 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  struc-
       ture 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 time-
       out 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 non-positive 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 back-
       ported  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 across a  fork(2)
       system call, 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 maxi-
       mum  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 alto-
       gether (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, &sop, 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 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-07-26                          SEMOP(2)