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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, 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 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
       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.

       ·  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 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.

       ·  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 caller's process ID.   In
       addition, the sem_otime is set to the current time.

       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().

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

       SVr4, POSIX.1-2001.

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  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 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

       This  page  is  part of release 3.65 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/.



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