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

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, contain-
       ing 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 pres-
       ence 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, distin-
       guished 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.  Oth-
       erwise, 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:

       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	thread	catches a signal: the value of semzcnt is decremented 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 pro-
       ceed 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 sema-
       phore  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:

       o  semval becomes greater than or equal to the absolute value of sem_op: the operation now
	  proceeds, as described above.

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

       o  The calling thread catches a signal: the value of semncnt is	decremented  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 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 sched-
       uling 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 oper-
       ations 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 indi-
       cating 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 nonpos-
	      itive 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  depen-
	      dent 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  per-
       formed.	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 3.53 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					    2013-04-17					 SEMOP(2)
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