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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, 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
       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 subtracts the	value sem_op  from  the	 sema-
       phore 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 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
       threads waiting until this semaphore's value becomes  zero)  is	incre-
       mented 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 EI-
	  DRM.

       o  The calling thread 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 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 oc-
       curs:

       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	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 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	struc-
       ture whose address is passed in the timeout argument.  (This sleep  in-
       terval  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 im-
	      plementation 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 ap-
       peared 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 pro-
       duced  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 sema-
       phore 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 re-
       quest to	semctl(2), the corresponding semadj values  in	all  processes
       are  cleared.   The  clone()  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  (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 pa-
       rameters.

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 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 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-09-21			      SEMOP(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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