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SEMOP(2)		   Linux Programmers Manual		     SEMOP(2)



NAME
       semop, semtimedop - semaphore operations

SYNOPSIS
       #include 
       #include 
       #include 

       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  pro
       cess 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
       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	semaphores  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 decre
	  mented 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 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  semaphores value to increase) is incremented by one and the pro
       cess 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 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.

	 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 decre
	  mented 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 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
	      isnt 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 doesnt 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 arent 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 semaphores 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
       semaphores 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)




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