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

NAME
       semop, semtimedop - 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);

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  atomically,  that
       is, the operations are performed at the same time, and only if they can
       all be simultaneously performed.	 The behaviour 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:

       ·      semval becomes 0, at which time the value of semzcnt  is	decre‐
	      mented.

       ·      The  semaphore  set is removed: semop() fails, with errno set to
	      EIDRM.

       ·      The calling process 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 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:

       ·      semval becomes greater than or equal to the  absolute  value  of
	      sem_op,  at  which time the value of semncnt is decremented, the
	      absolute 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
	      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 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 parameter.  If  the	speci‐
       fied  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 parameter 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 timeout parameters
	      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.

       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.

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.

       semtimedop() first appeared in Linux 2.5.52, and was subsequently back‐
       ported into kernel 2.4.22.

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.

CONFORMING TO
       SVr4, POSIX.1-2001.

SEE ALSO
       semctl(2),  semget(2),  sigaction(2), capabilities(7), sem_overview(7),
       svipc(7)

Linux 2.6.9			  2004-11-10			      SEMOP(2)
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