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);

   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 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 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 semaphore's value becomes zero) is
       incremented by one and the process 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 process 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  abso-
       lute  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 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:

       o  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.

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

       o  The calling process 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 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 structure whose address is passed in the timeout argument.  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
       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 process 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 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.

       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 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  per-
       formed  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 peri-
       ods.  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 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, sops, 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 3.27 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								    2008-10-04								  SEMOP(2)