semop

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
       is a structure that 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  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
          EIDRM.

       o  The calling thread catches a signal: the value of semzcnt is  decre-
          mented and semop() fails, with errno set to EINTR.

       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
       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  decre-
          mented and semop() fails, with errno set to EINTR.

       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()
       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
       interval 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().

       Note that if semtimeop() is interrupted by a signal, causing  the  call
       to  fail  with  the  error  EINTR,  the  contents  of  timeout are left
       unchanged.

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
              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
       POSIX.1-2001, POSIX.1-2008, SVr4.

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
       request to semctl(2), the corresponding semadj values in all  processes
       are  cleared.   The  clone(2)  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.
              Before  Linux  3.19,  the  default  value for this limit was 32.
              Since Linux 3.19, the default value  is  500.   On  Linux,  this
              limit   can  be  read  and  modified  via  the  third  field  of
              /proc/sys/kernel/sem.  Note: this limit  should  not  be  raised
              above  1000,  because  of the risk of that semop(2) fails due to
              kernel memory fragmentation when allocating memory to  copy  the
              sops array.

       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
       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 4.04 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                             2015-08-08                          SEMOP(2)
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