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;   /* PID of process that last

       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 semtimedop() 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 in the user namespace that  governs
              its IPC namespace.

       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()  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 4.15 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                          SEMOP(2)
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