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           #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


           Each semaphore in a System V semaphore set has the following associated
               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
           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
           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
           semaphore adjustment (semadj) value for this semaphore.  This operation
           ?  The calling thread 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 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
           ?  semval  becomes  greater  than  or  equal  to  the absolute value of
              sem_op: the operation now proceeds, as described above.
           ?  The semaphore set is removed from the system:  semop()  fails,  with
              errno set to EIDRM.
           ?  The  calling thread 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 caller's process ID.   In
           addition, the sem_otime is set to the current time.
           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().


           If successful semop() and semtimedop() return 0; otherwise they  return
           -1 with errno indicating the error.
           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.


           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.


           SVr4, POSIX.1-2001.


           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
           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
           semaphore 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
           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


           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.


           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) {


           clone(2),   semctl(2),   semget(2),   sigaction(2),    capabilities(7),
           sem_overview(7), svipc(7), time(7)

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