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+//////////////////////////////////////////////////////////////////////////////
+//
+// (C) Copyright Ion Gaztanaga 2011-2011. Distributed under the Boost
+// Software License, Version 1.0. (See accompanying file
+// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/libs/interprocess for documentation.
+//
+//////////////////////////////////////////////////////////////////////////////
+#ifndef BOOST_INTERPROCESS_SYNC_XSI_SIMPLE_XSI_SEMAPHORE_HPP
+#define BOOST_INTERPROCESS_SYNC_XSI_SIMPLE_XSI_SEMAPHORE_HPP
+
+/*
+ * Provide an simpler and easier to understand interface to the System V
+ * semaphore system calls. There are 7 routines available to the user:
+ *
+ * id = sem_create(key, initval); # create with initial value or open
+ * id = sem_open(key); # open (must already exist)
+ * sem_wait(id); # wait = P = down by 1
+ * sem_signal(id); # signal = V = up by 1
+ * sem_op(id, amount); # wait if (amount < 0)
+ * # signal if (amount > 0)
+ * sem_close(id); # close
+ * sem_rm(id); # remove (delete)
+ *
+ * We create and use a 3-member set for the requested semaphore.
+ * The first member, [0], is the actual semaphore value, and the second
+ * member, [1], is a counter used to know when all processes have finished
+ * with the semaphore. The counter is initialized to a large number,
+ * decremented on every create or open and incremented on every close.
+ * This way we can use the "adjust" feature provided by System V so that
+ * any process that exit's without calling sem_close() is accounted
+ * for. It doesn't help us if the last process does this (as we have
+ * no way of getting control to remove the semaphore) but it will
+ * work if any process other than the last does an exit (intentional
+ * or unintentional).
+ * The third member, [2], of the semaphore set is used as a lock variable
+ * to avoid any race conditions in the sem_create() and sem_close()
+ * functions.
+ */
+
+#include <sys/ipc.h>
+#include <sys/sem.h>
+#include <errno.h>
+
+namespace boost {
+namespace interprocess {
+namespace xsi {
+
+// Create a semaphore with a specified initial value.
+// If the semaphore already exists, we don't initialize it (of course).
+// We return the semaphore ID if all OK, else -1.
+
+inline bool simple_sem_open_or_create(::key_t key, int initval, int &semid, int perm)
+{
+ int id, semval;
+ semid = -1;
+
+ if (key == IPC_PRIVATE)
+ return false; //not intended for private semaphores
+
+ else if (key == (::key_t) -1)
+ return false; //probably an ftok() error by caller
+
+ again:
+ if ((id = ::semget(key, 1, (perm & 0x01FF) | IPC_CREAT)) < 0)
+ return false; //permission problem or tables full
+
+ semid = id;
+ return true;
+}
+
+/****************************************************************************
+ * Remove a semaphore.
+ * This call is intended to be called by a server, for example,
+ * when it is being shut down, as we do an IPC_RMID on the semaphore,
+ * regardless whether other processes may be using it or not.
+ * Most other processes should use sem_close() below.
+ */
+
+inline bool simple_sem_rm(int id)
+{
+ if (::semctl(id, 0, IPC_RMID, 0) < 0)
+ return false;
+ return true;
+}
+
+
+/****************************************************************************
+ * General semaphore operation. Increment or decrement by a user-specified
+ * amount (positive or negative; amount can't be zero).
+ */
+
+inline bool simple_sem_op(int id, int value, bool undo = true)
+{
+ ::sembuf op_op[1] = {
+ 0, 99, 0 // decrement or increment [0] with undo on exit
+ // the 99 is set to the actual amount to add
+ // or subtract (positive or negative)
+ };
+ if(undo){
+ op_op[0].sem_flg = SEM_UNDO;
+ }
+ if ((op_op[0].sem_op = value) == 0)
+ return false;
+
+ if (::semop(id, &op_op[0], 1) < 0)
+ return false;
+ return true;
+}
+
+} //namespace xsi {
+} //namespace interprocess {
+} //namespace boost {
+
+#endif //BOOST_INTERPROCESS_SYNC_XSI_SIMPLE_XSI_SEMAPHORE_HPP