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//////////////////////////////////////////////////////////////////////////////
//
// (C) Copyright Ion Gaztanaga 2011-2012. 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
#ifndef BOOST_CONFIG_HPP
# include <boost/config.hpp>
#endif
#
#if defined(BOOST_HAS_PRAGMA_ONCE)
# pragma once
#endif
/*
* 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
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