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/*
* 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.
*/
#ifndef BOOST_INTERPROCESS_SYNC_XSI_ADVANCED_XSI_SEMAPHORE_HPP
#define BOOST_INTERPROCESS_SYNC_XSI_ADVANCED_XSI_SEMAPHORE_HPP
#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 advanced_sem_open_or_create(::key_t key, int initval, int &semid, int perm)
{
semid = -1;
int id, semval;
union semun {
int val;
::semid_ds *buf;
ushort *array;
} semctl_arg;
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, 3, (perm & 0x01FF) | IPC_CREAT)) < 0)
return false; //permission problem or tables full
// When the semaphore is created, we know that the value of all
// 3 members is 0.
// Get a lock on the semaphore by waiting for [2] to equal 0,
// then increment it.
//
// There is a race condition here. There is a possibility that
// between the semget() above and the ::semop() below, another
// process can call our sem_close() function which can remove
// the semaphore if that process is the last one using it.
// Therefore, we handle the error condition of an invalid
// semaphore ID specially below, and if it does happen, we just
// go back and create it again.
struct sembuf op_lock[2] = {
{2, 0, 0}, // wait for [2] (lock) to equal 0
{2, 1, SEM_UNDO} // then increment [2] to 1 - this locks it
// UNDO to release the lock if processes exits
// before explicitly unlocking
};
if (::semop(id, &op_lock[0], 2) < 0) {
if (errno == EINVAL)
goto again;
}
// Get the value of the process counter. If it equals 0,
// then no one has initialized the semaphore yet.
if ((semval = ::semctl(id, 1, GETVAL, 0)) < 0)
return false;
if (semval == 0) {
// We could initialize by doing a SETALL, but that
// would clear the adjust value that we set when we
// locked the semaphore above. Instead, we'll do 2
// system calls to initialize [0] and [1].
semctl_arg.val = initval;
if (::semctl(id, 0, SETVAL, semctl_arg) < 0)
return false;
semctl_arg.val = 1;
if (::semctl(id, 1, SETVAL, semctl_arg) < 0)
return false;
}
// Decrement the process counter and then release the lock.
struct sembuf op_unlock[1] = {
2, -1, 0/*SEM_UNDO*/ // decrement [2] (lock) back to 0
};
if (::semop(id, &op_unlock[0], 1) < 0)
return false;
semid = id;
return true;
}
// Open a semaphore that must already exist.
// This function should be used, instead of sem_create(), if the caller
// knows that the semaphore must already exist. For example a client
// from a client-server pair would use this, if its the server's
// responsibility to create the semaphore.
// We return the semaphore ID if all OK, else -1.
/*
inline bool advanced_sem_open(key_t key, int &semid)
{
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
if ((semid = ::semget(key, 3, 0)) < 0)
return false; // doesn't exist, or tables full
// Decrement the process counter. We don't need a lock
struct sembuf op_open[1] = {
1, -1, SEM_UNDO // decrement [1] (proc counter) with undo on exit
};
if (::semop(id, &op_open[0], 1) < 0)
return false;
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 advanced_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 advanced_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_ADVANCED_XSI_SEMAPHORE_HPP