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// Copyright (C) 2010 Davis E. King (davis@dlib.net)
// License: Boost Software License See LICENSE.txt for the full license.
#ifndef DLIB_READ_WRITE_MUTEX_EXTENSIOn_
#define DLIB_READ_WRITE_MUTEX_EXTENSIOn_
#include "threads_kernel.h"
#include "read_write_mutex_extension_abstract.h"
namespace dlib
{
// ----------------------------------------------------------------------------------------
class read_write_mutex
{
/*!
INITIAL VALUE
- max_locks == defined by constructor
- available_locks == max_locks
- write_lock_in_progress == false
- write_lock_active == false
CONVENTION
- Each time someone gets a read only lock they take one of the "available locks"
and each write lock takes all possible locks (i.e. max_locks). The number of
available locks is recorded in available_locks. Any time you try to lock this
object and there aren't available locks you have to wait.
- max_locks == max_readonly_locks()
- if (some thread is on the process of obtaining a write lock) then
- write_lock_in_progress == true
- else
- write_lock_in_progress == false
- if (some thread currently has a write lock on this mutex) then
- write_lock_active == true
- else
- write_lock_active == false
!*/
public:
read_write_mutex (
) : s(m),
max_locks(0xFFFFFFFF),
available_locks(max_locks),
write_lock_in_progress(false),
write_lock_active(false)
{}
explicit read_write_mutex (
unsigned long max_locks_
) : s(m),
max_locks(max_locks_),
available_locks(max_locks_),
write_lock_in_progress(false),
write_lock_active(false)
{
// make sure requires clause is not broken
DLIB_ASSERT(max_locks > 0,
"\t read_write_mutex::read_write_mutex(max_locks)"
<< "\n\t You must give a non-zero value for max_locks"
<< "\n\t this: " << this
);
}
~read_write_mutex (
)
{}
void lock (
) const
{
m.lock();
// If another write lock is already in progress then wait for it to finish
// before we start trying to grab all the available locks. This way we
// don't end up fighting over the locks.
while (write_lock_in_progress)
s.wait();
// grab the right to perform a write lock
write_lock_in_progress = true;
// now start grabbing all the locks
unsigned long locks_obtained = available_locks;
available_locks = 0;
while (locks_obtained != max_locks)
{
s.wait();
locks_obtained += available_locks;
available_locks = 0;
}
write_lock_in_progress = false;
write_lock_active = true;
m.unlock();
}
void unlock (
) const
{
m.lock();
// only do something if there really was a lock in place
if (write_lock_active)
{
available_locks = max_locks;
write_lock_active = false;
s.broadcast();
}
m.unlock();
}
void lock_readonly (
) const
{
m.lock();
while (available_locks == 0)
s.wait();
--available_locks;
m.unlock();
}
void unlock_readonly (
) const
{
m.lock();
// If this condition is false then it means there are no more readonly locks
// to free. So we don't do anything.
if (available_locks != max_locks && !write_lock_active)
{
++available_locks;
// only perform broadcast when there is another thread that might be listening
if (available_locks == 1 || write_lock_in_progress)
{
s.broadcast();
}
}
m.unlock();
}
unsigned long max_readonly_locks (
) const
{
return max_locks;
}
private:
mutex m;
signaler s;
const unsigned long max_locks;
mutable unsigned long available_locks;
mutable bool write_lock_in_progress;
mutable bool write_lock_active;
// restricted functions
read_write_mutex(read_write_mutex&); // copy constructor
read_write_mutex& operator=(read_write_mutex&); // assignment operator
};
// ----------------------------------------------------------------------------------------
}
#endif // DLIB_READ_WRITE_MUTEX_EXTENSIOn_
|
