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/*
* The MIT License
*
* Copyright (C) 2010-2017 Alexander Saprykin <saprykin.spb@gmail.com>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* 'Software'), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
/**
* @file psemaphore.h
* @brief Semaphore routines
* @author Alexander Saprykin
*
* A semaphore is a synchronization primitive which controls access to shared
* data from the concurrently running threads. Unlike a mutex (which is a
* particular case of a binary semaphore) it allows concurrent access not to
* only the one thread.
*
* The semaphore has a counter which means the number of available resources
* (units). Before entering a critical section a thread must perform the so
* called P (acquire) operation: if the counter is positive it decrements the
* counter by 1 and continues execution; otherwise the thread is suspended until
* the counter becomes positive. Before leaving the critical section the thread
* must perform the so called V (release) operation: increments the counter by 1
* and wakes up a waiting thread from the queue (if any).
*
* You can think about the semaphore as a resource controller: the P operation
* takes one unit, while the V operation gives one unit back. The thread could
* not continue execution without taking a resource unit. By setting the initial
* semaphore counter value you can control how much concurrent threads can work
* with a shared resource.
*
* This semaphore implementation is process-wide so you can synchronize not only
* the threads. But it makes this IPC primitive (actually like any other IPC
* primitive, as well) relatively heavy. Consider using a mutex or a spinlock
* instead if you do not need to cross a process boundary.
*
* A process-wide semaphore is identified by its name across the system, thus it
* is also called a named semaphore. Use p_semaphore_new() to open the named
* semaphore and p_semaphore_free() to close it.
*
* Please note the following platform specific differences:
*
* - Windows doesn't own IPC objects (processes own them), which means that a
* semaphore will be removed from the system after the last process or thread
* closes it (or after terminating all the processes and threads holding open
* semaphore).
*
* - UNIX systems own IPC objects. Because of that UNIX IPC objects can survive
* an application crash: an already used semaphore can be opened in a locked
* state and an application can fail into a deadlock or an inconsistent state.
* This could happen if you have not closed all the open semaphores explicitly
* before terminating the application.
*
* - IRIX allows to open several instances of a semaphore within the single
* process, but it will close the object after the first close call from any of
* the threads within the process.
*
* - AmigaOS has process-wide semaphores without actual tracking of counter,
* which means that semaphore behaves the same way as recursive mutex.
*
* - OpenVMS (as of 8.4 release) has declared prototypes for process-wide named
* semaphores but the actual implementation is broken.
*
* - OS/2 lacks support for process-wide named semaphores.
*
* - Syllable lacks support for process-wide named semaphores.
*
* - BeOS lacks support for process-wide named semaphores.
*
* Use the third argument as #P_SEM_ACCESS_CREATE in p_semaphore_new() to reset
* a semaphore value while opening it. This argument is ignored on Windows. You
* can also take ownership of the semaphore with p_semaphore_take_ownership() to
* explicitly remove it from the system after closing.
*/
#if
!defined (
PLIBSYS_H_INSIDE
)
&&
!defined (
PLIBSYS_COMPILATION
)
# error
"Header files shouldn't be included directly, consider using <plibsys.h> instead."
#endif
#ifndef
PLIBSYS_HEADER_PSEMAPHORE_H
#define
PLIBSYS_HEADER_PSEMAPHORE_H
#include
<pmacros.h>
#include
<ptypes.h>
#include
<perror.h>
P_BEGIN_DECLS
/** Enum with semaphore creation modes. */
typedef
enum
PSemaphoreAccessMode_
{
P_SEM_ACCESS_OPEN
=
0
,
/**< Opens an existing semaphore or creates one with a given value. */
P_SEM_ACCESS_CREATE
=
1
/**< Creates semaphore, resets to a given value if exists. */
}
PSemaphoreAccessMode
;
/** Semaphore opaque data structure. */
typedef
struct
PSemaphore_
PSemaphore
;
/**
* @brief Creates a new #PSemaphore object.
* @param name Semaphore name.
* @param init_val Initial semaphore value.
* @param mode Creation mode.
* @param[out] error Error report object, NULL to ignore.
* @return Pointer to a newly created #PSemaphore object in case of success,
* NULL otherwise.
* @since 0.0.1
*
* The @a init_val is used only in one of following cases: a semaphore with the
* such name doesn't exist, or the semaphore with the such name exists but
* @a mode specified as #P_SEM_ACCESS_CREATE (non-Windows platforms only). In
* other cases @a init_val is ignored. The @a name is system-wide, so any other
* process can open that semaphore passing the same name.
*/
P_LIB_API
PSemaphore
*
p_semaphore_new
(
const
pchar
*
name
,
pint
init_val
,
PSemaphoreAccessMode
mode
,
PError
*
*
error
);
/**
* @brief Takes ownership of a semaphore.
* @param sem Semaphore to take ownership.
* @since 0.0.1
*
* If you take ownership of a semaphore object, p_semaphore_free() will try to
* completely unlink it and remove from the system. This is useful on UNIX
* systems where the semaphore can survive an application crash. On the Windows
* platform this call has no effect.
*
* The common usage of this call is upon application startup to ensure that the
* semaphore from the previous crash will be unlinked from the system. To do
* that, call p_semaphore_new(), take ownership of the semaphore object and
* remove it with the p_semaphore_free() call. After that, create it again.
*
* You can also do the same thing upon semaphore creation passing
* #P_SEM_ACCESS_CREATE to p_semaphore_new(). The only difference is that you
* should already know whether this semaphore object is from the previous crash
* or not.
*/
P_LIB_API
void
p_semaphore_take_ownership
(
PSemaphore
*
sem
);
/**
* @brief Acquires (P operation) a semaphore.
* @param sem #PSemaphore to acquire.
* @param[out] error Error report object, NULL to ignore.
* @return TRUE in case of success, FALSE otherwise.
* @since 0.0.1
*/
P_LIB_API
pboolean
p_semaphore_acquire
(
PSemaphore
*
sem
,
PError
*
*
error
);
/**
* @brief Releases (V operation) a semaphore.
* @param sem #PSemaphore to release.
* @param[out] error Error report object, NULL to ignore.
* @return TRUE in case of success, FALSE otherwise.
* @since 0.0.1
*/
P_LIB_API
pboolean
p_semaphore_release
(
PSemaphore
*
sem
,
PError
*
*
error
);
/**
* @brief Frees #PSemaphore object.
* @param sem #PSemaphore to free.
* @since 0.0.1
*
* It doesn't release an acquired semaphore, be careful to not to make a
* deadlock while removing the acquired semaphore.
*/
P_LIB_API
void
p_semaphore_free
(
PSemaphore
*
sem
);
P_END_DECLS
#endif
/* PLIBSYS_HEADER_PSEMAPHORE_H */
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