NAME

AG_Threads — agar threads support

SYNOPSIS

#include <agar/core.h>

DESCRIPTION

On platforms with threads support, Agar can be compiled with support for multithreading. In a threaded build, Agar API calls can be considered free-threaded (safe to use from different threads without need for application-level synchronization) unless documented otherwise.

Even though calls are free-threaded, application-level synchronization (calls to AG_ObjectLock(3)) may still be needed in some cases. See EXAMPLES for some examples of thread-unsafe vs. thread-safe usages.

CONVENTIONS

Agar function calls are free-threaded unless mentioned otherwise.

The AG_Object(3) system provides a per-object recursive mutex which is implicitely acquired before invoking object methods or processing events.

THREADS INTERFACE

When compiled with threads support, Agar provides a portable, minimal interface to the operating system's native threads interface. These functions follow Agar's standard error-handling style (see AG_Intro(3)).

MUTEXES

Mutexes (MUTual EXclusion devices) are commonly used to protect shared data structure against concurrent modifications.

void
AG_MutexInit(AG_Mutex *mutex);

int
AG_MutexTryInit(AG_Mutex *mutex);

void
AG_MutexInitRecursive(AG_Mutex *mutex);

int
AG_MutexTryInitRecursive(AG_Mutex *mutex);

void
AG_MutexDestroy(AG_Mutex *mutex);

void
AG_MutexLock(AG_Mutex *mutex);

int
AG_MutexTryLock(AG_Mutex *mutex);

void
AG_MutexUnlock(AG_Mutex *mutex);

The AG_MutexInit() function initializes a mutex structure. AG_MutexInitRecursive() initializes a recursive mutex (a mutex with a reference count), which allows nested AG_MutexLock() calls.

AG_MutexDestroy() frees all resources allocated for a mutex.

AG_MutexLock() and AG_MutexUnlock() respectively acquire and release a mutex.

AG_MutexTryLock() tries to acquire a mutex without blocking and immediately returns 0 on success. On failure, the function returns -1, but does not set any error message (so AG_GetError(3) should not be used).

CONDITION VARIABLES

void
AG_CondInit(AG_Cond *cv);

int
AG_CondTryInit(AG_Cond *cv);

void
AG_CondDestroy(AG_Cond *cv);

void
AG_CondBroadcast(AG_Cond *cv);

void
AG_CondSignal(AG_Cond *cv);

int
AG_CondWait(AG_Cond *cv, AG_Mutex *m);

int
AG_CondTimedWait(AG_Cond *cv, AG_Mutex *m, const struct timespec *t);

AG_CondInit() initializes a condition variable structure.

AG_CondDestroy() releases resources allocated for a condition variable.

AG_CondBroadcast() unblock all threads which are currently blocked waiting on cv. AG_CondSignal() unblocks at least one thread currently blocked waiting on cv.

AG_CondWait() blocks the calling thread until cv is signaled. The AG_CondTimedWait() variant will not block for more than the specified amount of time.

All of these functions will raise a fatal condition if an error is encountered.

THREADS

void
AG_ThreadCreate(AG_Thread *th, void *(*fn)(void *arg), void *arg);

int
AG_ThreadTryCreate(AG_Thread *th, void *(*fn)(void *arg), void *arg);

void
AG_ThreadCancel(AG_Thread th);

int
AG_ThreadTryCancel(AG_Thread th);

void
AG_ThreadJoin(AG_Thread th, void **exitVal);

int
AG_ThreadTryJoin(AG_Thread th, void **exitVal);

void
AG_ThreadExit(void *exitVal);

void
AG_ThreadKill(AG_Thread th, int signal);

AG_Thread
AG_ThreadSelf(void);

int
AG_ThreadEqual(AG_Thread a, AG_Thread b);

AG_ThreadCreate() creates a new thread executing fn. The optional argument arg is passed to fn.

The AG_ThreadCancel() routine requests that the specified thread be cancelled. If the given thread is invalid, a fatal error is raised.

The AG_ThreadJoin() function suspends the execution of the current thread until th terminates. When it does, the value passed to AG_ThreadExit() is made available in exitVal.

AG_ThreadExit() terminates the current thread. exitVal is an optional user pointer.

AG_ThreadKill() sends a signal to the specified thread.

AG_ThreadSelf() returns the identifier of the current (caller's) thread. AG_ThreadEqual() returns 1 if the identifiers a and b both refer to the same thread, or 0 if they differ.

THREAD-SPECIFIC VARIABLES

void
AG_ThreadKeyCreate(AG_ThreadKey *key, void (*destructor)(void *));

int
AG_ThreadKeyTryCreate(AG_ThreadKey *key, void (*destructor)(void *));

void
AG_ThreadKeyDelete(AG_ThreadKey key);

int
AG_ThreadKeyTryDelete(AG_ThreadKey key);

void *
AG_ThreadKeyGet(AG_ThreadKey key);

void
AG_ThreadKeySet(AG_ThreadKey key, const void *value);

int
AG_ThreadKeyTrySet(AG_ThreadKey key, const void *value);

AG_ThreadKeyCreate() initializes a key (i.e., a handle) to a thread-specific value. The handle itself is accessible to all threads. The thread-specific value (i.e., the value specified by AG_ThreadKeySet(), and which defaults to NULL) will persist only for the life of the thread. If an optional destructor is given, that function will be called (with the thread-specific value as its argument), when the thread exists.

The AG_ThreadKeyDelete() function releases resources allocated for a key.

AG_ThreadKeyGet() returns the thread-specific value associated with key.

AG_ThreadKeySet() sets a thread-specific value with key.

EXAMPLES

The following code uses the return value of a VFS lookup in a manner which is not thread-safe. A race condition exists between the AG_ObjectFind() call and the following access:

AG_Object *o;

o = AG_ObjectFind(root, "/Foo");
if (o != NULL) { /* ... */ }     /* UNSAFE access */

The following code accesses the returned object safely by acquiring the mutex of the VFS root object (which protects the entire VFS linkage):

AG_Object *o;

AG_ObjectLock(root);
o = AG_ObjectFind(root, "/Foo");
if (o != NULL) { /* ... */ }     /* Safe access */
AG_ObjectUnlock(root);

HISTORY

The AG_Threads interface first appeared in Agar 1.0