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Diffstat (limited to 'man3/pthread_mutex_init.3')
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diff --git a/man3/pthread_mutex_init.3 b/man3/pthread_mutex_init.3 new file mode 100644 index 0000000..8d9abf9 --- /dev/null +++ b/man3/pthread_mutex_init.3 @@ -0,0 +1,241 @@ +.\" Copyright, Xavier Leroy <Xavier.Leroy@inria.fr> +.\" Copyright 2023, Alejandro Colomar <alx@kernel.org> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.TH pthread_mutex_init 3 2024-02-26 "Linux man-pages 6.7" +. +. +.SH NAME +pthread_mutex_init, +pthread_mutex_lock, +pthread_mutex_trylock, +pthread_mutex_unlock, +pthread_mutex_destroy +\- +operations on mutexes +. +. +.SH SYNOPSIS +.B #include <pthread.h> +.P +.BI "pthread_mutex_t " fastmutex " = PTHREAD_MUTEX_INITIALIZER;" +.BI "pthread_mutex_t " recmutex " = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;" +.BI "pthread_mutex_t " errchkmutex " = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;" +.P +.BI "int pthread_mutex_init(pthread_mutex_t *" mutex ", const pthread_mutexattr_t *" mutexattr ");" +.BI "int pthread_mutex_lock(pthread_mutex_t *" mutex ");" +.BI "int pthread_mutex_trylock(pthread_mutex_t *" mutex ");" +.BI "int pthread_mutex_unlock(pthread_mutex_t *" mutex ");" +.BI "int pthread_mutex_destroy(pthread_mutex_t *" mutex ");" +. +. +.SH DESCRIPTION +A mutex is a MUTual EXclusion device, +and is useful for +protecting shared data structures from concurrent modifications, +and implementing critical sections and monitors. +.P +A mutex has two possible states: +unlocked (not owned by any thread), +and locked (owned by one thread). +A mutex can never be owned by two different threads simultaneously. +A thread attempting to lock a mutex +that is already locked by another thread +is suspended until the owning thread unlocks the mutex first. +.P +\fBpthread_mutex_init\fP initializes the mutex object pointed to by \fImutex\fP +according to the mutex attributes specified in \fImutexattr\fP. +If \fImutexattr\fP is \fBNULL\fP, +default attributes are used instead. +.P +The LinuxThreads implementation supports only one mutex attributes, +the \fImutex kind\fP, +which is either ``fast'', +``recursive'', +or ``error checking''. +The kind of a mutex determines +whether it can be locked again by a thread that already owns it. +The default kind is ``fast''. +See \fBpthread_mutexattr_init\fP(3) for more information on mutex attributes. +.P +Variables of type \fBpthread_mutex_t\fP can also be initialized statically, +using the constants +\fBPTHREAD_MUTEX_INITIALIZER\fP +(for fast mutexes), +\fBPTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP\fP +(for recursive mutexes), +and \fBPTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP\fP +(for error checking mutexes). +.P +\fBpthread_mutex_lock\fP locks the given mutex. +If the mutex is currently unlocked, +it becomes locked and owned by the calling thread, +and \fBpthread_mutex_lock\fP returns immediately. +If the mutex is already locked by another thread, +\fBpthread_mutex_lock\fP suspends the calling thread +until the mutex is unlocked. +.P +If the mutex is already locked by the calling thread, +the behavior of \fBpthread_mutex_lock\fP depends on the kind of the mutex. +If the mutex is of the ``fast'' kind, +the calling thread is suspended until the mutex is unlocked, +thus effectively causing the calling thread to deadlock. +If the mutex is of the ``error checking'' kind, +\fBpthread_mutex_lock\fP returns immediately with the error code \fBEDEADLK\fP. +If the mutex is of the ``recursive'' kind, +\fBpthread_mutex_lock\fP succeeds and returns immediately, +recording the number of times the calling thread has locked the mutex. +An equal number of \fBpthread_mutex_unlock\fP operations +must be performed before the mutex returns to the unlocked state. +.P +\fBpthread_mutex_trylock\fP behaves identically to \fBpthread_mutex_lock\fP, +except that it does not block the calling thread +if the mutex is already locked by another thread +(or by the calling thread in the case of a ``fast'' mutex). +Instead, +\fBpthread_mutex_trylock\fP returns immediately +with the error code \fBEBUSY\fP. +.P +\fBpthread_mutex_unlock\fP unlocks the given mutex. +The mutex is assumed to be locked and owned by the calling thread +on entrance to \fBpthread_mutex_unlock\fP. +If the mutex is of the ``fast'' kind, +\fBpthread_mutex_unlock\fP always returns it to the unlocked state. +If it is of the ``recursive'' kind, +it decrements the locking count of the mutex +(number of \fBpthread_mutex_lock\fP operations +performed on it by the calling thread), +and only when this count reaches zero is the mutex actually unlocked. +.P +On ``error checking'' and ``recursive'' mutexes, +\fBpthread_mutex_unlock\fP actually checks at run-time +that the mutex is locked on entrance, +and that it was locked by the same thread +that is now calling \fBpthread_mutex_unlock\fP. +If these conditions are not met, +an error code is returned and the mutex remains unchanged. +``Fast'' mutexes perform no such checks, +thus allowing a locked mutex to be +unlocked by a thread other than its owner. +This is non-portable behavior and must not be relied upon. +.P +\fBpthread_mutex_destroy\fP destroys a mutex object, +freeing the resources it might hold. +The mutex must be unlocked on entrance. +In the LinuxThreads implementation, +no resources are associated with mutex objects, +thus \fBpthread_mutex_destroy\fP actually does nothing +except checking that the mutex is unlocked. +. +. +.SH CANCELLATION +None of the mutex functions is a cancelation point, +not even \fBpthread_mutex_lock\fP, +in spite of the fact that it can suspend a thread for arbitrary durations. +This way, +the status of mutexes at cancelation points is predictable, +allowing cancelation handlers +to unlock precisely those mutexes that need to be unlocked +before the thread stops executing. +Consequently, +threads using deferred cancelation +should never hold a mutex for extended periods of time. +. +. +.SH "ASYNC-SIGNAL SAFETY" +The mutex functions are not async-signal safe. +What this means is that they should not be called from a signal handler. +In particular, +calling \fBpthread_mutex_lock\fP or \fBpthread_mutex_unlock\fP +from a signal handler +may deadlock the calling thread. +. +. +.SH "RETURN VALUE" +\fBpthread_mutex_init\fP always returns 0. +The other mutex functions +return 0 on success and a non-zero error code on error. +. +. +.SH ERRORS +The \fBpthread_mutex_lock\fP function returns +the following error code on error: +.RS +.TP +\fBEINVAL\fP +The mutex has not been properly initialized. +.TP +\fBEDEADLK\fP +The mutex is already locked by the calling thread +(``error checking'' mutexes only). +.RE +.P +The \fBpthread_mutex_trylock\fP function returns +the following error codes on error: +.RS +.TP +\fBEBUSY\fP +The mutex could not be acquired because it was currently locked. +.TP +\fBEINVAL\fP +The mutex has not been properly initialized. +.RE +.P +The \fBpthread_mutex_unlock\fP function returns +the following error code on error: +.RS +.TP +\fBEINVAL\fP +The mutex has not been properly initialized. +.TP +\fBEPERM\fP +The calling thread does not own the mutex +(``error checking'' mutexes only). +.RE +.P +The \fBpthread_mutex_destroy\fP function returns +the following error code on error: +.RS +.TP +\fBEBUSY\fP +The mutex is currently locked. +.RE +. +. +.SH "SEE ALSO" +\fBpthread_mutexattr_init\fP(3), +\fBpthread_mutexattr_setkind_np\fP(3), +\fBpthread_cancel\fP(3). +. +. +.SH EXAMPLE +A shared global variable \fIx\fP can be protected by a mutex as follows: +.P +.RS +.ft 3 +.nf +.sp +int x; +pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; +.ft +.P +.RE +.fi +.P +All accesses and modifications to \fIx\fP +should be bracketed by calls to +\fBpthread_mutex_lock\fP and \fBpthread_mutex_unlock\fP +as follows: +.P +.RS +.ft 3 +.nf +.sp +pthread_mutex_lock(&mut); +/* operate on x */ +pthread_mutex_unlock(&mut); +.ft +.P +.RE +.fi |