This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The mutex object referenced by mutex shall be locked by a call to
pthread_mutex_lock() that returns zero or [EOWNERDEAD]. If the mutex
is already locked by another thread, the calling thread shall block
until the mutex becomes available. This operation shall return with
the mutex object referenced by mutex in the locked state with the
calling thread as its owner. If a thread attempts to relock a mutex
that it has already locked, pthread_mutex_lock() shall behave as
described in the Relock column of the following table. If a thread
attempts to unlock a mutex that it has not locked or a mutex which is
unlocked, pthread_mutex_unlock() shall behave as described in the
Unlock When Not Owner column of the following table.
│Mutex Type │ Robustness │ Relock │ Unlock When Not Owner │
│NORMAL │ non-robust │ deadlock │ undefined behavior │
│NORMAL │ robust │ deadlock │ error returned │
│ERRORCHECK │ either │ error returned │ error returned │
│RECURSIVE │ either │ recursive │ error returned │
│ │ │ (see below) │ │
│DEFAULT │ non-robust │ undefined │ undefined behavior† │
│ │ │ behavior† │ │
│DEFAULT │ robust │ undefined │ error returned │
│ │ │ behavior† │ │
† If the mutex type is PTHREAD_MUTEX_DEFAULT, the behavior of
pthread_mutex_lock() may correspond to one of the three other
standard mutex types as described in the table above. If it
does not correspond to one of those three, the behavior is
undefined for the cases marked †.
Where the table indicates recursive behavior, the mutex shall
maintain the concept of a lock count. When a thread successfully
acquires a mutex for the first time, the lock count shall be set to
one. Every time a thread relocks this mutex, the lock count shall be
incremented by one. Each time the thread unlocks the mutex, the lock
count shall be decremented by one. When the lock count reaches zero,
the mutex shall become available for other threads to acquire.
The pthread_mutex_trylock() function shall be equivalent to
pthread_mutex_lock(), except that if the mutex object referenced by
mutex is currently locked (by any thread, including the current
thread), the call shall return immediately. If the mutex type is
PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the
calling thread, the mutex lock count shall be incremented by one and
the pthread_mutex_trylock() function shall immediately return
The pthread_mutex_unlock() function shall release the mutex object
referenced by mutex. The manner in which a mutex is released is
dependent upon the mutex's type attribute. If there are threads
blocked on the mutex object referenced by mutex when
pthread_mutex_unlock() is called, resulting in the mutex becoming
available, the scheduling policy shall determine which thread shall
acquire the mutex.
(In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall
become available when the count reaches zero and the calling thread
no longer has any locks on this mutex.)
If a signal is delivered to a thread waiting for a mutex, upon return
from the signal handler the thread shall resume waiting for the mutex
as if it was not interrupted.
If mutex is a robust mutex and the process containing the owning
thread terminated while holding the mutex lock, a call to
pthread_mutex_lock() shall return the error value [EOWNERDEAD]. If
mutex is a robust mutex and the owning thread terminated while
holding the mutex lock, a call to pthread_mutex_lock() may return the
error value [EOWNERDEAD] even if the process in which the owning
thread resides has not terminated. In these cases, the mutex is
locked by the thread but the state it protects is marked as
inconsistent. The application should ensure that the state is made
consistent for reuse and when that is complete call
pthread_mutex_consistent(). If the application is unable to recover
the state, it should unlock the mutex without a prior call to
pthread_mutex_consistent(), after which the mutex is marked
If mutex does not refer to an initialized mutex object, the behavior
of pthread_mutex_lock(), pthread_mutex_trylock(), and
pthread_mutex_unlock() is undefined.
The pthread_mutex_lock() and pthread_mutex_trylock() functions shall
EAGAIN The mutex could not be acquired because the maximum number of
recursive locks for mutex has been exceeded.
EINVAL The mutex was created with the protocol attribute having the
value PTHREAD_PRIO_PROTECT and the calling thread's priority
is higher than the mutex's current priority ceiling.
The state protected by the mutex is not recoverable.
The mutex is a robust mutex and the process containing the
previous owning thread terminated while holding the mutex
lock. The mutex lock shall be acquired by the calling thread
and it is up to the new owner to make the state consistent.
The pthread_mutex_lock() function shall fail if:
The mutex type is PTHREAD_MUTEX_ERRORCHECK and the current
thread already owns the mutex.
The pthread_mutex_trylock() function shall fail if:
EBUSY The mutex could not be acquired because it was already locked.
The pthread_mutex_unlock() function shall fail if:
EPERM The mutex type is PTHREAD_MUTEX_ERRORCHECK or
PTHREAD_MUTEX_RECURSIVE, or the mutex is a robust mutex, and
the current thread does not own the mutex.
The pthread_mutex_lock() and pthread_mutex_trylock() functions may
The mutex is a robust mutex and the previous owning thread
terminated while holding the mutex lock. The mutex lock shall
be acquired by the calling thread and it is up to the new
owner to make the state consistent.
The pthread_mutex_lock() function may fail if:
A deadlock condition was detected.
These functions shall not return an error code of [EINTR].
The following sections are informative.
Applications that have assumed that non-zero return values are errors
will need updating for use with robust mutexes, since a valid return
for a thread acquiring a mutex which is protecting a currently
inconsistent state is [EOWNERDEAD]. Applications that do not check
the error returns, due to ruling out the possibility of such errors
arising, should not use robust mutexes. If an application is supposed
to work with normal and robust mutexes it should check all return
values for error conditions and if necessary take appropriate action.
Mutex objects are intended to serve as a low-level primitive from
which other thread synchronization functions can be built. As such,
the implementation of mutexes should be as efficient as possible, and
this has ramifications on the features available at the interface.
The mutex functions and the particular default settings of the mutex
attributes have been motivated by the desire to not preclude fast,
inlined implementations of mutex locking and unlocking.
Since most attributes only need to be checked when a thread is going
to be blocked, the use of attributes does not slow the (common)
Likewise, while being able to extract the thread ID of the owner of a
mutex might be desirable, it would require storing the current thread
ID when each mutex is locked, and this could incur unacceptable
levels of overhead. Similar arguments apply to a mutex_tryunlock
For further rationale on the extended mutex types, see the Rationale
(Informative) volume of POSIX.1‐2008, Threads Extensions.
If an implementation detects that the value specified by the mutex
argument does not refer to an initialized mutex object, it is
recommended that the function should fail and report an [EINVAL]
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 PTHREAD_MUTEX_LOCK(3P)