PTHREAD_MUTEX_LOCK(3)      POSIX Programmer's Manual     PTHREAD_MUTEX_LOCK(3)



PPRROOLLOOGG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.


NNAAMMEE
       pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock -- lock
       and unlock a mutex

SSYYNNOOPPSSIISS
       #include <pthread.h>

       int pthread_mutex_lock(pthread_mutex_t *_m_u_t_e_x);
       int pthread_mutex_trylock(pthread_mutex_t *_m_u_t_e_x);
       int pthread_mutex_unlock(pthread_mutex_t *_m_u_t_e_x);

DDEESSCCRRIIPPTTIIOONN
       The mutex object referenced by _m_u_t_e_x shall be locked by a call to
       _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k() that returns zero or [[EEOOWWNNEERRDDEEAADD]].  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 _m_u_t_e_x in the locked state with the calling
       thread as its owner. If a thread attempts to relock a mutex that it has
       already locked, _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k() shall behave as described in the
       RReelloocckk column of the following table. If a thread attempts to unlock a
       mutex that it has not locked or a mutex which is unlocked,
       _p_t_h_r_e_a_d___m_u_t_e_x___u_n_l_o_c_k() shall behave as described in the UUnnlloocckk WWhheenn NNoott
       OOwwnneerr column of the following table.

         +-----------+------------+----------------+-----------------------+
         |MMuutteexx TTyyppee | RRoobbuussttnneessss |     RReelloocckk     | UUnnlloocckk WWhheenn NNoott OOwwnneerr |
         +-----------+------------+----------------+-----------------------+
         |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
             _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k() 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 _p_t_h_r_e_a_d___m_u_t_e_x___t_r_y_l_o_c_k() function shall be equivalent to
       _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k(), except that if the mutex object referenced by
       _m_u_t_e_x 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
       _p_t_h_r_e_a_d___m_u_t_e_x___t_r_y_l_o_c_k() function shall immediately return success.

       The _p_t_h_r_e_a_d___m_u_t_e_x___u_n_l_o_c_k() function shall release the mutex object
       referenced by _m_u_t_e_x.  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 _m_u_t_e_x when _p_t_h_r_e_a_d___m_u_t_e_x___u_n_l_o_c_k() 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 _m_u_t_e_x is a robust mutex and the process containing the owning thread
       terminated while holding the mutex lock, a call to _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k()
       shall return the error value [[EEOOWWNNEERRDDEEAADD]].  If _m_u_t_e_x is a robust mutex
       and the owning thread terminated while holding the mutex lock, a call
       to _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k() may return the error value [[EEOOWWNNEERRDDEEAADD]] 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
       _p_t_h_r_e_a_d___m_u_t_e_x___c_o_n_s_i_s_t_e_n_t().  If the application is unable to recover
       the state, it should unlock the mutex without a prior call to
       _p_t_h_r_e_a_d___m_u_t_e_x___c_o_n_s_i_s_t_e_n_t(), after which the mutex is marked permanently
       unusable.

       If _m_u_t_e_x does not refer to an initialized mutex object, the behavior of
       _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k(), _p_t_h_r_e_a_d___m_u_t_e_x___t_r_y_l_o_c_k(), and
       _p_t_h_r_e_a_d___m_u_t_e_x___u_n_l_o_c_k() is undefined.

RREETTUURRNN VVAALLUUEE
       If successful, the _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k(), _p_t_h_r_e_a_d___m_u_t_e_x___t_r_y_l_o_c_k(), and
       _p_t_h_r_e_a_d___m_u_t_e_x___u_n_l_o_c_k() functions shall return zero; otherwise, an error
       number shall be returned to indicate the error.

EERRRROORRSS
       The _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k() and _p_t_h_r_e_a_d___m_u_t_e_x___t_r_y_l_o_c_k() functions shall
       fail if:

       EEAAGGAAIINN The mutex could not be acquired because the maximum number of
              recursive locks for _m_u_t_e_x has been exceeded.

       EEIINNVVAALL The _m_u_t_e_x 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.

       EENNOOTTRREECCOOVVEERRAABBLLEE
              The state protected by the mutex is not recoverable.

       EEOOWWNNEERRDDEEAADD
              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 _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k() function shall fail if:

       EEDDEEAADDLLKK
              The mutex type is PTHREAD_MUTEX_ERRORCHECK and the current
              thread already owns the mutex.

       The _p_t_h_r_e_a_d___m_u_t_e_x___t_r_y_l_o_c_k() function shall fail if:

       EEBBUUSSYY  The _m_u_t_e_x could not be acquired because it was already locked.

       The _p_t_h_r_e_a_d___m_u_t_e_x___u_n_l_o_c_k() function shall fail if:

       EEPPEERRMM  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 _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k() and _p_t_h_r_e_a_d___m_u_t_e_x___t_r_y_l_o_c_k() functions may fail
       if:

       EEOOWWNNEERRDDEEAADD
              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 _p_t_h_r_e_a_d___m_u_t_e_x___l_o_c_k() function may fail if:

       EEDDEEAADDLLKK
              A deadlock condition was detected.

       These functions shall not return an error code of [[EEIINNTTRR]].

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
       None.

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       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 [[EEOOWWNNEERRDDEEAADD]].  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.

RRAATTIIOONNAALLEE
       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) mutex-
       locking case.

       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 _m_u_t_e_x___t_r_y_u_n_l_o_c_k operation.

       For further rationale on the extended mutex types, see the Rationale
       (Informative) volume of POSIX.1-2008, _T_h_r_e_a_d_s _E_x_t_e_n_s_i_o_n_s.

       If an implementation detects that the value specified by the _m_u_t_e_x
       argument does not refer to an initialized mutex object, it is
       recommended that the function should fail and report an [[EEIINNVVAALL]] error.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _p_t_h_r_e_a_d___m_u_t_e_x___c_o_n_s_i_s_t_e_n_t(), _p_t_h_r_e_a_d___m_u_t_e_x___d_e_s_t_r_o_y(),
       _p_t_h_r_e_a_d___m_u_t_e_x___t_i_m_e_d_l_o_c_k(), _p_t_h_r_e_a_d___m_u_t_e_x_a_t_t_r___g_e_t_r_o_b_u_s_t()

       The Base Definitions volume of POSIX.1-2008, _S_e_c_t_i_o_n _4_._1_1, _M_e_m_o_r_y
       _S_y_n_c_h_r_o_n_i_z_a_t_i_o_n, <<pptthhrreeaadd..hh>>

CCOOPPYYRRIIGGHHTT
       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2013                PTHREAD_MUTEX_LOCK(3)
