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NAME

       pthread_mutexattr_getrobust,  pthread_mutexattr_setrobust  -  get  and  set the robustness
       attribute of a mutex attributes object

LIBRARY

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS

       #include <pthread.h>

       int pthread_mutexattr_getrobust(const pthread_mutexattr_t *attr,
                                       int *robustness);
       int pthread_mutexattr_setrobust(pthread_mutexattr_t *attr,
                                       int robustness);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       pthread_mutexattr_getrobust(), pthread_mutexattr_setrobust():
           _POSIX_C_SOURCE >= 200809L

DESCRIPTION

       The pthread_mutexattr_getrobust() function places the value of the robustness attribute of
       the    mutex    attributes   object   referred   to   by   attr   in   *robustness.    The
       pthread_mutexattr_setrobust() function sets the value of the robustness attribute  of  the
       mutex attributes object referred to by attr to the value specified in *robustness.

       The  robustness  attribute specifies the behavior of the mutex when the owning thread dies
       without unlocking the mutex.  The following values are valid for robustness:

       PTHREAD_MUTEX_STALLED
              This is the default value for a mutex attributes object.  If a mutex is initialized
              with  the  PTHREAD_MUTEX_STALLED attribute and its owner dies without unlocking it,
              the  mutex  remains  locked  afterwards   and   any   future   attempts   to   call
              pthread_mutex_lock(3) on the mutex will block indefinitely.

       PTHREAD_MUTEX_ROBUST
              If  a  mutex  is  initialized with the PTHREAD_MUTEX_ROBUST attribute and its owner
              dies without unlocking it, any future attempts  to  call  pthread_mutex_lock(3)  on
              this  mutex  will succeed and return EOWNERDEAD to indicate that the original owner
              no longer exists and  the  mutex  is  in  an  inconsistent  state.   Usually  after
              EOWNERDEAD  is  returned, the next owner should call pthread_mutex_consistent(3) on
              the acquired mutex to make it consistent again before using it any further.

              If the next owner unlocks the mutex using pthread_mutex_unlock(3) before making  it
              consistent,  the  mutex will be permanently unusable and any subsequent attempts to
              lock it using pthread_mutex_lock(3) will fail with the error ENOTRECOVERABLE.   The
              only permitted operation on such a mutex is pthread_mutex_destroy(3).

              If  the  next  owner terminates before calling pthread_mutex_consistent(3), further
              pthread_mutex_lock(3) operations on this mutex will still return EOWNERDEAD.

       Note    that    the     attr     argument     of     pthread_mutexattr_getrobust()     and
       pthread_mutexattr_setrobust()   should  refer  to  a  mutex  attributes  object  that  was
       initialized by pthread_mutexattr_init(3), otherwise the behavior is undefined.

RETURN VALUE

       On success, these functions return 0.  On error, they return a positive error number.

       In the glibc implementation, pthread_mutexattr_getrobust() always return zero.

ERRORS

       EINVAL A value other than PTHREAD_MUTEX_STALLED  or  PTHREAD_MUTEX_ROBUST  was  passed  to
              pthread_mutexattr_setrobust().

VERSIONS

       pthread_mutexattr_getrobust() and pthread_mutexattr_setrobust() were added in glibc 2.12.

STANDARDS

       POSIX.1-2008.

NOTES

       In  the  Linux  implementation, when using process-shared robust mutexes, a waiting thread
       also receives the EOWNERDEAD notification if the owner  of  a  robust  mutex  performs  an
       execve(2)  without  first  unlocking the mutex.  POSIX.1 does not specify this detail, but
       the same behavior also occurs in at least some other implementations.

       Before the addition of pthread_mutexattr_getrobust() and pthread_mutexattr_setrobust()  to
       POSIX,  glibc  defined  the  following equivalent nonstandard functions if _GNU_SOURCE was
       defined:

       [[deprecated]]
       int pthread_mutexattr_getrobust_np(const pthread_mutexattr_t *attr,
                                          int *robustness);
       [[deprecated]]
       int pthread_mutexattr_setrobust_np(const pthread_mutexattr_t *attr,
                                          int robustness);

       Correspondingly, the constants PTHREAD_MUTEX_STALLED_NP and  PTHREAD_MUTEX_ROBUST_NP  were
       also defined.

       These  GNU-specific  APIs,  which  first  appeared in glibc 2.4, are nowadays obsolete and
       should not be used in new programs; since glibc 2.34 these APIs are marked as deprecated.

EXAMPLES

       The program below demonstrates the use of the robustness attribute of a  mutex  attributes
       object.   In  this  program, a thread holding the mutex dies prematurely without unlocking
       the mutex.  The main thread subsequently acquires the  mutex  successfully  and  gets  the
       error EOWNERDEAD, after which it makes the mutex consistent.

       The following shell session shows what we see when running this program:

           $ ./a.out
           [original owner] Setting lock...
           [original owner] Locked. Now exiting without unlocking.
           [main] Attempting to lock the robust mutex.
           [main] pthread_mutex_lock() returned EOWNERDEAD
           [main] Now make the mutex consistent
           [main] Mutex is now consistent; unlocking

   Program source
       #include <errno.h>
       #include <pthread.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define handle_error_en(en, msg) \
               do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

       static pthread_mutex_t mtx;

       static void *
       original_owner_thread(void *ptr)
       {
           printf("[original owner] Setting lock...\n");
           pthread_mutex_lock(&mtx);
           printf("[original owner] Locked. Now exiting without unlocking.\n");
           pthread_exit(NULL);
       }

       int
       main(void)
       {
           pthread_t thr;
           pthread_mutexattr_t attr;
           int s;

           pthread_mutexattr_init(&attr);

           pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST);

           pthread_mutex_init(&mtx, &attr);

           pthread_create(&thr, NULL, original_owner_thread, NULL);

           sleep(2);

           /* "original_owner_thread" should have exited by now. */

           printf("[main] Attempting to lock the robust mutex.\n");
           s = pthread_mutex_lock(&mtx);
           if (s == EOWNERDEAD) {
               printf("[main] pthread_mutex_lock() returned EOWNERDEAD\n");
               printf("[main] Now make the mutex consistent\n");
               s = pthread_mutex_consistent(&mtx);
               if (s != 0)
                   handle_error_en(s, "pthread_mutex_consistent");
               printf("[main] Mutex is now consistent; unlocking\n");
               s = pthread_mutex_unlock(&mtx);
               if (s != 0)
                   handle_error_en(s, "pthread_mutex_unlock");

               exit(EXIT_SUCCESS);
           } else if (s == 0) {
               printf("[main] pthread_mutex_lock() unexpectedly succeeded\n");
               exit(EXIT_FAILURE);
           } else {
               printf("[main] pthread_mutex_lock() unexpectedly failed\n");
               handle_error_en(s, "pthread_mutex_lock");
           }
       }

SEE ALSO

       get_robust_list(2), set_robust_list(2), pthread_mutex_consistent(3),
       pthread_mutex_init(3), pthread_mutex_lock(3), pthreads(7)