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NAME

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

SYNOPSIS

       #include <pthread.h>

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

       Compile and link with -pthread.

   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 to glibc in
       version 2.12.

CONFORMING TO

       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:

       int pthread_mutexattr_getrobust_np(const pthread_mutexattr_t *attr,
                                          int *robustness);
       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.

EXAMPLE

       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 thread] Attempting to lock the robust mutex.
           [main thread] pthread_mutex_lock() returned EOWNERDEAD
           [main thread] Now make the mutex consistent
           [main thread] Mutex is now consistent; unlocking

   Program source
       #include <stdlib.h>
       #include <stdio.h>
       #include <unistd.h>
       #include <pthread.h>
       #include <errno.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(int argc, char *argv[])
       {
           pthread_t thr;
           pthread_mutexattr_t attr;
           int s;

           pthread_mutexattr_init(&attr);
                                       /* initialize the attributes object */
           pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST);
                                      /* set robustness */

           pthread_mutex_init(&mtx, &attr);   /* initialize the mutex */

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

           sleep(2);

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

           printf("[main thread] Attempting to lock the robust mutex.\n");
           s = pthread_mutex_lock(&mtx);
           if (s == EOWNERDEAD) {
               printf("[main thread] pthread_mutex_lock() returned EOWNERDEAD\n");
               printf("[main thread] Now make the mutex consistent\n");
               s = pthread_mutex_consistent(&mtx);
               if (s != 0)
                   handle_error_en(s, "pthread_mutex_consistent");
               printf("[main thread] 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 thread] pthread_mutex_lock() unexpectedly succeeded\n");
               exit(EXIT_FAILURE);
           } else {
               printf("[main thread] 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_init(3),
       pthread_mutex_consistent(3), pthread_mutex_lock(3), pthreads(7)

COLOPHON

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       project, information about reporting bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.