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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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