Section (3) pthread_mutexattr_setrobust
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) ; |
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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
orPTHREAD_MUTEX_ROBUST
was passed topthread_mutexattr_setrobust
().
VERSIONS
pthread_mutexattr_getrobust
() and
pthread_mutexattr_setrobust
()
were added to glibc in version 2.12.
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... ); pthread_mutex_lock(&mtx); printf([original owner] Locked. Now exiting without unlocking. ); 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. ); s = pthread_mutex_lock(&mtx); if (s == EOWNERDEAD) { printf([main thread] pthread_mutex_lock() returned EOWNERDEAD ); printf([main thread] Now make the mutex consistent ); s = pthread_mutex_consistent(&mtx); if (s != 0) handle_error_en(s, pthread_mutex_consistent); printf([main thread] Mutex is now consistent; unlocking ); 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 ); exit(EXIT_FAILURE); } else { printf([main thread] pthread_mutex_lock() unexpectedly failed ); 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)
COLOPHON
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description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright (c) 2017, Yubin Ruan <ablacktshirtgmail.com> and Copyright (c) 2017, Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |