man-pages/man3/pthread_mutexattr_setrobust.3

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.TH PTHREAD_MUTEXATTR_SETROBUST 3 2017-08-20 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_mutexattr_getrobust, pthread_mutexattr_setrobust
\- get and set the robustness attribute of a mutex attribute object
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.PP
.BI "int pthread_mutexattr_getrobust(const pthread_mutexattr_t *" attr ,
.BI "                                int *" robustness ");"
.BI "int pthread_mutexattr_setrobust(const pthread_mutexattr_t *" attr ,
.BI "                                int " robustness ");"
.fi
.PP
Compile and link with \fI\-pthread\fP.
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR pthread_mutexattr_getrobust (),
.BR pthread_mutexattr_setrobust ():
.br
.RS 4
.ad l
_POSIX_C_SOURCE >= 200809L
.\" FIXME .
.\" But see https://sourceware.org/bugzilla/show_bug.cgi?id=22125
.RE
.ad
.SH DESCRIPTION
The
.BR pthread_mutexattr_getrobust ()
and
.BR pthread_mutexattr_setrobust ()
functions get and set the robustness attribute of an
initialized mutex attribute object.
The robustness attribute specifies the behavior of the mutex
when its owner dies without unlocking it.
.PP
Currently there are only two possible values for the
.IR robustness
attribute:
.TP
.BR PTHREAD_MUTEX_STALLED
This is the default value for a mutex attribute object.
If a mutex is initialized with a
.BR PTHREAD_MUTEX_STALLED
attribute object and its owner dies without unlocking it, it is kept locked
afterwards and any future attempts to call
.BR pthread_mutex_lock (3)
on this mutex will block indefinitely.
.TP
.B PTHREAD_MUTEX_ROBUST
can be set on a mutex attribute object so that when the owner of the mutex
dies or when the process containing such a locked mutex performs
.BR execve (2),
any future attempts to call
.BR pthread_mutex_lock (3)
on this mutex will succeed and return
.B EOWNERDEAD
to indicate that the original owner no longer exists and the mutex is left in
an inconsistent state.
Usually after
.B EOWNERDEAD
is returned, the next owner should call
.BR pthread_mutex_consistent (3)
on the acquired mutex to make it consistent again before using it any further.
If the next owner unlocks it using
.BR pthread_mutex_unlock (3)
before making it consistent, the mutex will be unusable permanently and any
subsequent attempts to lock it using
.BR pthread_mutex_lock (3)
will fail with the error
.BR ENOTRECOVERABLE .
If the next owner terminates before calling
.BR pthread_mutex_consistent (3),
further
.BR pthread_mutex_lock (3)
on this mutex will still return
.BR EOWNERDEAD .
.PP
Note that the
.IR attr
argument of
.BR pthread_mutexattr_getrobust ()
and
.BR pthread_mutexattr_setrobust ()
should refer to a mutex attribute object that was initialized by
.BR pthread_mutexattr_init (3),
otherwise the behavior is undefined.
.SH RETURN VALUE
On success, zero is returned by
.BR pthread_mutexattr_getrobust ()
and the value pointed to by the
.IR robustness
parameter is set to the robustness attribute of
.IR attr .
Otherwise, an error number shall be returned.
On success,
.BR pthread_mutexattr_setrobust()
sets the robustness attribute into the mutex attribute object
.IR attr
and returns zero, otherwise an error number is returned to indicate the error.
.SS Glibc\-specified features
In glibc's implementation,
.BR pthread_mutexattr_getrobust ()
always return zero.
.SH ERRORS
.TP
.B EINVAL
A value other than
.B PTHREAD_MUTEX_STALLED
or
.B PTHREAD_MUTEX_ROBUST
was passed to
.BR pthread_mutexattr_setrobust ().
.SH CONFORMING TO
POSIX.1-2008.
.SH NOTES
Before the addition of
.BR pthread_mutexattr_getrobust ()
and
.BR pthread_mutexattr_setrobust ()
to POSIX,
glibc defined the following equivalent nonstandard functions if
.BR _GNU_SOURCE
was defined:
.PP
.nf
.BI "int pthread_mutexattr_getrobust_np(const pthread_mutexattr_t *" attr ,
.BI "                                   int *" robustness ");"
.BI "int pthread_mutexattr_setrobust_np(const pthread_mutexattr_t *" attr ,
.BI "                                   int " robustness ");"
.fi
.PP
Correspondingly, the constants
.B PTHREAD_MUTEX_STALLED_NP
and
.B PTHREAD_MUTEX_ROBUST_NP
were also defined.
.PP
These GNU-specific APIs, which first appeared in glibc 2.4,
are nowadays obsolete and should not be used in new programs.
.SH EXAMPLE
.PP
The program demonstrates a simple use of the robustness attribute of a
pthread mutex attribute object.
In this program, a thread holding the mutex
dies prematurely without unlocking the mutex.
Another thread acquires it
successfully and gets the error
.BR EOWNERDEAD .
.SS Program source
.EX
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <errno.h>

static pthread_mutex_t lock;

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

int
main(int argc, char *argv[])
{
    pthread_t lock_getter;
    pthread_mutexattr_t attr;
    int ret_code;

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

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

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

    sleep(2);

    /* Original_owner_thread should have exited by now */

    printf("Attempting to acquire the unlocked robust mutex.\\n");
    ret_code = pthread_mutex_lock(&lock);
    if (EOWNERDEAD == ret_code) {
        printf("EOWNERDEAD returned. Make the mutex consistent now\\n");
        pthread_mutex_consistent(&lock);
    }

    pthread_mutex_unlock(&lock);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.ad l
.nh
.BR pthread_mutex_init (3),
.BR pthread_mutex_consistent (3),
.BR pthread_mutex_lock (3),
.BR pthreads (7)