Hi Loïc,
Loic Domaigne wrote:
Gidday Michael,
one more review. The page rocks!
Thanks! Aside from reading the source code, writing some test
programs (4 pthreads bugs reported to date), and reading POSIX.1,
I've also been checking man pages on some other systems to see if
there's any points that I've missed. At the end of that, and
with some review help, I hope the Linux pages will be at
least as good as those on other systems (and of course,
hopefully better).
.BI "int pthread_(pthread_t " thread );
pthread_cancel ;-)
Ah, yeah, like I said: "with some review help" ;-).
Fixed.
[...]
Asynchronous cancelability
means that the thread can be canceled at any time
(usually immediately, but the system does not guarantee this).
Deferred cancelability means that cancellation will be delayed until
the thread next calls a function that is a
.IR "cancellation point" .
A list of functions that are or may be cancellation points is provided in
.IR pthreads (7).
It is very important to document the list of functions that are/are not
CP in the "may be a CP" list: this is system specific and belongs to the
system documentation.
For man-pages-3.14, I have added POSIX.1's lists of "are" and "may be"
cancellation points to pthreads.7.
However, it unclear to me how one determines the list of functions
that are cancellation points under glibc. Do you have some ideas
about this?
Furthermore we should perhaps add a sentence about the fact that when a
trace is created, cancellation is enabled and deferred by default ? (I
have noticed that you mention this fact in pthread_setcancelstate.3 )
I added some words to note the defaults.
[...]
.PP
After a canceled thread has terminated,
a join with that thread using
.BR pthread_join (3)
obtains
.B PTHREAD_CANCELED
as the thread's exit status.
Joining the canceled thread is the only way to know that cancellation
has completed.
Good idea -- that should be mentioned. I added your words more
or less as you gave them above.
[...]
.SH NOTES
On Linux, cancellation is implemented using signals.
Under the NPTL threading implementation,
the first real-time signal (i.e., signal 32) is used for this purpose.
Hmmm... You are right: NPTL uses the first real-time signal (32)
provided by the *kernel*. As a matter of fact, Glibc reserves kernel
real-time signals 32 and 33 for NPTL; real-time queued signals available
to the application ranges from SIGRTMIN (34) to SIGRTMAX(64).
Yes... exactly.
On LinuxThreads, the second real-time signal is used,
if real-time signals are available, otherwise
.B SIGUSR2
is used.
IIRC, this was true on 'older LinuxThreads'. Never used real-time queued
signals as well ? (To verify...)
I'm not quite sure what you want to say there. Can you say some
more please.
.SH EXAMPLE
[...]
sleep(3) is a CP on Linux (AFAIR, sleep is a "may be CP").
Not sure what you are wanting to saying here; should something
be changed in the page? (BTW: sleep() *is* a CP in POSIX.1.)
[...]
Thanks for these comments Loic. New version of the page is below.
Cheers,
Michael
.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
.\" <mtk.manpages@xxxxxxxxx>
.\"
.\" 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.
.\"
.TH PTHREAD_CANCEL 3 2008-11-17 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_cancel \- send a cancellation request to a thread
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.BI "int pthread_cancel(pthread_t " thread );
.sp
Compile and link with \fI\-pthread\fP.
.SH DESCRIPTION
The
.BR pthread_cancel ()
function sends a cancellation request to the thread
.IR thread .
Whether and when the target thread
reacts to the cancellation request depends on
two attributes that are under the control of that thread:
its cancelability \fIstate\fP and \fItype\fP.
A thread's cancelability state, determined by
.BR pthread_setcancelstate (3),
can be
.I enabled
(the default for new threads) or
.IR disabled .
If a thread has disabled cancellation,
then a cancellation request remains queued until the thread
enables cancellation.
If a thread has enabled cancellation,
then its cancelability type determines when cancellation occurs.
A thread's cancellation type, determined by
.BR pthread_setcanceltype (3),
may be either
.IR asynchronous
or
.IR deferred
(the default for new threads).
Asynchronous cancelability
means that the thread can be canceled at any time
(usually immediately, but the system does not guarantee this).
Deferred cancelability means that cancellation will be delayed until
the thread next calls a function that is a
.IR "cancellation point" .
A list of functions that are or may be cancellation points is provided in
.IR pthreads (7).
When a cancellation requested is acted on, the following steps occur for
.IR thread
(in this order):
.IP 1. 3
Cancellation clean-up handlers are popped
(in the reverse of the order in which they were pushed) and called.
(See
.BR pthread_cleanup_push (3).)
.IP 2.
Thread-specific data destructors are called,
in an unspecified order.
(See
.BR pthread_key_create (3).)
.IP 3.
The thread is terminated.
(See
.BR pthread_exit (3).)
.PP
The above steps happen asynchronously with respect to the
.BR pthread_cancel ()
call;
the return status of
.BR pthread_cancel ()
merely informs the caller whether the cancellation request
was successfully queued.
.PP
After a canceled thread has terminated,
a join with that thread using
.BR pthread_join (3)
obtains
.B PTHREAD_CANCELED
as the thread's exit status.
(Joining with a thread is the only way to know that cancellation
has completed.)
.SH RETURN VALUE
On success,
.BR pthread_cancel ()
returns 0;
on error, it returns a non-zero error number.
.SH ERRORS
.TP
.B ESRCH
No thread with the ID
.I thread
could be found.
.\" .SH VERSIONS
.\" Available since glibc 2.0
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
On Linux, cancellation is implemented using signals.
Under the NPTL threading implementation,
the first real-time signal (i.e., signal 32) is used for this purpose.
On LinuxThreads, the second real-time signal is used,
if real-time signals are available, otherwise
.B SIGUSR2
is used.
.SH EXAMPLE
The program below creates a thread and then cancels it.
The main thread joins with the canceled thread to check
that its exit status was
.BR PTHREAD_CANCELED .
The following shell session shows what happens when we run the program:
.in +4n
.nf
$ ./a.out
thread_func(): started; cancellation disabled
main(): sending cancellation request
thread_func(): about to enable cancellation
main(): thread was canceled
.fi
.in
.SS Program source
\&
.nf
#include <pthread.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#define handle_error_en(en, msg) \\
do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
static void *
thread_func(void *ignored_argument)
{
int s;
/* Disable cancellation for a while, so that we don\(aqt
immediately react to a cancellation request */
s = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
if (s != 0)
handle_error_en(s, "pthread_setcancelstate");
printf("thread_func(): started; cancellation disabled\\n");
sleep(5);
printf("thread_func(): about to enable cancellation\\n");
s = pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
if (s != 0)
handle_error_en(s, "pthread_setcancelstate");
/* sleep() is a cancellation point */
sleep(1000); /* Should get canceled while we sleep */
/* Should never get here */
printf("thread_func(): not canceled!\\n");
return NULL;
}
int
main(void)
{
pthread_t thr;
void *res;
int s;
/* Start a thread and then send it a cancellation request */
s = pthread_create(&thr, NULL, &thread_func, NULL);
if (s != 0)
handle_error_en(s, "pthread_create");
sleep(2); /* Give thread a chance to get started */
printf("main(): sending cancellation request\\n");
s = pthread_cancel(thr);
if (s != 0)
handle_error_en(s, "pthread_cancel");
/* Join with thread to see what its exit status was */
s = pthread_join(thr, &res);
if (s != 0)
handle_error_en(s, "pthread_join");
if (res == PTHREAD_CANCELED)
printf("main(): thread was canceled\\n");
else
printf("main(): thread wasn\(aqt canceled (shouldn\(aqt happen!)\\n");
exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR pthread_cleanup_push (3),
.BR pthread_create (3),
.BR pthread_exit (3),
.BR pthread_join (3),
.BR pthread_key_create (3),
.BR pthread_setcancelstate (3),
.BR pthread_setcanceltype (3),
.BR pthread_testcancel (3),
.BR pthreads (7)
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