.TH PTHREAD_CANCEL 3 LinuxThreads

.XREF pthread_setcancelstate
.XREF pthread_setcanceltype
.XREF pthread_testcancel

.SH NAME
pthread_cancel, pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel \- thread cancellation

.SH SYNOPSIS
#include <pthread.h>

int pthread_cancel(pthread_t thread);

int pthread_setcancelstate(int state, int *oldstate);

int pthread_setcanceltype(int type, int *oldtype);

void pthread_testcancel(void);

.SH DESCRIPTION

Cancellation is the mechanism by which a thread can terminate the
execution of another thread. More precisely, a thread can send a
cancellation request to another thread. Depending on its settings, the
target thread can then either ignore the request, honor it
immediately, or defer it till it reaches a cancellation point.

When a thread eventually honors a cancellation request, it performs as
if !pthread_exit(PTHREAD_CANCELED)! has been called at that point:
all cleanup handlers are executed in reverse order, finalization
functions for thread-specific data are called, and finally the thread
stops executing with the return value !PTHREAD_CANCELED!. See
!pthread_exit!(3) for more information.

!pthread_cancel! sends a cancellation request to the thread denoted
by the |thread| argument.

!pthread_setcancelstate! changes the cancellation state for the
calling thread -- that is, whether cancellation requests are ignored
or not. The |state| argument is the new cancellation state: either
!PTHREAD_CANCEL_ENABLE! to enable cancellation, or
!PTHREAD_CANCEL_DISABLE! to disable cancellation (cancellation
requests are ignored). If |oldstate| is not !NULL!, the previous
cancellation state is stored in the location pointed to by |oldstate|,
and can thus be restored later by another call to
!pthread_setcancelstate!.

!pthread_setcanceltype! changes the type of responses to cancellation
requests for the calling thread: asynchronous (immediate) or deferred.
The |type| argument is the new cancellation type: either
!PTHREAD_CANCEL_ASYNCHRONOUS! to cancel the calling thread as soon as
the cancellation request is received, or !PTHREAD_CANCEL_DEFERRED! to
keep the cancellation request pending until the next cancellation
point. If |oldtype| is not !NULL!, the previous
cancellation state is stored in the location pointed to by |oldtype|,
and can thus be restored later by another call to
!pthread_setcanceltype!.

Threads are always created by !pthread_create!(3) with cancellation
enabled and deferred. That is, the initial cancellation state is
!PTHREAD_CANCEL_ENABLE! and the initial type is
!PTHREAD_CANCEL_DEFERRED!.

Cancellation points are those points in the program execution where a
test for pending cancellation requests is performed and cancellation
is executed if positive. The following POSIX threads functions
are cancellation points:

!pthread_join!(3)
.br
!pthread_cond_wait!(3)
.br
!pthread_cond_timedwait!(3)
.br
!pthread_testcancel!(3)
.br
!sem_wait!(3)
.br
!sigwait!(3)

All other POSIX threads functions are guaranteed not to be
cancellation points. That is, they never perform cancellation in
deferred cancellation mode.

!pthread_testcancel! does nothing except testing for pending
cancellation and executing it. Its purpose is to introduce explicit
checks for cancellation in long sequences of code that do not call
cancellation point functions otherwise.

.SH "RETURN VALUE"

!pthread_cancel!, !pthread_setcancelstate! and
!pthread_setcanceltype! return 0 on success and a non-zero error code
on error.

.SH ERRORS
!pthread_cancel! returns the following error code on error:
.RS
.TP
!ESRCH!
no thread could be found corresponding to that specified by the |thread| ID.
.RE

!pthread_setcancelstate! returns the following error code on error:
.RS
.TP
!EINVAL!
the |state| argument is not !PTHREAD_CANCEL_ENABLE! nor
!PTHREAD_CANCEL_DISABLE!
.RE

!pthread_setcanceltype! returns the following error code on error:
.RS
.TP
!EINVAL!
the |type| argument is not !PTHREAD_CANCEL_DEFERRED! nor
!PTHREAD_CANCEL_ASYNCHRONOUS!
.RE

.SH AUTHOR
Xavier Leroy <Xavier.Leroy@inria.fr>

.SH "SEE ALSO"
!pthread_exit!(3),
!pthread_cleanup_push!(3),
!pthread_cleanup_pop!(3).

.SH BUGS

POSIX specifies that a number of system calls (basically, all
system calls that may block, such as !read!(2), !write!(2), !wait!(2),
etc.) and library functions that may call these system calls (e.g.
!fprintf!(3)) are cancellation points.  LinuxThreads is not yet
integrated enough with the C library to implement this, and thus none
of the C library functions is a cancellation point.

For system calls at least, there is a workaround. Cancellation
requests are transmitted to the target thread by sending it a
signal. That signal will interrupt all blocking system calls, causing
them to return immediately with the !EINTR! error. So, checking for
cancellation during a !read! system call, for instance, can be
achieved as follows:

.RS
.ft 3
.nf
.sp
pthread_testcancel();
retcode = read(fd, buffer, length);
pthread_testcancel();
.ft
.LP
.RE
.fi