12.7. Cancel Options

Two thread attributes that are not included in the pthread_attr_t structure are the cancelability state and the cancelability type. These attributes affect the behavior of a thread in response to a call to pthread_cancel (Section 11.5).

The cancelability state attribute can be either PTHREAD_CANCEL_ENABLE or PTHREAD_CANCEL_DISABLE. A thread can change its cancelability state by calling pthread_setcancelstate.

#include <pthread.h> int pthread_setcancelstate(int state, int *oldstate);
Returns: 0 if OK, error number on failure

In one atomic operation, pthread_setcancelstate sets the current cancelability state to state and stores the previous cancelability state in the memory location pointed to by oldstate.

Recall from Section 11.5 that a call to pthread_cancel doesn't wait for a thread to terminate. In the default case, a thread will continue to execute after a cancellation request is made, until the thread reaches a cancellation point. A cancellation point is a place where the thread checks to see whether it has been canceled, and then acts on the request. POSIX.1 guarantees that cancellation points will occur when a thread calls any of the functions listed in Figure 12.14.

Figure 12.14. Cancellation points defined by POSIX.1

accept	mq_timedsend	putpmsg	sigsuspend
aio_suspend	msgrcv	pwrite	sigtimedwait
clock_nanosleep	msgsnd	read	sigwait
close	msync	readv	sigwaitinfo
connect	nanosleep	recv	sleep
creat	open	recvfrom	system
fcntl2	pause	recvmsg	tcdrain
fsync	poll	select	usleep
getmsg	pread	sem_timedwait	wait
getpmsg	pthread_cond_timedwait	sem_wait	waitid
lockf	pthread_cond_wait	send	waitpid
mq_receive	pthread_join	sendmsg	write
mq_send	pthread_testcancel	sendto	writev
mq_timedreceive	putmsg	sigpause

A thread starts with a default cancelability state of PTHREAD_CANCEL_ENABLE. When the state is set to PTHREAD_CANCEL_DISABLE, a call to pthread_cancel will not kill the thread. Instead, the cancellation request remains pending for the thread. When the state is enabled again, the thread will act on any pending cancellation requests at the next cancellation point.

In addition to the functions listed in Figure 12.14, POSIX.1 specifies the functions listed in Figure 12.15 as optional cancellation points.

Figure 12.15. Optional cancellation points defined by POSIX.1

catclose	ftell	getwc	printf
catgets	ftello	getwchar	putc
catopen	ftw	getwd	putc_unlocked
closedir	fwprintf	glob	putchar
closelog	fwrite	iconv_close	putchar_unlocked
ctermid	fwscanf	iconv_open	puts
dbm_close	getc	ioctl	pututxline
dbm_delete	getc_unlocked	lseek	putwc
dbm_fetch	getchar	mkstemp	putwchar
dbm_nextkey	getchar_unlocked	nftw	readdir
dbm_open	getcwd	opendir	readdir_r
dbm_store	getdate	openlog	remove
dlclose	getgrent	pclose	rename
dlopen	getgrgid	perror	rewind
endgrent	getgrgid_r	popen	rewinddir
endhostent	getgrnam	posix_fadvise	scanf
endnetent	getgrnam_r	posix_fallocate	seekdir
endprotoent	gethostbyaddr	posix_madvise	semop
endpwent	gethostbyname	posix_spawn	setgrent
endservent	gethostent	posix_spawnp	sethostent
endutxent	gethostname	posix_trace_clear	setnetent
fclose	getlogin	posix_trace_close	setprotoent
fcntl	getlogin_r	posix_trace_create	setpwent
fflush	getnetbyaddr	posix_trace_create_withlog	setservent
fgetc	getnetbyname	posix_trace_eventtypelist_getnext_id	setutxent
fgetpos	getnetent	posix_trace_eventtypelist_rewind	strerror
fgets	getprotobyname	posix_trace_flush	syslog
fgetwc	getprotobynumber	posix_trace_get_attr	tmpfile
fgetws	getprotoent	posix_trace_get_filter	tmpnam
fopen	getpwent	posix_trace_get_status	ttyname
fprintf	getpwnam	posix_trace_getnext_event	ttyname_r
fputc	getpwnam_r	posix_trace_open	ungetc
fputs	getpwuid	posix_trace_rewind	ungetwc
fputwc	getpwuid_r	posix_trace_set_filter	unlink
fputws	gets	posix_trace_shutdown	vfprintf
fread	getservbyname	posix_trace_timedgetnext_event	vfwprintf
freopen	getservbyport	posix_typed_mem_open	vprintf
fscanf	getservent	pthread_rwlock_rdlock	vwprintf
fseek	getutxent	pthread_rwlock_timedrdlock	wprintf
fseeko	getutxid	pthread_rwlock_timedwrlock	wscanf
fsetpos	getutxline	pthread_rwlock_wrlock

Note that several of the functions listed in Figure 12.15 are not discussed further in this text. Many are optional in the Single UNIX Specification.

If your application doesn't call one of the functions in Figure 12.14 or Figure 12.15 for a long period of time (if it is compute-bound, for example), then you can call pthread_testcancel to add your own cancellation points to the program.

#include <pthread.h> void pthread_testcancel(void);

When you call pthread_testcancel, if a cancellation request is pending and if cancellation has not been disabled, the thread will be canceled. When cancellation is disabled, however, calls to pthread_testcancel have no effect.

The default cancellation type we have been describing is known as deferred cancellation. After a call to pthread_cancel, the actual cancellation doesn't occur until the thread hits a cancellation point. We can change the cancellation type by calling pthread_setcanceltype.

#include <pthread.h> int pthread_setcanceltype(int type, int *oldtype);
Returns: 0 if OK, error number on failure

The type parameter can be either PTHREAD_CANCEL_DEFERRED or PTHREAD_CANCEL_ASYNCHRONOUS. The pthread_setcanceltype function sets the cancellation type to type and returns the previous type in the integer pointed to by oldtype.

Asynchronous cancellation differs from deferred cancellation in that the thread can be canceled at any time. The thread doesn't necessarily need to hit a cancellation point for it to be canceled.