Section 12.7. Cancel Options

team bbl


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.

    team bbl



    Advanced Programming in the UNIX Environment
    Advanced Programming in the UNIX Environment, Second Edition (Addison-Wesley Professional Computing Series)
    ISBN: 0321525949
    EAN: 2147483647
    Year: 2005
    Pages: 370

    Similar book on Amazon

    flylib.com © 2008-2017.
    If you may any questions please contact us: flylib@qtcs.net