man3/pthread_setcancelstate.3

   1 .\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
   2 .\"     <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH PTHREAD_SETCANCELSTATE 3 2021-03-22 "Linux man-pages (unreleased)"
   7 .SH NAME
   8 pthread_setcancelstate, pthread_setcanceltype \-
   9 set cancelability state and type
  10 .SH LIBRARY
  11 POSIX threads library
  12 .RI ( libpthread ", " \-lpthread )
  13 .SH SYNOPSIS
  14 .nf
  15 .B #include <pthread.h>
  16 .PP
  17 .BI "int pthread_setcancelstate(int " state ", int *" oldstate );
  18 .BI "int pthread_setcanceltype(int " type ", int *" oldtype );
  19 .fi
  20 .SH DESCRIPTION
  21 The
  22 .BR pthread_setcancelstate ()
  23 sets the cancelability state of the calling thread to the value
  24 given in
  25 .IR state .
  26 The previous cancelability state of the thread is returned
  27 in the buffer pointed to by
  28 .IR oldstate .
  29 The
  30 .I state
  31 argument must have one of the following values:
  32 .TP
  33 .B PTHREAD_CANCEL_ENABLE
  34 The thread is cancelable.
  35 This is the default cancelability state in all new threads,
  36 including the initial thread.
  37 The thread's cancelability type determines when a cancelable thread
  38 will respond to a cancelation request.
  39 .TP
  40 .B PTHREAD_CANCEL_DISABLE
  41 The thread is not cancelable.
  42 If a cancelation request is received,
  43 it is blocked until cancelability is enabled.
  44 .PP
  45 The
  46 .BR pthread_setcanceltype ()
  47 sets the cancelability type of the calling thread to the value
  48 given in
  49 .IR type .
  50 The previous cancelability type of the thread is returned
  51 in the buffer pointed to by
  52 .IR oldtype .
  53 The
  54 .I type
  55 argument must have one of the following values:
  56 .TP
  57 .B PTHREAD_CANCEL_DEFERRED
  58 A cancelation request is deferred until the thread next calls
  59 a function that is a cancelation point (see
  60 .BR pthreads (7)).
  61 This is the default cancelability type in all new threads,
  62 including the initial thread.
  63 .IP
  64 Even with deferred cancelation, a
  65 cancelation point in an asynchronous signal handler may still
  66 be acted upon and the effect is as if it was an asynchronous
  67 cancelation.
  68 .TP
  69 .B PTHREAD_CANCEL_ASYNCHRONOUS
  70 The thread can be canceled at any time.
  71 (Typically,
  72 it will be canceled immediately upon receiving a cancelation request,
  73 but the system doesn't guarantee this.)
  74 .PP
  75 The set-and-get operation performed by each of these functions
  76 is atomic with respect to other threads in the process
  77 calling the same function.
  78 .SH RETURN VALUE
  79 On success, these functions return 0;
  80 on error, they return a nonzero error number.
  81 .SH ERRORS
  82 The
  83 .BR pthread_setcancelstate ()
  84 can fail with the following error:
  85 .TP
  86 .B EINVAL
  87 Invalid value for
  88 .IR state .
  89 .PP
  90 The
  91 .BR pthread_setcanceltype ()
  92 can fail with the following error:
  93 .TP
  94 .B EINVAL
  95 Invalid value for
  96 .IR type .
  97 .\" .SH VERSIONS
  98 .\" Available since glibc 2.0
  99 .SH ATTRIBUTES
 100 For an explanation of the terms used in this section, see
 101 .BR attributes (7).
 102 .ad l
 103 .nh
 104 .TS
 105 allbox;
 106 lbx lb lb
 107 l l l.
 108 Interface       Attribute       Value
 109 T{
 110 .BR pthread_setcancelstate (),
 111 .BR pthread_setcanceltype ()
 112 T}      Thread safety   T{
 113 MT-Safe
 114 T}
 115 T{
 116 .BR pthread_setcancelstate (),
 117 .BR pthread_setcanceltype ()
 118 T}      Async-cancel safety     T{
 119 AC-Safe
 120 T}
 121 .TE
 122 .hy
 123 .ad
 124 .sp 1
 125 .hy
 126 .SH STANDARDS
 127 POSIX.1-2001, POSIX.1-2008.
 128 .SH NOTES
 129 For details of what happens when a thread is canceled, see
 130 .BR pthread_cancel (3).
 131 .PP
 132 Briefly disabling cancelability is useful
 133 if a thread performs some critical action
 134 that must not be interrupted by a cancelation request.
 135 Beware of disabling cancelability for long periods,
 136 or around operations that may block for long periods,
 137 since that will render the thread unresponsive to cancelation requests.
 138 .SS Asynchronous cancelability
 139 Setting the cancelability type to
 140 .B PTHREAD_CANCEL_ASYNCHRONOUS
 141 is rarely useful.
 142 Since the thread could be canceled at
 143 .I any
 144 time, it cannot safely reserve resources (e.g., allocating memory with
 145 .BR malloc (3)),
 146 acquire mutexes, semaphores, or locks, and so on.
 147 Reserving resources is unsafe because the application has no way of
 148 knowing what the state of these resources is when the thread is canceled;
 149 that is, did cancelation occur before the resources were reserved,
 150 while they were reserved, or after they were released?
 151 Furthermore, some internal data structures
 152 (e.g., the linked list of free blocks managed by the
 153 .BR malloc (3)
 154 family of functions) may be left in an inconsistent state
 155 if cancelation occurs in the middle of the function call.
 156 Consequently, clean-up handlers cease to be useful.
 157 .PP
 158 Functions that can be safely asynchronously canceled are called
 159 .IR "async-cancel-safe functions" .
 160 POSIX.1-2001 and POSIX.1-2008 require only that
 161 .BR pthread_cancel (3),
 162 .BR pthread_setcancelstate (),
 163 and
 164 .BR pthread_setcanceltype ()
 165 be async-cancel-safe.
 166 In general, other library functions
 167 can't be safely called from an asynchronously cancelable thread.
 168 .PP
 169 One of the few circumstances in which asynchronous cancelability is useful
 170 is for cancelation of a thread that is in a pure compute-bound loop.
 171 .SS Portability notes
 172 The Linux threading implementations permit the
 173 .I oldstate
 174 argument of
 175 .BR pthread_setcancelstate ()
 176 to be NULL, in which case the information about the previous
 177 cancelability state is not returned to the caller.
 178 Many other implementations also permit a NULL
 179 .I oldstat
 180 argument,
 181 .\" It looks like at least Solaris, FreeBSD and Tru64 support this.
 182 but POSIX.1 does not specify this point,
 183 so portable applications should always specify a non-NULL value in
 184 .IR oldstate .
 185 A precisely analogous set of statements applies for the
 186 .I oldtype
 187 argument of
 188 .BR pthread_setcanceltype ().
 189 .SH EXAMPLES
 190 See
 191 .BR pthread_cancel (3).
 192 .SH SEE ALSO
 193 .BR pthread_cancel (3),
 194 .BR pthread_cleanup_push (3),
 195 .BR pthread_testcancel (3),
 196 .BR pthreads (7)