man3/setjmp.3

   1 .\" Copyright (C) 2016 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\"
   3 .\" SPDX-License-Identifier: GPL-2.0-or-later
   4 .\"
   5 .TH setjmp 3 (date) "Linux man-pages (unreleased)"
   6 .SH NAME
   7 setjmp, sigsetjmp, longjmp, siglongjmp  \- performing a nonlocal goto
   8 .SH LIBRARY
   9 Standard C library
  10 .RI ( libc ", " \-lc )
  11 .SH SYNOPSIS
  12 .nf
  13 .B #include <setjmp.h>
  14 .PP
  15 .BI "int setjmp(jmp_buf " env );
  16 .BI "int sigsetjmp(sigjmp_buf " env ", int " savesigs );
  17 .PP
  18 .BI "noreturn void longjmp(jmp_buf " env ", int " val );
  19 .BI "noreturn void siglongjmp(sigjmp_buf " env ", int " val );
  20 .fi
  21 .PP
  22 .RS -4
  23 Feature Test Macro Requirements for glibc (see
  24 .BR feature_test_macros (7)):
  25 .RE
  26 .PP
  27 .BR setjmp ():
  28 see NOTES.
  29 .PP
  30 .BR sigsetjmp ():
  31 .nf
  32     _POSIX_C_SOURCE
  33 .fi
  34 .SH DESCRIPTION
  35 The functions described on this page are used for performing "nonlocal gotos":
  36 transferring execution from one function to a predetermined location
  37 in another function.
  38 The
  39 .BR setjmp ()
  40 function dynamically establishes the target to which control
  41 will later be transferred, and
  42 .BR longjmp ()
  43 performs the transfer of execution.
  44 .PP
  45 The
  46 .BR setjmp ()
  47 function saves various information about the calling environment
  48 (typically, the stack pointer, the instruction pointer,
  49 possibly the values of other registers and the signal mask)
  50 in the buffer
  51 .I env
  52 for later use by
  53 .BR longjmp ().
  54 In this case,
  55 .BR setjmp ()
  56 returns 0.
  57 .PP
  58 The
  59 .BR longjmp ()
  60 function uses the information saved in
  61 .I env
  62 to transfer control back to the point where
  63 .BR setjmp ()
  64 was called and to restore ("rewind") the stack to its state at the time of the
  65 .BR setjmp ()
  66 call.
  67 In addition, and depending on the implementation (see NOTES),
  68 the values of some other registers and the process signal mask
  69 may be restored to their state at the time of the
  70 .BR setjmp ()
  71 call.
  72 .PP
  73 Following a successful
  74 .BR longjmp (),
  75 execution continues as if
  76 .BR setjmp ()
  77 had returned for a second time.
  78 This "fake" return can be distinguished from a true
  79 .BR setjmp ()
  80 call because the "fake" return returns the value provided in
  81 .IR val .
  82 If the programmer mistakenly passes the value 0 in
  83 .IR val ,
  84 the "fake" return will instead return 1.
  85 .SS sigsetjmp() and siglongjmp()
  86 .BR sigsetjmp ()
  87 and
  88 .BR siglongjmp ()
  89 also perform nonlocal gotos, but provide predictable handling of
  90 the process signal mask.
  91 .PP
  92 If, and only if, the
  93 .I savesigs
  94 argument provided to
  95 .BR sigsetjmp ()
  96 is nonzero, the process's current signal mask is saved in
  97 .I env
  98 and will be restored if a
  99 .BR siglongjmp ()
 100 is later performed with this
 101 .IR env .
 102 .SH RETURN VALUE
 103 .BR setjmp ()
 104 and
 105 .BR sigsetjmp ()
 106 return 0 when called directly;
 107 on the "fake" return that occurs after
 108 .BR longjmp ()
 109 or
 110 .BR siglongjmp (),
 111 the nonzero value specified in
 112 .I val
 113 is returned.
 114 .PP
 115 The
 116 .BR longjmp ()
 117 or
 118 .BR siglongjmp ()
 119 functions do not return.
 120 .SH ATTRIBUTES
 121 For an explanation of the terms used in this section, see
 122 .BR attributes (7).
 123 .ad l
 124 .nh
 125 .TS
 126 allbox;
 127 lbx lb lb
 128 l l l.
 129 Interface       Attribute       Value
 130 T{
 131 .BR setjmp (),
 132 .BR sigsetjmp ()
 133 T}      Thread safety   MT-Safe
 134 T{
 135 .BR longjmp (),
 136 .BR siglongjmp ()
 137 T}      Thread safety   MT-Safe
 138 .TE
 139 .hy
 140 .ad
 141 .sp 1
 142 .SH STANDARDS
 143 .BR setjmp (),
 144 .BR longjmp ():
 145 POSIX.1-2001, POSIX.1-2008, C89, C99.
 146 .PP
 147 .BR sigsetjmp (),
 148 .BR siglongjmp ():
 149 POSIX.1-2001, POSIX.1-2008.
 150 .SH NOTES
 151 POSIX does not specify whether
 152 .BR setjmp ()
 153 will save the signal mask
 154 (to be later restored during
 155 .BR longjmp ()).
 156 In System V it will not.
 157 In 4.3BSD it will, and there
 158 is a function
 159 .BR _setjmp ()
 160 that will not.
 161 The behavior under Linux depends on the glibc version
 162 and the setting of feature test macros.
 163 On Linux with glibc versions before 2.19,
 164 .BR setjmp ()
 165 follows the System V behavior by default,
 166 but the BSD behavior is provided if the
 167 .B _BSD_SOURCE
 168 feature test macro is explicitly defined
 169 .\" so that _FAVOR_BSD is triggered
 170 and none of
 171 .BR _POSIX_SOURCE ,
 172 .BR _POSIX_C_SOURCE ,
 173 .BR _XOPEN_SOURCE ,
 174 .\" .BR _XOPEN_SOURCE_EXTENDED ,
 175 .BR _GNU_SOURCE ,
 176 or
 177 .B _SVID_SOURCE
 178 is defined.
 179 Since glibc 2.19,
 180 .I <setjmp.h>
 181 exposes only the System V version of
 182 .BR setjmp ().
 183 Programs that need the BSD semantics should replace calls to
 184 .BR setjmp ()
 185 with calls to
 186 .BR sigsetjmp ()
 187 with a nonzero
 188 .I savesigs
 189 argument.
 190 .PP
 191 .BR setjmp ()
 192 and
 193 .BR longjmp ()
 194 can be useful for dealing with errors inside deeply nested function calls
 195 or to allow a signal handler to pass control to
 196 a specific point in the program,
 197 rather than returning to the point where the handler interrupted
 198 the main program.
 199 In the latter case,
 200 if you want to portably save and restore signal masks, use
 201 .BR sigsetjmp ()
 202 and
 203 .BR siglongjmp ().
 204 See also the discussion of program readability below.
 205 .PP
 206 The compiler may optimize variables into registers, and
 207 .BR longjmp ()
 208 may restore the values of other registers in addition to the
 209 stack pointer and program counter.
 210 Consequently, the values of automatic variables are unspecified
 211 after a call to
 212 .BR longjmp ()
 213 if they meet all the following criteria:
 214 .IP \(bu 3
 215 they are local to the function that made the corresponding
 216 .BR setjmp ()
 217 call;
 218 .IP \(bu
 219 their values are changed between the calls to
 220 .BR setjmp ()
 221 and
 222 .BR longjmp ();
 223 and
 224 .IP \(bu
 225 they are not declared as
 226 .IR volatile .
 227 .PP
 228 Analogous remarks apply for
 229 .BR siglongjmp ().
 230 .\"
 231 .SS Nonlocal gotos and program readability
 232 While it can be abused,
 233 the traditional C "goto" statement at least has the benefit that lexical cues
 234 (the goto statement and the target label)
 235 allow the programmer to easily perceive the flow of control.
 236 Nonlocal gotos provide no such cues: multiple
 237 .BR setjmp ()
 238 calls might employ the same
 239 .I jmp_buf
 240 variable so that the content of the variable may change
 241 over the lifetime of the application.
 242 Consequently, the programmer may be forced to perform detailed
 243 reading of the code to determine the dynamic target of a particular
 244 .BR longjmp ()
 245 call.
 246 (To make the programmer's life easier, each
 247 .BR setjmp ()
 248 call should employ a unique
 249 .I jmp_buf
 250 variable.)
 251 .PP
 252 Adding further difficulty, the
 253 .BR setjmp ()
 254 and
 255 .BR longjmp ()
 256 calls may not even be in the same source code module.
 257 .PP
 258 In summary, nonlocal gotos can make programs harder to understand
 259 and maintain, and an alternative should be used if possible.
 260 .\"
 261 .SS Caveats
 262 If the function which called
 263 .BR setjmp ()
 264 returns before
 265 .BR longjmp ()
 266 is called, the behavior is undefined.
 267 Some kind of subtle or unsubtle chaos is sure to result.
 268 .PP
 269 If, in a multithreaded program, a
 270 .BR longjmp ()
 271 call employs an
 272 .I env
 273 buffer that was initialized by a call to
 274 .BR setjmp ()
 275 in a different thread, the behavior is undefined.
 276 .\"
 277 .\" The following statement appeared in versions up to POSIX.1-2008 TC1,
 278 .\" but is set to be removed in POSIX.1-2008 TC2:
 279 .\"
 280 .\"     According to POSIX.1, if a
 281 .\"     .BR longjmp ()
 282 .\"     call is performed from a nested signal handler
 283 .\"     (i.e., from a handler that was invoked in response to a signal that was
 284 .\"     generated while another signal was already in the process of being
 285 .\"     handled), the behavior is undefined.
 286 .PP
 287 POSIX.1-2008 Technical Corrigendum 2 adds
 288 .\" http://austingroupbugs.net/view.php?id=516#c1195
 289 .BR longjmp ()
 290 and
 291 .BR siglongjmp ()
 292 to the list of async-signal-safe functions.
 293 However, the standard recommends avoiding the use of these functions
 294 from signal handlers and goes on to point out that
 295 if these functions are called from a signal handler that interrupted
 296 a call to a non-async-signal-safe function (or some equivalent,
 297 such as the steps equivalent to
 298 .BR exit (3)
 299 that occur upon a return from the initial call to
 300 .IR main ()),
 301 the behavior is undefined if the program subsequently makes a call to
 302 a non-async-signal-safe function.
 303 The only way of avoiding undefined behavior is to ensure one of the following:
 304 .IP \(bu 3
 305 After long jumping from the signal handler,
 306 the program does not call any non-async-signal-safe functions
 307 and does not return from the initial call to
 308 .IR main ().
 309 .IP \(bu
 310 Any signal whose handler performs a long jump must be blocked during
 311 .I every
 312 call to a non-async-signal-safe function and
 313 no non-async-signal-safe functions are called after
 314 returning from the initial call to
 315 .IR main ().
 316 .SH SEE ALSO
 317 .BR signal (7),
 318 .BR signal\-safety (7)