man3/setjmp.3

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  24 .TH SETJMP 3 2017-03-13 "" "Linux Programmer's Manual"
  25 .SH NAME
  26 setjmp, sigsetjmp, longjmp, siglongjmp  \- performing a nonlocal goto
  27 .SH SYNOPSIS
  28 .nf
  29 .B #include <setjmp.h>
  30 .PP
  31 .BI "int setjmp(jmp_buf " env );
  32 .BI "int sigsetjmp(sigjmp_buf " env ", int " savesigs );
  33 .PP
  34 .BI "void longjmp(jmp_buf " env ", int " val );
  35 .BI "void siglongjmp(sigjmp_buf " env ", int " val );
  36 .fi
  37 .PP
  38 .in -4n
  39 Feature Test Macro Requirements for glibc (see
  40 .BR feature_test_macros (7)):
  41 .in
  42 .PP
  43 .BR setjmp ():
  44 see NOTES.
  45 .PP
  46 .BR sigsetjmp ():
  47 _POSIX_C_SOURCE
  48 .SH DESCRIPTION
  49 The functions described on this page are used for performing "nonlocal gotos":
  50 transferring execution from one function to a predetermined location
  51 in another function.
  52 The
  53 .BR setjmp ()
  54 function dynamically establishes the target to which control
  55 will later be transferred, and
  56 .BR longjmp ()
  57 performs the transfer of execution.
  58 .PP
  59 The
  60 .BR setjmp ()
  61 function saves various information about the calling environment
  62 (typically, the stack pointer, the instruction pointer,
  63 possibly the values of other registers and the signal mask)
  64 in the buffer
  65 .IR env
  66 for later use by
  67 .BR longjmp ().
  68 In this case,
  69 .BR setjmp ()
  70 returns 0.
  71 .PP
  72 The
  73 .BR longjmp ()
  74 function uses the information saved in
  75 .IR env
  76 to transfer control back to the point where
  77 .BR setjmp ()
  78 was called and to restore ("rewind") the stack to its state at the time of the
  79 .BR setjmp ()
  80 call.
  81 In addition, and depending on the implementation (see NOTES),
  82 the values of some other registers and the process signal mask
  83 may be restored to their state at the time of the
  84 .BR setjmp ()
  85 call.
  86 .PP
  87 Following a successful
  88 .BR longjmp (),
  89 execution continues as if
  90 .BR setjmp ()
  91 had returned for a second time.
  92 This "fake" return can be distinguished from a true
  93 .BR setjmp ()
  94 call because the "fake" return returns the value provided in
  95 .IR val .
  96 If the programmer mistakenly passes the value 0 in
  97 .IR val ,
  98 the "fake" return will instead return 1.
  99 .PP
 100 .SS sigsetjmp() and siglongjmp()
 101 .BR sigsetjmp ()
 102 and
 103 .BR siglongjmp ()
 104 also perform nonlocal gotos, but provide predictable handling of
 105 the process signal mask.
 106 .PP
 107 If, and only if, the
 108 .I savesigs
 109 argument provided to
 110 .BR sigsetjmp ()
 111 is nonzero, the process's current signal mask is saved in
 112 .I env
 113 and will be restored if a
 114 .BR siglongjmp ()
 115 is later performed with this
 116 .IR env .
 117 .SH RETURN VALUE
 118 .BR setjmp ()
 119 and
 120 .BR sigsetjmp ()
 121 return 0 when called directly;
 122 on the "fake" return that occurs after
 123 .BR longjmp ()
 124 or
 125 .BR siglongjmp (),
 126 the nonzero value specified in
 127 .I val
 128 is returned.
 129 .PP
 130 The
 131 .BR longjmp ()
 132 or
 133 .BR siglongjmp ()
 134 functions do not return.
 135 .SH ATTRIBUTES
 136 For an explanation of the terms used in this section, see
 137 .BR attributes (7).
 138 .TS
 139 allbox;
 140 lbw23 lb lb
 141 l l l.
 142 Interface       Attribute       Value
 143 T{
 144 .BR setjmp (),
 145 .BR sigsetjmp ()
 146 T}      Thread safety   MT-Safe
 147 T{
 148 .BR longjmp (),
 149 .BR siglongjmp ()
 150 T}      Thread safety   MT-Safe
 151 .TE
 152 .PP
 153 .SH CONFORMING TO
 154 .BR setjmp (),
 155 .BR longjmp ():
 156 POSIX.1-2001, POSIX.1-2008, C89, C99.
 157 .PP
 158 .BR sigsetjmp (),
 159 .BR siglongjmp ():
 160 POSIX.1-2001, POSIX.1-2008.
 161 .SH NOTES
 162 POSIX does not specify whether
 163 .BR setjmp ()
 164 will save the signal mask
 165 (to be later restored during
 166 .BR longjmp ()).
 167 In System V it will not.
 168 In 4.3BSD it will, and there
 169 is a function
 170 .BR _setjmp ()
 171 that will not.
 172 The behavior under Linux depends on the glibc version
 173 and the setting of feature test macros.
 174 On Linux with glibc versions before 2.19,
 175 .BR setjmp ()
 176 follows the System V behavior by default,
 177 but the BSD behavior is provided if the
 178 .BR _BSD_SOURCE
 179 feature test macro is explicitly defined
 180 .\" so that _FAVOR_BSD is triggered
 181 and none of
 182 .BR _POSIX_SOURCE ,
 183 .BR _POSIX_C_SOURCE ,
 184 .BR _XOPEN_SOURCE ,
 185 .\" .BR _XOPEN_SOURCE_EXTENDED ,
 186 .BR _GNU_SOURCE ,
 187 or
 188 .B _SVID_SOURCE
 189 is defined.
 190 Since glibc 2.19,
 191 .IR <setjmp.h>
 192 exposes only the System V version of
 193 .BR setjmp ().
 194 Programs that need the BSD semantics should replace calls to
 195 .BR setjmp ()
 196 with calls to
 197 .BR sigsetjmp ()
 198 with a nonzero
 199 .I savesigs
 200 argument.
 201 .PP
 202 .BR setjmp ()
 203 and
 204 .BR longjmp ()
 205 can be useful for dealing with errors inside deeply nested function calls
 206 or to allow a signal handler to pass control to
 207 a specific point in the program,
 208 rather than returning to the point where the handler interrupted
 209 the main program.
 210 In the latter case,
 211 if you want to portably save and restore signal masks, use
 212 .BR sigsetjmp ()
 213 and
 214 .BR siglongjmp ().
 215 See also the discussion of program readability below.
 216 .PP
 217 The compiler may optimize variables into registers, and
 218 .BR longjmp ()
 219 may restore the values of other registers in addition to the
 220 stack pointer and program counter.
 221 Consequently, the values of automatic variables are unspecified
 222 after a call to
 223 .BR longjmp ()
 224 if they meet all the following criteria:
 225 .IP \(bu 3
 226 they are local to the function that made the corresponding
 227 .BR setjmp ()
 228 call;
 229 .IP \(bu
 230 their values are changed between the calls to
 231 .BR setjmp ()
 232 and
 233 .BR longjmp ();
 234 and
 235 .IP \(bu
 236 they are not declared as
 237 .IR volatile .
 238 .PP
 239 Analogous remarks apply for
 240 .BR siglongjmp ().
 241 .\"
 242 .SS Nonlocal gotos and program readability
 243 While it can be abused,
 244 the traditional C "goto" statement at least has the benefit that lexical cues
 245 (the goto statement and the target label)
 246 allow the programmer to easily perceive the flow of control.
 247 Nonlocal gotos provide no such cues: multiple
 248 .BR setjmp ()
 249 calls might employ the same
 250 .IR jmp_buf
 251 variable so that the content of the variable may change
 252 over the lifetime of the application.
 253 Consequently, the programmer may be forced to perform detailed
 254 reading of the code to determine the dynamic target of a particular
 255 .BR longjmp ()
 256 call.
 257 (To make the programmer's life easier, each
 258 .BR setjmp ()
 259 call should employ a unique
 260 .IR jmp_buf
 261 variable.)
 262 .PP
 263 Adding further difficulty, the
 264 .BR setjmp ()
 265 and
 266 .BR longjmp ()
 267 calls may not even be in the same source code module.
 268 .PP
 269 In summary, nonlocal gotos can make programs harder to understand
 270 and maintain, and an alternative should be used if possible.
 271 .\"
 272 .SS Caveats
 273 If the function which called
 274 .BR setjmp ()
 275 returns before
 276 .BR longjmp ()
 277 is called, the behavior is undefined.
 278 Some kind of subtle or unsubtle chaos is sure to result.
 279 .PP
 280 If, in a multithreaded program, a
 281 .BR longjmp ()
 282 call employs an
 283 .I env
 284 buffer that was initialized by a call to
 285 .BR setjmp ()
 286 in a different thread, the behavior is undefined.
 287 .\"
 288 .\" The following statement appeared in versions up to POSIX.1-2008 TC1,
 289 .\" but is set to be removed in POSIX.1-2008 TC2:
 290 .\"
 291 .\"     According to POSIX.1, if a
 292 .\"     .BR longjmp ()
 293 .\"     call is performed from a nested signal handler
 294 .\"     (i.e., from a handler that was invoked in response to a signal that was
 295 .\"     generated while another signal was already in the process of being
 296 .\"     handled), the behavior is undefined.
 297 .PP
 298 POSIX.1-2008 Technical Corrigendum 2 adds
 299 .\" http://austingroupbugs.net/view.php?id=516#c1195
 300 .BR longjmp ()
 301 and
 302 .BR siglongjmp ()
 303 to the list of async-signal-safe functions.
 304 However, the standard recommends avoiding the use of these functions
 305 from signal handlers and goes on to point out that
 306 if these functions are called from a signal handler that interrupted
 307 a call to a non-async-signal-safe function (or some equivalent,
 308 such as the steps equivalent to
 309 .BR exit (3)
 310 that occur upon a return from the initial call to
 311 .IR main ()),
 312 the behavior is undefined if the program subsequently makes a call to
 313 a non-async-signal-safe function.
 314 The only way of avoiding undefined behavior is to ensure one of the following:
 315 .IP * 3
 316 After long jumping from the signal handler,
 317 the program does not call any non-async-signal-safe functions
 318 and does not return from the initial call to
 319 .IR main ().
 320 .IP *
 321 Any signal whose handler performs a long jump must be blocked during
 322 .I every
 323 call to a non-async-signal-safe function and
 324 no non-async-signal-safe functions are called after
 325 returning from the initial call to
 326 .IR main ().
 327 .SH SEE ALSO
 328 .BR signal (7),
 329 .BR signal-safety (7)