man2/signal.2

   1 .\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
   2 .\" and Copyright (c) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\" and Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk
   4 .\"      <mtk.manpages@gmail.com>
   5 .\" based on work by Rik Faith <faith@cs.unc.edu>
   6 .\" and Mike Battersby <mike@starbug.apana.org.au>.
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  30 .\" Modified 2004-11-19, mtk:
  31 .\" added pointer to sigaction.2 for details of ignoring SIGCHLD
  32 .\" 2007-06-03, mtk: strengthened portability warning, and rewrote
  33 .\"     various sections.
  34 .\" 2008-07-11, mtk: rewrote and expanded portability discussion.
  35 .\"
  36 .TH SIGNAL 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  37 .SH NAME
  38 signal \- ANSI C signal handling
  39 .SH SYNOPSIS
  40 .B #include <signal.h>
  41 .PP
  42 .B typedef void (*sighandler_t)(int);
  43 .PP
  44 .BI "sighandler_t signal(int " signum ", sighandler_t " handler );
  45 .SH DESCRIPTION
  46 The behavior of
  47 .BR signal ()
  48 varies across UNIX versions,
  49 and has also varied historically across different versions of Linux.
  50 \fBAvoid its use\fP: use
  51 .BR sigaction (2)
  52 instead.
  53 See \fIPortability\fP below.
  54 .PP
  55 .BR signal ()
  56 sets the disposition of the signal
  57 .I signum
  58 to
  59 .IR handler ,
  60 which is either
  61 .BR SIG_IGN ,
  62 .BR SIG_DFL ,
  63 or the address of a programmer-defined function (a "signal handler").
  64 .PP
  65 If the signal
  66 .I signum
  67 is delivered to the process, then one of the following happens:
  68 .TP 3
  69 *
  70 If the disposition is set to
  71 .BR SIG_IGN ,
  72 then the signal is ignored.
  73 .TP
  74 *
  75 If the disposition is set to
  76 .BR SIG_DFL ,
  77 then the default action associated with the signal (see
  78 .BR signal (7))
  79 occurs.
  80 .TP
  81 *
  82 If the disposition is set to a function,
  83 then first either the disposition is reset to
  84 .BR SIG_DFL ,
  85 or the signal is blocked (see \fIPortability\fP below), and then
  86 .I handler
  87 is called with argument
  88 .IR signum .
  89 If invocation of the handler caused the signal to be blocked,
  90 then the signal is unblocked upon return from the handler.
  91 .PP
  92 The signals
  93 .B SIGKILL
  94 and
  95 .B SIGSTOP
  96 cannot be caught or ignored.
  97 .SH RETURN VALUE
  98 .BR signal ()
  99 returns the previous value of the signal handler, or
 100 .B SIG_ERR
 101 on error.
 102 In the event of an error,
 103 .I errno
 104 is set to indicate the cause.
 105 .SH ERRORS
 106 .TP
 107 .B EINVAL
 108 .I signum
 109 is invalid.
 110 .SH CONFORMING TO
 111 POSIX.1-2001, POSIX.1-2008, C89, C99.
 112 .SH NOTES
 113 The effects of
 114 .BR signal ()
 115 in a multithreaded process are unspecified.
 116 .PP
 117 According to POSIX, the behavior of a process is undefined after it
 118 ignores a
 119 .BR SIGFPE ,
 120 .BR SIGILL ,
 121 or
 122 .B SIGSEGV
 123 signal that was not generated by
 124 .BR kill (2)
 125 or
 126 .BR raise (3).
 127 Integer division by zero has undefined result.
 128 On some architectures it will generate a
 129 .B SIGFPE
 130 signal.
 131 (Also dividing the most negative integer by \-1 may generate
 132 .BR SIGFPE .)
 133 Ignoring this signal might lead to an endless loop.
 134 .PP
 135 See
 136 .BR sigaction (2)
 137 for details on what happens when the disposition
 138 .B SIGCHLD
 139 is set to
 140 .BR SIG_IGN .
 141 .PP
 142 See
 143 .BR signal-safety (7)
 144 for a list of the async-signal-safe functions that can be
 145 safely called from inside a signal handler.
 146 .PP
 147 The use of
 148 .I sighandler_t
 149 is a GNU extension, exposed if
 150 .B _GNU_SOURCE
 151 is defined;
 152 .\" libc4 and libc5 define
 153 .\" .IR SignalHandler ;
 154 glibc also defines (the BSD-derived)
 155 .I sig_t
 156 if
 157 .B _BSD_SOURCE
 158 (glibc 2.19 and earlier)
 159 or
 160 .BR _DEFAULT_SOURCE
 161 (glibc 2.19 and later)
 162 is defined.
 163 Without use of such a type, the declaration of
 164 .BR signal ()
 165 is the somewhat harder to read:
 166 .PP
 167 .in +4n
 168 .EX
 169 .BI "void ( *" signal "(int " signum ", void (*" handler ")(int)) ) (int);"
 170 .EE
 171 .in
 172 .SS Portability
 173 The only portable use of
 174 .BR signal ()
 175 is to set a signal's disposition to
 176 .BR SIG_DFL
 177 or
 178 .BR SIG_IGN .
 179 The semantics when using
 180 .BR signal ()
 181 to establish a signal handler vary across systems
 182 (and POSIX.1 explicitly permits this variation);
 183 .B do not use it for this purpose.
 184 .PP
 185 POSIX.1 solved the portability mess by specifying
 186 .BR sigaction (2),
 187 which provides explicit control of the semantics when a
 188 signal handler is invoked; use that interface instead of
 189 .BR signal ().
 190 .PP
 191 In the original UNIX systems, when a handler that was established using
 192 .BR signal ()
 193 was invoked by the delivery of a signal,
 194 the disposition of the signal would be reset to
 195 .BR SIG_DFL ,
 196 and the system did not block delivery of further instances of the signal.
 197 This is equivalent to calling
 198 .BR sigaction (2)
 199 with the following flags:
 200 .PP
 201 .EX
 202     sa.sa_flags = SA_RESETHAND | SA_NODEFER;
 203 .EE
 204 .PP
 205 System\ V also provides these semantics for
 206 .BR signal ().
 207 This was bad because the signal might be delivered again
 208 before the handler had a chance to reestablish itself.
 209 Furthermore, rapid deliveries of the same signal could
 210 result in recursive invocations of the handler.
 211 .PP
 212 BSD improved on this situation, but unfortunately also
 213 changed the semantics of the existing
 214 .BR signal ()
 215 interface while doing so.
 216 On BSD, when a signal handler is invoked,
 217 the signal disposition is not reset,
 218 and further instances of the signal are blocked from
 219 being delivered while the handler is executing.
 220 Furthermore, certain blocking system calls are automatically
 221 restarted if interrupted by a signal handler (see
 222 .BR signal (7)).
 223 The BSD semantics are equivalent to calling
 224 .BR sigaction (2)
 225 with the following flags:
 226 .PP
 227 .EX
 228     sa.sa_flags = SA_RESTART;
 229 .EE
 230 .PP
 231 The situation on Linux is as follows:
 232 .IP * 2
 233 The kernel's
 234 .BR signal ()
 235 system call provides System\ V semantics.
 236 .IP *
 237 By default, in glibc 2 and later, the
 238 .BR signal ()
 239 wrapper function does not invoke the kernel system call.
 240 Instead, it calls
 241 .BR sigaction (2)
 242 using flags that supply BSD semantics.
 243 This default behavior is provided as long as a suitable
 244 feature test macro is defined:
 245 .B _BSD_SOURCE
 246 on glibc 2.19 and earlier or
 247 .BR _DEFAULT_SOURCE
 248 in glibc 2.19 and later.
 249 (By default, these macros are defined; see
 250 .BR feature_test_macros (7)
 251 for details.)
 252 If such a feature test macro is not defined, then
 253 .BR signal ()
 254 provides System\ V semantics.
 255 .\"
 256 .\" System V semantics are also provided if one uses the separate
 257 .\" .BR sysv_signal (3)
 258 .\" function.
 259 .\" .IP *
 260 .\" The
 261 .\" .BR signal ()
 262 .\" function in Linux libc4 and libc5 provide System\ V semantics.
 263 .\" If one on a libc5 system includes
 264 .\" .I <bsd/signal.h>
 265 .\" instead of
 266 .\" .IR <signal.h> ,
 267 .\" then
 268 .\" .BR signal ()
 269 .\" provides BSD semantics.
 270 .SH SEE ALSO
 271 .BR kill (1),
 272 .BR alarm (2),
 273 .BR kill (2),
 274 .BR pause (2),
 275 .BR sigaction (2),
 276 .BR signalfd (2),
 277 .BR sigpending (2),
 278 .BR sigprocmask (2),
 279 .BR sigsuspend (2),
 280 .BR bsd_signal (3),
 281 .BR killpg (3),
 282 .BR raise (3),
 283 .BR siginterrupt (3),
 284 .BR sigqueue (3),
 285 .BR sigsetops (3),
 286 .BR sigvec (3),
 287 .BR sysv_signal (3),
 288 .BR signal (7)