gen/FreeBSD/signal.3

   1 .\" Copyright (c) 1980, 1991, 1993
   2 .\"     The Regents of the University of California.  All rights reserved.
   3 .\"
   4 .\" Redistribution and use in source and binary forms, with or without
   5 .\" modification, are permitted provided that the following conditions
   6 .\" are met:
   7 .\" 1. Redistributions of source code must retain the above copyright
   8 .\"    notice, this list of conditions and the following disclaimer.
   9 .\" 2. Redistributions in binary form must reproduce the above copyright
  10 .\"    notice, this list of conditions and the following disclaimer in the
  11 .\"    documentation and/or other materials provided with the distribution.
  12 .\" 3. All advertising materials mentioning features or use of this software
  13 .\"    must display the following acknowledgement:
  14 .\"     This product includes software developed by the University of
  15 .\"     California, Berkeley and its contributors.
  16 .\" 4. Neither the name of the University nor the names of its contributors
  17 .\"    may be used to endorse or promote products derived from this software
  18 .\"    without specific prior written permission.
  19 .\"
  20 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  21 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  22 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  23 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  24 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  25 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  26 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  27 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  28 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  29 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  30 .\" SUCH DAMAGE.
  31 .\"
  32 .\"     @(#)signal.3    8.3 (Berkeley) 4/19/94
  33 .\" $FreeBSD: src/lib/libc/gen/signal.3,v 1.38 2004/07/03 22:30:08 ru Exp $
  34 .\"
  35 .Dd June 7, 2004
  36 .Dt SIGNAL 3
  37 .Os
  38 .Sh NAME
  39 .Nm signal
  40 .Nd simplified software signal facilities
  41 .Sh LIBRARY
  42 .Lb libc
  43 .Sh SYNOPSIS
  44 .In signal.h
  45 .\" The following is Quite Ugly, but syntactically correct.
  46 .\" Don't try to
  47 .\" fix it.
  48 .Ft void \*(lp*
  49 .Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint"
  50 .Pp
  51 or in
  52 .Fx Ns 's
  53 equivalent but easier to read typedef'd version:
  54 .Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
  55 .Ft sig_t
  56 .Fn signal "int sig" "sig_t func"
  57 .Sh DESCRIPTION
  58 This
  59 .Fn signal
  60 facility
  61 is a simplified interface to the more general
  62 .Xr sigaction 2
  63 facility.
  64 .Pp
  65 Signals allow the manipulation of a process from outside its
  66 domain as well as allowing the process to manipulate itself or
  67 copies of itself (children).
  68 There are two general types of signals:
  69 those that cause termination of a process and those that do not.
  70 Signals which cause termination of a program might result from
  71 an irrecoverable error or might be the result of a user at a terminal
  72 typing the `interrupt' character.
  73 Signals are used when a process is stopped because it wishes to access
  74 its control terminal while in the background (see
  75 .Xr tty 4 ) .
  76 Signals are optionally generated
  77 when a process resumes after being stopped,
  78 when the status of child processes changes,
  79 or when input is ready at the control terminal.
  80 Most signals result in the termination of the process receiving them
  81 if no action
  82 is taken; some signals instead cause the process receiving them
  83 to be stopped, or are simply discarded if the process has not
  84 requested otherwise.
  85 Except for the
  86 .Dv SIGKILL
  87 and
  88 .Dv SIGSTOP
  89 signals, the
  90 .Fn signal
  91 function allows for a signal to be caught, to be ignored, or to generate
  92 an interrupt.
  93 These signals are defined in the file
  94 .In signal.h :
  95 .Bl -column No ".Dv SIGVTALRM" "create core image"
  96 .It Sy "No      Name    Default Action  Description"
  97 .It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
  98 .It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program"
  99 .It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program"
 100 .It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction"
 101 .It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap"
 102 .It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program"
 103 (formerly
 104 .Dv SIGIOT )
 105 .It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
 106 .It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception"
 107 .It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program"
 108 .It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error"
 109 .It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
 110 .It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
 111 .It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
 112 .It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
 113 .It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal"
 114 .It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
 115 .It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
 116 .It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
 117 .It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop"
 118 .It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
 119 .It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from"
 120 control terminal
 121 .It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to"
 122 control terminal
 123 .It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O"
 124 is possible on a descriptor (see
 125 .Xr fcntl 2 )
 126 .It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
 127 .Xr setrlimit 2 )
 128 .It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
 129 .Xr setrlimit 2 )
 130 .It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
 131 .Xr setitimer 2 )
 132 .It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
 133 .Xr setitimer 2 )
 134 .It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change"
 135 .It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
 136 .It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
 137 .It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
 138 .It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt"
 139 .El
 140 .Pp
 141 The
 142 .Fa sig
 143 argument specifies which signal was received.
 144 The
 145 .Fa func
 146 procedure allows a user to choose the action upon receipt of a signal.
 147 To set the default action of the signal to occur as listed above,
 148 .Fa func
 149 should be
 150 .Dv SIG_DFL .
 151 A
 152 .Dv SIG_DFL
 153 resets the default action.
 154 To ignore the signal
 155 .Fa func
 156 should be
 157 .Dv SIG_IGN .
 158 This will cause subsequent instances of the signal to be ignored
 159 and pending instances to be discarded.
 160 If
 161 .Dv SIG_IGN
 162 is not used,
 163 further occurrences of the signal are
 164 automatically blocked and
 165 .Fa func
 166 is called.
 167 .Pp
 168 The handled signal is unblocked when the
 169 function returns and
 170 the process continues from where it left off when the signal occurred.
 171 .Bf -symbolic
 172 Unlike previous signal facilities, the handler
 173 func() remains installed after a signal has been delivered.
 174 .Ef
 175 .Pp
 176 For some system calls, if a signal is caught while the call is
 177 executing and the call is prematurely terminated,
 178 the call is automatically restarted.
 179 (The handler is installed using the
 180 .Dv SA_RESTART
 181 flag with
 182 .Xr sigaction 2 . )
 183 The affected system calls include
 184 .Xr read 2 ,
 185 .Xr write 2 ,
 186 .Xr sendto 2 ,
 187 .Xr recvfrom 2 ,
 188 .Xr sendmsg 2
 189 and
 190 .Xr recvmsg 2
 191 on a communications channel or a low speed device
 192 and during a
 193 .Xr ioctl 2
 194 or
 195 .Xr wait 2 .
 196 However, calls that have already committed are not restarted,
 197 but instead return a partial success (for example, a short read count).
 198 These semantics could be changed with
 199 .Xr siginterrupt 3 .
 200 .Pp
 201 When a process which has installed signal handlers forks,
 202 the child process inherits the signals.
 203 All caught signals may be reset to their default action by a call
 204 to the
 205 .Xr execve 2
 206 function;
 207 ignored signals remain ignored.
 208 .Pp
 209 If a process explicitly specifies
 210 .Dv SIG_IGN
 211 as the action for the signal
 212 .Dv SIGCHLD ,
 213 the system will not create zombie processes when children
 214 of the calling process exit.
 215 As a consequence, the system will discard the exit status
 216 from the child processes.
 217 If the calling process subsequently issues a call to
 218 .Xr wait 2
 219 or equivalent, it will block until all of the calling process's
 220 children terminate, and then return a value of \-1 with
 221 .Va errno
 222 set to
 223 .Er ECHILD .
 224 .Pp
 225 See
 226 .Xr sigaction 2
 227 for a list of functions
 228 that are considered safe for use in signal handlers.
 229 .Sh RETURN VALUES
 230 The previous action is returned on a successful call.
 231 Otherwise, SIG_ERR is returned and the global variable
 232 .Va errno
 233 is set to indicate the error.
 234 .Sh ERRORS
 235 The
 236 .Fn signal
 237 function
 238 will fail and no action will take place if one of the
 239 following occur:
 240 .Bl -tag -width Er
 241 .It Bq Er EINVAL
 242 The
 243 .Fa sig
 244 argument
 245 is not a valid signal number.
 246 .It Bq Er EINVAL
 247 An attempt is made to ignore or supply a handler for
 248 .Dv SIGKILL
 249 or
 250 .Ev SIGSTOP .
 251 .El
 252 .Sh SEE ALSO
 253 .Xr kill 1 ,
 254 .Xr kill 2 ,
 255 .Xr ptrace 2 ,
 256 .Xr sigaction 2 ,
 257 .Xr sigaltstack 2 ,
 258 .Xr sigprocmask 2 ,
 259 .Xr sigsuspend 2 ,
 260 .Xr wait 2 ,
 261 .Xr fpsetmask 3 ,
 262 .Xr setjmp 3 ,
 263 .Xr siginterrupt 3 ,
 264 .Xr tty 4
 265 .Sh HISTORY
 266 This
 267 .Fn signal
 268 facility appeared in
 269 .Bx 4.0 .
 270 The option to avoid the creation of child zombies through ignoring
 271 .Dv SIGCHLD
 272 appeared in
 273 .Fx 5.0 .