gen/signal.3

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  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 $
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  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 the equivalent but easier to read typedef'd version:
  52 .Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
  53 .Ft sig_t
  54 .Fn signal "int sig" "sig_t func"
  55 .Sh DESCRIPTION
  56 This
  57 .Fn signal
  58 facility
  59 is a simplified interface to the more general
  60 .Xr sigaction 2
  61 facility.
  62 .Pp
  63 Signals allow the manipulation of a process from outside its domain,
  64 as well as allowing the process to manipulate itself
  65 or copies of itself (children).
  66 There are two general types of signals:
  67 those that cause termination of a process and those that do not.
  68 Signals which cause termination of a program might result from
  69 an irrecoverable error or might be the result of a user at a terminal
  70 typing the `interrupt' character.
  71 Signals are used when a process is stopped because it wishes to access
  72 its control terminal while in the background (see
  73 .Xr tty 4 ) .
  74 Signals are optionally generated
  75 when a process resumes after being stopped,
  76 when the status of child processes changes,
  77 or when input is ready at the control terminal.
  78 Most signals result in the termination of the process receiving them,
  79 if no action is taken;
  80 some signals instead cause the process receiving them to be stopped,
  81 or are simply discarded if the process has not requested otherwise.
  82 Except for the
  83 .Dv SIGKILL
  84 and
  85 .Dv SIGSTOP
  86 signals, the
  87 .Fn signal
  88 function allows for a signal to be caught, to be ignored, or to generate
  89 an interrupt.
  90 These signals are defined in the file
  91 .In signal.h :
  92 .Bl -column No ".Dv SIGVTALRM" "create core image"
  93 .It Sy "No      Name    Default Action  Description"
  94 .It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
  95 .It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program"
  96 .It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program"
  97 .It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction"
  98 .It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap"
  99 .It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program"
 100 (formerly
 101 .Dv SIGIOT )
 102 .It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
 103 .It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception"
 104 .It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program"
 105 .It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error"
 106 .It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
 107 .It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
 108 .It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
 109 .It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
 110 .It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal"
 111 .It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
 112 .It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
 113 .It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
 114 .It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop"
 115 .It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
 116 .It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from"
 117 control terminal
 118 .It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to"
 119 control terminal
 120 .It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O"
 121 is possible on a descriptor (see
 122 .Xr fcntl 2 )
 123 .It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
 124 .Xr setrlimit 2 )
 125 .It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
 126 .Xr setrlimit 2 )
 127 .It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
 128 .Xr setitimer 2 )
 129 .It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
 130 .Xr setitimer 2 )
 131 .It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change"
 132 .It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
 133 .It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
 134 .It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
 135 .El
 136 .Pp
 137 The
 138 .Fa sig
 139 argument specifies which signal was received.
 140 The
 141 .Fa func
 142 procedure allows a user to choose the action upon receipt of a signal.
 143 To set the default action of the signal to occur as listed above,
 144 .Fa func
 145 should be
 146 .Dv SIG_DFL .
 147 A
 148 .Dv SIG_DFL
 149 resets the default action.
 150 To ignore the signal,
 151 .Fa func
 152 should be
 153 .Dv SIG_IGN .
 154 This will cause subsequent instances of the signal to be ignored
 155 and pending instances to be discarded.
 156 If
 157 .Dv SIG_IGN
 158 is not used,
 159 further occurrences of the signal are
 160 automatically blocked and
 161 .Fa func
 162 is called.
 163 .Pp
 164 The handled signal is unblocked when the
 165 function returns and
 166 the process continues from where it left off when the signal occurred.
 167 .Bf -symbolic
 168 Unlike previous signal facilities, the handler
 169 func() remains installed after a signal has been delivered.
 170 .Ef
 171 .Pp
 172 For some system calls, if a signal is caught while the call is
 173 executing and the call is prematurely terminated,
 174 the call is automatically restarted.
 175 (The handler is installed using the
 176 .Dv SA_RESTART
 177 flag with
 178 .Xr sigaction 2 . )
 179 The affected system calls include
 180 .Xr read 2 ,
 181 .Xr write 2 ,
 182 .Xr sendto 2 ,
 183 .Xr recvfrom 2 ,
 184 .Xr sendmsg 2 ,
 185 and
 186 .Xr recvmsg 2
 187 on a communications channel or a low speed device
 188 and during a
 189 .Xr ioctl 2
 190 or
 191 .Xr wait 2 .
 192 However, calls that have already committed are not restarted,
 193 but instead return a partial success (for example, a short read count).
 194 These semantics could be changed with
 195 .Xr siginterrupt 3 .
 196 .Pp
 197 When a process which has installed signal handlers forks,
 198 the child process inherits the signals.
 199 All caught signals may be reset to their default action by a call
 200 to the
 201 .Xr execve 2
 202 function;
 203 ignored signals remain ignored.
 204 .Pp
 205 If a process explicitly specifies
 206 .Dv SIG_IGN
 207 as the action for the signal
 208 .Dv SIGCHLD ,
 209 the system will not create zombie processes when children
 210 of the calling process exit.
 211 As a consequence, the system will discard the exit status
 212 from the child processes.
 213 If the calling process subsequently issues a call to
 214 .Xr wait 2
 215 or equivalent, it will block until all of the calling process's
 216 children terminate, and then return a value of \-1 with
 217 .Va errno
 218 set to
 219 .Er ECHILD .
 220 .Pp
 221 See
 222 .Xr sigaction 2
 223 for a list of functions
 224 that are considered safe for use in signal handlers.
 225 .Sh RETURN VALUES
 226 The previous action is returned on a successful call.
 227 Otherwise, SIG_ERR is returned and the global variable
 228 .Va errno
 229 is set to indicate the error.
 230 .Sh ERRORS
 231 The
 232 .Fn signal
 233 function
 234 will fail and no action will take place if one of the
 235 following occur:
 236 .Bl -tag -width Er
 237 .It Bq Er EINVAL
 238 The
 239 .Fa sig
 240 argument
 241 is not a valid signal number.
 242 .It Bq Er EINVAL
 243 An attempt is made to ignore or supply a handler for
 244 .Dv SIGKILL
 245 or
 246 .Ev SIGSTOP .
 247 .El
 248 .Sh SEE ALSO
 249 .Xr kill 1 ,
 250 .Xr kill 2 ,
 251 .Xr ptrace 2 ,
 252 .Xr sigaction 2 ,
 253 .Xr sigaltstack 2 ,
 254 .Xr sigprocmask 2 ,
 255 .Xr sigsuspend 2 ,
 256 .Xr wait 2 ,
 257 .Xr fpsetmask 3 ,
 258 .Xr setjmp 3 ,
 259 .Xr siginterrupt 3 ,
 260 .Xr tty 4
 261 .Sh HISTORY
 262 This
 263 .Fn signal
 264 facility appeared in
 265 .Bx 4.0 .
 266 The option to avoid the creation of child zombies through ignoring
 267 .Dv SIGCHLD
 268 appeared in
 269 .Fx 5.0 .