gen/signal.3

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  28 .\"     @(#)signal.3    8.3 (Berkeley) 4/19/94
  29 .\" $FreeBSD: src/lib/libc/gen/signal.3,v 1.43 2008/07/17 21:54:23 simon Exp $
  30 .\"
  31 .Dd June 7, 2004
  32 .Dt SIGNAL 3
  33 .Os
  34 .Sh NAME
  35 .Nm signal
  36 .Nd simplified software signal facilities
  37 .Sh LIBRARY
  38 .Lb libc
  39 .Sh SYNOPSIS
  40 .In signal.h
  41 .\" XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX
  42 .\" The prototype for signal(3) cannot be cleanly marked up in -mdoc
  43 .\" without the following lower-level tweak.
  44 .nr in-synopsis-section 0
  45 .Pp
  46 .Ft "void \*(lp*" Ns
  47 .Fo signal
  48 .Fa "int sig"
  49 .Fa "void \*(lp*func\*(rp\*(lpint\*(rp"
  50 .Fc Ns
  51 .Ft "\*(rp\*(lpint\*(rp" ;
  52 .Pp
  53 .nr in-synopsis-section 1
  54 .\" XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX
  55 or in the equivalent but easier to read typedef'd version:
  56 .Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
  57 .Pp
  58 .Ft sig_t
  59 .Fn signal "int sig" "sig_t func"
  60 .Sh DESCRIPTION
  61 This
  62 .Fn signal
  63 facility
  64 is a simplified interface to the more general
  65 .Xr sigaction 2
  66 facility.
  67 .Pp
  68 Signals allow the manipulation of a process from outside its domain,
  69 as well as allowing the process to manipulate itself
  70 or copies of itself (children).
  71 There are two general types of signals:
  72 those that cause termination of a process and those that do not.
  73 Signals which cause termination of a program might result from
  74 an irrecoverable error or might be the result of a user at a terminal
  75 typing the `interrupt' character.
  76 Signals are used when a process is stopped because it wishes to access
  77 its control terminal while in the background (see
  78 .Xr tty 4 ) .
  79 Signals are optionally generated
  80 when a process resumes after being stopped,
  81 when the status of child processes changes,
  82 or when input is ready at the control terminal.
  83 Most signals result in the termination of the process receiving them,
  84 if no action is taken;
  85 some signals instead cause the process receiving them to be stopped,
  86 or are simply discarded if the process has not requested otherwise.
  87 Except for the
  88 .Dv SIGKILL
  89 and
  90 .Dv SIGSTOP
  91 signals, the
  92 .Fn signal
  93 function allows for a signal to be caught, to be ignored, or to generate
  94 an interrupt.
  95 These signals are defined in the file
  96 .In signal.h :
  97 .Bl -column No ".Dv SIGVTALRM" "create core image"
  98 .It Sy "No      Name    Default Action  Description"
  99 .It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
 100 .It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program"
 101 .It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program"
 102 .It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction"
 103 .It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap"
 104 .It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program"
 105 (formerly
 106 .Dv SIGIOT )
 107 .It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
 108 .It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception"
 109 .It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program"
 110 .It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error"
 111 .It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
 112 .It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
 113 .It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
 114 .It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
 115 .It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal"
 116 .It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
 117 .It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
 118 .It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
 119 .It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop"
 120 .It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
 121 .It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from"
 122 control terminal
 123 .It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to"
 124 control terminal
 125 .It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O"
 126 is possible on a descriptor (see
 127 .Xr fcntl 2 )
 128 .It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
 129 .Xr setrlimit 2 )
 130 .It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
 131 .Xr setrlimit 2 )
 132 .It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
 133 .Xr setitimer 2 )
 134 .It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
 135 .Xr setitimer 2 )
 136 .It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change"
 137 .It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
 138 .It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
 139 .It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
 140 .El
 141 .Pp
 142 The
 143 .Fa sig
 144 argument specifies which signal was received.
 145 The
 146 .Fa func
 147 procedure allows a user to choose the action upon receipt of a signal.
 148 To set the default action of the signal to occur as listed above,
 149 .Fa func
 150 should be
 151 .Dv SIG_DFL .
 152 A
 153 .Dv SIG_DFL
 154 resets the default action.
 155 To ignore the signal,
 156 .Fa func
 157 should be
 158 .Dv SIG_IGN .
 159 This will cause subsequent instances of the signal to be ignored
 160 and pending instances to be discarded.
 161 If
 162 .Dv SIG_IGN
 163 is not used,
 164 further occurrences of the signal are
 165 automatically blocked and
 166 .Fa func
 167 is called.
 168 .Pp
 169 The handled signal is unblocked when the
 170 function returns and
 171 the process continues from where it left off when the signal occurred.
 172 .Bf -symbolic
 173 Unlike previous signal facilities, the handler
 174 func() remains installed after a signal has been delivered.
 175 .Ef
 176 .Pp
 177 For some system calls, if a signal is caught while the call is
 178 executing and the call is prematurely terminated,
 179 the call is automatically restarted.
 180 Any handler installed with
 181 .Xr signal 3
 182 will have the
 183 .Dv SA_RESTART
 184 flag set, meaning that any restartable system call will not return on
 185 receipt of a signal.
 186 The affected system calls include
 187 .Xr read 2 ,
 188 .Xr write 2 ,
 189 .Xr sendto 2 ,
 190 .Xr recvfrom 2 ,
 191 .Xr sendmsg 2 ,
 192 and
 193 .Xr recvmsg 2
 194 on a communications channel or a low speed device
 195 and during a
 196 .Xr ioctl 2
 197 or
 198 .Xr wait 2 .
 199 However, calls that have already committed are not restarted,
 200 but instead return a partial success (for example, a short read count).
 201 These semantics could be changed with
 202 .Xr siginterrupt 3 .
 203 .Pp
 204 When a process which has installed signal handlers forks,
 205 the child process inherits the signals.
 206 All caught signals may be reset to their default action by a call
 207 to the
 208 .Xr execve 2
 209 function;
 210 ignored signals remain ignored.
 211 .Pp
 212 If a process explicitly specifies
 213 .Dv SIG_IGN
 214 as the action for the signal
 215 .Dv SIGCHLD ,
 216 the system will not create zombie processes when children
 217 of the calling process exit.
 218 As a consequence, the system will discard the exit status
 219 from the child processes.
 220 If the calling process subsequently issues a call to
 221 .Xr wait 2
 222 or equivalent, it will block until all of the calling process's
 223 children terminate, and then return a value of \-1 with
 224 .Va errno
 225 set to
 226 .Er ECHILD .
 227 .Pp
 228 See
 229 .Xr sigaction 2
 230 for a list of functions
 231 that are considered safe for use in signal handlers.
 232 .Sh RETURN VALUES
 233 The previous action is returned on a successful call.
 234 Otherwise, SIG_ERR is returned and the global variable
 235 .Va errno
 236 is set to indicate the error.
 237 .Sh ERRORS
 238 The
 239 .Fn signal
 240 function
 241 will fail and no action will take place if one of the
 242 following occur:
 243 .Bl -tag -width Er
 244 .It Bq Er EINVAL
 245 The
 246 .Fa sig
 247 argument
 248 is not a valid signal number.
 249 .It Bq Er EINVAL
 250 An attempt is made to ignore or supply a handler for
 251 .Dv SIGKILL
 252 or
 253 .Dv SIGSTOP .
 254 .El
 255 .Sh SEE ALSO
 256 .Xr kill 1 ,
 257 .Xr kill 2 ,
 258 .Xr ptrace 2 ,
 259 .Xr sigaction 2 ,
 260 .Xr sigaltstack 2 ,
 261 .Xr sigprocmask 2 ,
 262 .Xr sigsuspend 2 ,
 263 .Xr wait 2 ,
 264 .Xr fpsetmask 3 ,
 265 .Xr setjmp 3 ,
 266 .Xr siginterrupt 3 ,
 267 .Xr tty 4
 268 .Sh HISTORY
 269 The
 270 .Nm
 271 facility appeared in
 272 .Bx 4.0 .
 273 The option to avoid the creation of child zombies through ignoring
 274 .Dv SIGCHLD
 275 appeared in
 276 .Fx 5.0 .