gen/FreeBSD/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
  56 .Fx Ap s
  57 equivalent but easier to read typedef'd version:
  58 .Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
  59 .Pp
  60 .Ft sig_t
  61 .Fn signal "int sig" "sig_t func"
  62 .Sh DESCRIPTION
  63 This
  64 .Fn signal
  65 facility
  66 is a simplified interface to the more general
  67 .Xr sigaction 2
  68 facility.
  69 .Pp
  70 Signals allow the manipulation of a process from outside its
  71 domain as well as allowing the process to manipulate itself or
  72 copies of itself (children).
  73 There are two general types of signals:
  74 those that cause termination of a process and those that do not.
  75 Signals which cause termination of a program might result from
  76 an irrecoverable error or might be the result of a user at a terminal
  77 typing the `interrupt' character.
  78 Signals are used when a process is stopped because it wishes to access
  79 its control terminal while in the background (see
  80 .Xr tty 4 ) .
  81 Signals are optionally generated
  82 when a process resumes after being stopped,
  83 when the status of child processes changes,
  84 or when input is ready at the control terminal.
  85 Most signals result in the termination of the process receiving them
  86 if no action
  87 is taken; some signals instead cause the process receiving them
  88 to be stopped, or are simply discarded if the process has not
  89 requested otherwise.
  90 Except for the
  91 .Dv SIGKILL
  92 and
  93 .Dv SIGSTOP
  94 signals, the
  95 .Fn signal
  96 function allows for a signal to be caught, to be ignored, or to generate
  97 an interrupt.
  98 These signals are defined in the file
  99 .In signal.h :
 100 .Bl -column No ".Dv SIGVTALRM" "create core image"
 101 .It Sy "No      Name    Default Action  Description"
 102 .It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
 103 .It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program"
 104 .It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program"
 105 .It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction"
 106 .It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap"
 107 .It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program"
 108 (formerly
 109 .Dv SIGIOT )
 110 .It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
 111 .It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception"
 112 .It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program"
 113 .It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error"
 114 .It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
 115 .It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
 116 .It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
 117 .It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
 118 .It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal"
 119 .It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
 120 .It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
 121 .It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
 122 .It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop"
 123 .It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
 124 .It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from"
 125 control terminal
 126 .It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to"
 127 control terminal
 128 .It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O"
 129 is possible on a descriptor (see
 130 .Xr fcntl 2 )
 131 .It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
 132 .Xr setrlimit 2 )
 133 .It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
 134 .Xr setrlimit 2 )
 135 .It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
 136 .Xr setitimer 2 )
 137 .It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
 138 .Xr setitimer 2 )
 139 .It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change"
 140 .It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
 141 .It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
 142 .It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
 143 .It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt"
 144 .El
 145 .Pp
 146 The
 147 .Fa sig
 148 argument specifies which signal was received.
 149 The
 150 .Fa func
 151 procedure allows a user to choose the action upon receipt of a signal.
 152 To set the default action of the signal to occur as listed above,
 153 .Fa func
 154 should be
 155 .Dv SIG_DFL .
 156 A
 157 .Dv SIG_DFL
 158 resets the default action.
 159 To ignore the signal
 160 .Fa func
 161 should be
 162 .Dv SIG_IGN .
 163 This will cause subsequent instances of the signal to be ignored
 164 and pending instances to be discarded.
 165 If
 166 .Dv SIG_IGN
 167 is not used,
 168 further occurrences of the signal are
 169 automatically blocked and
 170 .Fa func
 171 is called.
 172 .Pp
 173 The handled signal is unblocked when the
 174 function returns and
 175 the process continues from where it left off when the signal occurred.
 176 .Bf -symbolic
 177 Unlike previous signal facilities, the handler
 178 func() remains installed after a signal has been delivered.
 179 .Ef
 180 .Pp
 181 For some system calls, if a signal is caught while the call is
 182 executing and the call is prematurely terminated,
 183 the call is automatically restarted.
 184 Any handler installed with
 185 .Xr signal 3
 186 will have the
 187 .Dv SA_RESTART
 188 flag set, meaning that any restartable system call will not return on
 189 receipt of a signal.
 190 The affected system calls include
 191 .Xr read 2 ,
 192 .Xr write 2 ,
 193 .Xr sendto 2 ,
 194 .Xr recvfrom 2 ,
 195 .Xr sendmsg 2
 196 and
 197 .Xr recvmsg 2
 198 on a communications channel or a low speed device
 199 and during a
 200 .Xr ioctl 2
 201 or
 202 .Xr wait 2 .
 203 However, calls that have already committed are not restarted,
 204 but instead return a partial success (for example, a short read count).
 205 These semantics could be changed with
 206 .Xr siginterrupt 3 .
 207 .Pp
 208 When a process which has installed signal handlers forks,
 209 the child process inherits the signals.
 210 All caught signals may be reset to their default action by a call
 211 to the
 212 .Xr execve 2
 213 function;
 214 ignored signals remain ignored.
 215 .Pp
 216 If a process explicitly specifies
 217 .Dv SIG_IGN
 218 as the action for the signal
 219 .Dv SIGCHLD ,
 220 the system will not create zombie processes when children
 221 of the calling process exit.
 222 As a consequence, the system will discard the exit status
 223 from the child processes.
 224 If the calling process subsequently issues a call to
 225 .Xr wait 2
 226 or equivalent, it will block until all of the calling process's
 227 children terminate, and then return a value of \-1 with
 228 .Va errno
 229 set to
 230 .Er ECHILD .
 231 .Pp
 232 See
 233 .Xr sigaction 2
 234 for a list of functions
 235 that are considered safe for use in signal handlers.
 236 .Sh RETURN VALUES
 237 The previous action is returned on a successful call.
 238 Otherwise, SIG_ERR is returned and the global variable
 239 .Va errno
 240 is set to indicate the error.
 241 .Sh ERRORS
 242 The
 243 .Fn signal
 244 function
 245 will fail and no action will take place if one of the
 246 following occur:
 247 .Bl -tag -width Er
 248 .It Bq Er EINVAL
 249 The
 250 .Fa sig
 251 argument
 252 is not a valid signal number.
 253 .It Bq Er EINVAL
 254 An attempt is made to ignore or supply a handler for
 255 .Dv SIGKILL
 256 or
 257 .Dv SIGSTOP .
 258 .El
 259 .Sh SEE ALSO
 260 .Xr kill 1 ,
 261 .Xr kill 2 ,
 262 .Xr ptrace 2 ,
 263 .Xr sigaction 2 ,
 264 .Xr sigaltstack 2 ,
 265 .Xr sigprocmask 2 ,
 266 .Xr sigsuspend 2 ,
 267 .Xr wait 2 ,
 268 .Xr fpsetmask 3 ,
 269 .Xr setjmp 3 ,
 270 .Xr siginterrupt 3 ,
 271 .Xr tty 4
 272 .Sh HISTORY
 273 The
 274 .Nm
 275 facility appeared in
 276 .Bx 4.0 .
 277 The option to avoid the creation of child zombies through ignoring
 278 .Dv SIGCHLD
 279 appeared in
 280 .Fx 5.0 .