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 1.28 2001/10/01 16:08:51 ru Exp $
  34 .\"
  35 .Dd April 19, 1994
  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.  Don't try to
  46 .\" fix it.
  47 .Ft void \*(lp*
  48 .Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint"
  49 .Pp
  50 or in
  51 .Fx Ns 's
  52 equivalent but easier to read typedef'd version:
  53 .Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp"
  54 .Ft sig_t
  55 .Fn signal "int sig" "sig_t func"
  56 .Sh DESCRIPTION
  57 This
  58 .Fn signal
  59 facility
  60 is a simplified interface to the more general
  61 .Xr sigaction 2
  62 facility.
  63 .Pp
  64 Signals allow the manipulation of a process from outside its
  65 domain as well as allowing the process to manipulate itself or
  66 copies of itself (children). 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
  80 is taken; some signals instead cause the process receiving them
  81 to be stopped, or are simply discarded if the process has not
  82 requested otherwise.
  83 Except for the
  84 .Dv SIGKILL
  85 and
  86 .Dv SIGSTOP
  87 signals, the
  88 .Fn signal
  89 function allows for a signal to be caught, to be ignored, or to generate
  90 an interrupt.
  91 These signals are defined in the file
  92 .Aq Pa signal.h :
  93 .Bl -column SIGVTALARMXX "create core imagexxx"
  94 .It Sy "Name    Default Action  Description"
  95 .It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
  96 .It Dv SIGINT Ta "terminate process" Ta "interrupt program"
  97 .It Dv SIGQUIT Ta "create core image" Ta "quit program"
  98 .It Dv SIGILL Ta "create core image" Ta "illegal instruction"
  99 .It Dv SIGTRAP Ta "create core image" Ta "trace trap"
 100 .It Dv SIGABRT Ta "create core image" Ta "abort program"
 101 (formerly
 102 .Dv SIGIOT )
 103 .It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
 104 .It Dv SIGFPE Ta "create core image" Ta "floating-point exception"
 105 .It Dv SIGKILL Ta "terminate process" Ta "kill program"
 106 .It Dv SIGBUS Ta "create core image" Ta "bus error"
 107 .It Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
 108 .It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
 109 .It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
 110 .It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
 111 .It Dv SIGTERM Ta "terminate process" Ta "software termination signal"
 112 .It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
 113 .It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
 114 .It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
 115 .It Dv SIGCONT Ta "discard signal" Ta "continue after stop"
 116 .It Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
 117 .It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal"
 118 .It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal"
 119 .It Dv SIGIO Ta "discard signal" Ta Tn "I/O"
 120 is possible on a descriptor (see
 121 .Xr fcntl 2 )
 122 .It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
 123 .Xr setrlimit 2 )
 124 .It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
 125 .Xr setrlimit 2 )
 126 .It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
 127 .Xr setitimer 2 )
 128 .It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
 129 .Xr setitimer 2 )
 130 .It Dv SIGWINCH Ta "discard signal" Ta "Window size change"
 131 .It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
 132 .It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
 133 .It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
 134 .El
 135 .Pp
 136 The
 137 .Fa sig
 138 parameter specifies which signal was received.
 139 The
 140 .Fa func
 141 procedure allows a user to choose the action upon receipt of a signal.
 142 To set the default action of the signal to occur as listed above,
 143 .Fa func
 144 should be
 145 .Dv SIG_DFL .
 146 A
 147 .Dv SIG_DFL
 148 resets the default action.
 149 To ignore the signal
 150 .Fa func
 151 should be
 152 .Dv SIG_IGN .
 153 This will cause subsequent instances of the signal to be ignored
 154 and pending instances to be discarded.
 155 If
 156 .Dv SIG_IGN
 157 is not used,
 158 further occurrences of the signal are
 159 automatically blocked and
 160 .Fa func
 161 is called.
 162 .Pp
 163 The handled signal is unblocked when the
 164 function returns and
 165 the process continues from where it left off when the signal occurred.
 166 .Bf -symbolic
 167 Unlike previous signal facilities, the handler
 168 func() remains installed after a signal has been delivered.
 169 .Ef
 170 .Pp
 171 For some system calls, if a signal is caught while the call is
 172 executing and the call is prematurely terminated,
 173 the call is automatically restarted.
 174 (The handler is installed using the
 175 .Dv SA_RESTART
 176 flag with
 177 .Xr sigaction 2 . )
 178 The affected system calls include
 179 .Xr read 2 ,
 180 .Xr write 2 ,
 181 .Xr sendto 2 ,
 182 .Xr recvfrom 2 ,
 183 .Xr sendmsg 2
 184 and
 185 .Xr recvmsg 2
 186 on a communications channel or a low speed device
 187 and during a
 188 .Xr ioctl 2
 189 or
 190 .Xr wait 2 .
 191 However, calls that have already committed are not restarted,
 192 but instead return a partial success (for example, a short read count).
 193 This semantics could be changed with
 194 .Xr siginterrupt 3 .
 195 .Pp
 196 When a process which has installed signal handlers forks,
 197 the child process inherits the signals.
 198 All caught signals may be reset to their default action by a call
 199 to the
 200 .Xr execve 2
 201 function;
 202 ignored signals remain ignored.
 203 .Pp
 204 See
 205 .Xr sigaction 2
 206 for a list of functions
 207 that are considered safe for use in signal handler.
 208 .Sh RETURN VALUES
 209 The previous action is returned on a successful call.
 210 Otherwise, SIG_ERR is returned and  the global variable
 211 .Va errno
 212 is set to indicate the error.
 213 .Sh ERRORS
 214 .Fn Signal
 215 will fail and no action will take place if one of the
 216 following occur:
 217 .Bl -tag -width Er
 218 .It Bq Er EINVAL
 219 .Em Sig
 220 is not a valid signal number.
 221 .It Bq Er EINVAL
 222 An attempt is made to ignore or supply a handler for
 223 .Dv SIGKILL
 224 or
 225 .Ev SIGSTOP .
 226 .El
 227 .Sh SEE ALSO
 228 .Xr kill 1 ,
 229 .Xr kill 2 ,
 230 .Xr ptrace 2 ,
 231 .Xr sigaction 2 ,
 232 .Xr sigaltstack 2 ,
 233 .Xr sigprocmask 2 ,
 234 .Xr sigsuspend 2 ,
 235 .Xr fpsetmask 3 ,
 236 .Xr setjmp 3 ,
 237 .Xr siginterrupt 3 ,
 238 .Xr tty 4
 239 .Sh HISTORY
 240 This
 241 .Fn signal
 242 facility appeared in
 243 .Bx 4.0 .