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32 .\" @(#)signal.3 8.3 (Berkeley) 4/19/94
33 .\" $FreeBSD: src/lib/libc/gen/signal.3,v 1.17.2.9 2003/03/13 18:05:37 trhodes Exp $
34 .\" $DragonFly: src/lib/libc/gen/signal.3,v 1.6 2007/09/08 20:50:49 swildner Exp $
35 .\"
36 .Dd April 19, 1994
37 .Dt SIGNAL 3
38 .Os
39 .Sh NAME
40 .Nm signal
41 .Nd simplified software signal facilities
42 .Sh LIBRARY
43 .Lb libc
44 .Sh SYNOPSIS
45 .In signal.h
46 .\" The following is Quite Ugly, but syntactically correct. 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 .Dx 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 ".Dv SIGCKPTEXIT" "create core imagexxx"
96 .It Sy "Name Default Action Description"
97 .It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
98 .It Dv SIGINT Ta "terminate process" Ta "interrupt program"
99 .It Dv SIGQUIT Ta "create core image" Ta "quit program"
100 .It Dv SIGILL Ta "create core image" Ta "illegal instruction"
101 .It Dv SIGTRAP Ta "create core image" Ta "trace trap"
102 .It Dv SIGABRT Ta "create core image" Ta "abort program"
103 (formerly
104 .Dv SIGIOT )
105 .It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
106 .It Dv SIGFPE Ta "create core image" Ta "floating-point exception"
107 .It Dv SIGKILL Ta "terminate process" Ta "kill program"
108 .It Dv SIGBUS Ta "create core image" Ta "bus error"
109 .It Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
110 .It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
111 .It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
112 .It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
113 .It Dv SIGTERM Ta "terminate process" Ta "software termination signal"
114 .It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
115 .It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
116 .It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
117 .It Dv SIGCONT Ta "discard signal" Ta "continue after stop"
118 .It Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
119 .It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal"
120 .It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal"
121 .It Dv SIGIO Ta "discard signal" Ta Tn "I/O"
122 is possible on a descriptor (see
123 .Xr fcntl 2 )
124 .It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
125 .Xr setrlimit 2 )
126 .It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
127 .Xr setrlimit 2 )
128 .It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
129 .Xr setitimer 2 )
130 .It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
131 .Xr setitimer 2 )
132 .It Dv SIGWINCH Ta "discard signal" Ta "Window size change"
133 .It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
134 .It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
135 .It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
136 .It Dv SIGCKPT Ta "checkpoint process" Ta "Checkpoint"
137 .It Dv SIGCKPTEXIT Ta "terminate process" Ta "Checkpoint and exit"
138 .El
139 .Pp
140 The
141 .Fa sig
142 argument specifies which signal was received.
143 The
144 .Fa func
145 procedure allows a user to choose the action upon receipt of a signal.
146 To set the default action of the signal to occur as listed above,
147 .Fa func
148 should be
149 .Dv SIG_DFL .
150 A
151 .Dv SIG_DFL
152 resets the default action.
153 To ignore the signal
154 .Fa func
155 should be
156 .Dv SIG_IGN .
157 This will cause subsequent instances of the signal to be ignored
158 and pending instances to be discarded.
159 If
160 .Dv SIG_IGN
161 is not used,
162 further occurrences of the signal are
163 automatically blocked and
164 .Fa func
165 is called.
166 .Pp
167 The handled signal is unblocked when the
168 function returns and
169 the process continues from where it left off when the signal occurred.
170 .Bf -symbolic
171 Unlike previous signal facilities, the handler
172 func() remains installed after a signal has been delivered.
173 .Ef
174 .Pp
175 For some system calls, if a signal is caught while the call is
176 executing and the call is prematurely terminated,
177 the call is automatically restarted.
178 (The handler is installed using the
179 .Dv SA_RESTART
180 flag with
181 .Xr sigaction 2 . )
182 The affected system calls include
183 .Xr read 2 ,
184 .Xr write 2 ,
185 .Xr sendto 2 ,
186 .Xr recvfrom 2 ,
187 .Xr sendmsg 2
188 and
189 .Xr recvmsg 2
190 on a communications channel or a low speed device
191 and during a
192 .Xr ioctl 2
193 or
194 .Xr wait 2 .
195 However, calls that have already committed are not restarted,
196 but instead return a partial success (for example, a short read count).
197 These semantics could be changed with
198 .Xr siginterrupt 3 .
199 .Pp
200 When a process which has installed signal handlers forks,
201 the child process inherits the signals.
202 All caught signals may be reset to their default action by a call
203 to the
204 .Xr execve 2
205 function;
206 ignored signals remain ignored.
207 .Pp
208 See
209 .Xr sigaction 2
210 for a list of functions
211 that are considered safe for use in signal handlers.
212 .Sh RETURN VALUES
213 The previous action is returned on a successful call.
214 Otherwise,
215 .Dv SIG_ERR
216 is returned and the global variable
217 .Va errno
218 is set to indicate the error.
219 .Sh ERRORS
220 The
221 .Fn signal
222 function
223 will fail and no action will take place if one of the
224 following occur:
225 .Bl -tag -width Er
226 .It Bq Er EINVAL
227 The
228 .Fa sig
229 argument
230 is not a valid signal number.
231 .It Bq Er EINVAL
232 An attempt is made to ignore or supply a handler for
233 .Dv SIGKILL
234 or
235 .Dv SIGSTOP .
236 .El
237 .Sh SEE ALSO
238 .Xr kill 1 ,
239 .Xr kill 2 ,
240 .Xr ptrace 2 ,
241 .Xr sigaction 2 ,
242 .Xr sigaltstack 2 ,
243 .Xr sigprocmask 2 ,
244 .Xr sigsuspend 2 ,
245 .Xr fpsetmask 3 ,
246 .Xr setjmp 3 ,
247 .Xr siginterrupt 3 ,
248 .Xr tty 4
249 .Sh HISTORY
250 This
251 .Fn signal
252 facility appeared in
253 .Bx 4.0 .