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28 .\" @(#)signal.3 8.3 (Berkeley) 4/19/94
29 .\" $FreeBSD: src/lib/libc/gen/signal.3,v 1.17.2.9 2003/03/13 18:05:37 trhodes Exp $
30 .\" $DragonFly: src/lib/libc/gen/signal.3,v 1.6 2007/09/08 20:50:49 swildner Exp $
37 .Nd simplified software signal facilities
42 .\" The following is Quite Ugly, but syntactically correct. Don't try to
45 .Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint"
49 equivalent but easier to read typedef'd version:
50 .Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
52 .Fn signal "int sig" "sig_t func"
57 is a simplified interface to the more general
61 Signals allow the manipulation of a process from outside its
62 domain as well as allowing the process to manipulate itself or
63 copies of itself (children).
64 There are two general types of signals:
65 those that cause termination of a process and those that do not.
66 Signals which cause termination of a program might result from
67 an irrecoverable error or might be the result of a user at a terminal
68 typing the `interrupt' character.
69 Signals are used when a process is stopped because it wishes to access
70 its control terminal while in the background (see
72 Signals are optionally generated
73 when a process resumes after being stopped,
74 when the status of child processes changes,
75 or when input is ready at the control terminal.
76 Most signals result in the termination of the process receiving them
78 is taken; some signals instead cause the process receiving them
79 to be stopped, or are simply discarded if the process has not
87 function allows for a signal to be caught, to be ignored, or to generate
89 These signals are defined in the file
91 .Bl -column ".Dv SIGCKPTEXIT" "create core imagexxx"
92 .It Sy "Name Default Action Description"
93 .It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
94 .It Dv SIGINT Ta "terminate process" Ta "interrupt program"
95 .It Dv SIGQUIT Ta "create core image" Ta "quit program"
96 .It Dv SIGILL Ta "create core image" Ta "illegal instruction"
97 .It Dv SIGTRAP Ta "create core image" Ta "trace trap"
98 .It Dv SIGABRT Ta "create core image" Ta "abort program"
101 .It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
102 .It Dv SIGFPE Ta "create core image" Ta "floating-point exception"
103 .It Dv SIGKILL Ta "terminate process" Ta "kill program"
104 .It Dv SIGBUS Ta "create core image" Ta "bus error"
105 .It Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
106 .It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
107 .It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
108 .It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
109 .It Dv SIGTERM Ta "terminate process" Ta "software termination signal"
110 .It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
111 .It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
112 .It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
113 .It Dv SIGCONT Ta "discard signal" Ta "continue after stop"
114 .It Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
115 .It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal"
116 .It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal"
117 .It Dv SIGIO Ta "discard signal" Ta Tn "I/O"
118 is possible on a descriptor (see
120 .It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
122 .It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
124 .It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
126 .It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
128 .It Dv SIGWINCH Ta "discard signal" Ta "Window size change"
129 .It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
130 .It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
131 .It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
132 .It Dv SIGCKPT Ta "checkpoint process" Ta "Checkpoint"
133 .It Dv SIGCKPTEXIT Ta "terminate process" Ta "Checkpoint and exit"
138 argument specifies which signal was received.
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,
148 resets the default action.
153 This will cause subsequent instances of the signal to be ignored
154 and pending instances to be discarded.
158 further occurrences of the signal are
159 automatically blocked and
163 The handled signal is unblocked when the
165 the process continues from where it left off when the signal occurred.
167 Unlike previous signal facilities, the handler
168 func() remains installed after a signal has been delivered.
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
178 The affected system calls include
186 on a communications channel or a low speed device
191 However, calls that have already committed are not restarted,
192 but instead return a partial success (for example, a short read count).
193 These semantics could be changed with
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
202 ignored signals remain ignored.
206 for a list of functions
207 that are considered safe for use in signal handlers.
209 The previous action is returned on a successful call.
212 is returned and the global variable
214 is set to indicate the error.
219 will fail and no action will take place if one of the
226 is not a valid signal number.
228 An attempt is made to ignore or supply a handler for