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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 1.5 2006/05/26 21:39:36 swildner Exp $
41 .Nd simplified software signal facilities
46 .\" The following is Quite Ugly, but syntactically correct. Don't try to
49 .Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint"
53 equivalent but easier to read typedef'd version:
54 .Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
56 .Fn signal "int sig" "sig_t func"
61 is a simplified interface to the more general
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). There are two general types of signals:
68 those that cause termination of a process and those that do not.
69 Signals which cause termination of a program might result from
70 an irrecoverable error or might be the result of a user at a terminal
71 typing the `interrupt' character.
72 Signals are used when a process is stopped because it wishes to access
73 its control terminal while in the background (see
75 Signals are optionally generated
76 when a process resumes after being stopped,
77 when the status of child processes changes,
78 or when input is ready at the control terminal.
79 Most signals result in the termination of the process receiving them
81 is taken; some signals instead cause the process receiving them
82 to be stopped, or are simply discarded if the process has not
90 function allows for a signal to be caught, to be ignored, or to generate
92 These signals are defined in the file
94 .Bl -column SIGVTALARMXX "create core imagexxx"
95 .It Sy "Name Default Action Description"
96 .It Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
97 .It Dv SIGINT Ta "terminate process" Ta "interrupt program"
98 .It Dv SIGQUIT Ta "create core image" Ta "quit program"
99 .It Dv SIGILL Ta "create core image" Ta "illegal instruction"
100 .It Dv SIGTRAP Ta "create core image" Ta "trace trap"
101 .It Dv SIGABRT Ta "create core image" Ta "abort program"
104 .It Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
105 .It Dv SIGFPE Ta "create core image" Ta "floating-point exception"
106 .It Dv SIGKILL Ta "terminate process" Ta "kill program"
107 .It Dv SIGBUS Ta "create core image" Ta "bus error"
108 .It Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
109 .It Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
110 .It Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
111 .It Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
112 .It Dv SIGTERM Ta "terminate process" Ta "software termination signal"
113 .It Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
114 .It Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
115 .It Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
116 .It Dv SIGCONT Ta "discard signal" Ta "continue after stop"
117 .It Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
118 .It Dv SIGTTIN Ta "stop process" Ta "background read attempted from control terminal"
119 .It Dv SIGTTOU Ta "stop process" Ta "background write attempted to control terminal"
120 .It Dv SIGIO Ta "discard signal" Ta Tn "I/O"
121 is possible on a descriptor (see
123 .It Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
125 .It Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
127 .It Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
129 .It Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
131 .It Dv SIGWINCH Ta "discard signal" Ta "Window size change"
132 .It Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
133 .It Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
134 .It Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
135 .It Dv SIGCKPT Ta "checkpoint process" Ta "Checkpoint"
136 .It Dv SIGCKPTEXIT Ta "terminate process" Ta "Checkpoint and exit"
141 argument specifies which signal was received.
144 procedure allows a user to choose the action upon receipt of a signal.
145 To set the default action of the signal to occur as listed above,
151 resets the default action.
156 This will cause subsequent instances of the signal to be ignored
157 and pending instances to be discarded.
161 further occurrences of the signal are
162 automatically blocked and
166 The handled signal is unblocked when the
168 the process continues from where it left off when the signal occurred.
170 Unlike previous signal facilities, the handler
171 func() remains installed after a signal has been delivered.
174 For some system calls, if a signal is caught while the call is
175 executing and the call is prematurely terminated,
176 the call is automatically restarted.
177 (The handler is installed using the
181 The affected system calls include
189 on a communications channel or a low speed device
194 However, calls that have already committed are not restarted,
195 but instead return a partial success (for example, a short read count).
196 These semantics could be changed with
199 When a process which has installed signal handlers forks,
200 the child process inherits the signals.
201 All caught signals may be reset to their default action by a call
205 ignored signals remain ignored.
209 for a list of functions
210 that are considered safe for use in signal handlers.
212 The previous action is returned on a successful call.
213 Otherwise, SIG_ERR is returned and the global variable
215 is set to indicate the error.
220 will fail and no action will take place if one of the
227 is not a valid signal number.
229 An attempt is made to ignore or supply a handler for