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28 .\" @(#)signal.3 8.3 (Berkeley) 4/19/94
36 .Nd simplified software signal facilities
41 .\" The following is Quite Ugly, but syntactically correct.
42 .\" Don't try to fix it.
44 .Fn \*(lp*signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint"
48 equivalent but easier to read typedef'd version:
49 .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 No ".Dv SIGVTALRM" "create core image"
92 .It Sy "Num" Ta Sy "Name" Ta Sy "Default Action" Ta Sy "Description"
93 .It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
94 .It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program"
95 .It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program"
96 .It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction"
97 .It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap"
98 .It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program"
101 .It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
102 .It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception"
103 .It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program"
104 .It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error"
105 .It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
106 .It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
107 .It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
108 .It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
109 .It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal"
110 .It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
111 .It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
112 .It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
113 .It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop"
114 .It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
115 .It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from"
117 .It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to"
119 .It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O"
120 is possible on a descriptor (see
122 .It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
124 .It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
126 .It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
128 .It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
130 .It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change"
131 .It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
132 .It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
133 .It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
134 .It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt"
135 .It 33 Ta Dv SIGLIBRT Ta "terminate process" Ta "real-time library interrupt"
140 argument specifies which signal was received.
143 procedure allows a user to choose the action upon receipt of a signal.
144 To set the default action of the signal to occur as listed above,
150 resets the default action.
155 This will cause subsequent instances of the signal to be ignored
156 and pending instances to be discarded.
160 further occurrences of the signal are
161 automatically blocked and
165 The handled signal is unblocked when the
167 the process continues from where it left off when the signal occurred.
169 Unlike previous signal facilities, the handler
170 func() remains installed after a signal has been delivered.
173 For some system calls, if a signal is caught while the call is
174 executing and the call is prematurely terminated,
175 the call is automatically restarted.
176 Any handler installed with
180 flag set, meaning that any restartable system call will not return on
182 The affected system calls include
190 on a communications channel or a low speed device
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
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
206 ignored signals remain ignored.
208 If a process explicitly specifies
210 as the action for the signal
212 the system will not create zombie processes when children
213 of the calling process exit.
214 As a consequence, the system will discard the exit status
215 from the child processes.
216 If the calling process subsequently issues a call to
218 or equivalent, it will block until all of the calling process's
219 children terminate, and then return a value of \-1 with
226 for a list of functions
227 that are considered safe for use in signal handlers.
229 The previous action is returned on a successful call.
230 Otherwise, SIG_ERR is returned and the global variable
232 is set to indicate the error.
237 will fail and no action will take place if one of the
244 is not a valid signal number.
246 An attempt is made to ignore or supply a handler for
269 The option to avoid the creation of child zombies through ignoring