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32 .\" @(#)signal.3 8.3 (Berkeley) 4/19/94
40 .Nd simplified software signal facilities
45 .\" XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX
46 .\" The prototype for signal(3) cannot be cleanly marked up in -mdoc
47 .\" without the following lower-level tweak.
48 .nr in-synopsis-section 0
53 .Fa "void \*(lp*func\*(rp\*(lpint\*(rp"
55 .Ft "\*(rp\*(lpint\*(rp" ;
57 .nr in-synopsis-section 1
58 .\" XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX
61 equivalent but easier to read typedef'd version:
62 .Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
64 .Fn signal "int sig" "sig_t func"
69 is a simplified interface to the more general
73 Signals allow the manipulation of a process from outside its
74 domain as well as allowing the process to manipulate itself or
75 copies of itself (children).
76 There are two general types of signals:
77 those that cause termination of a process and those that do not.
78 Signals which cause termination of a program might result from
79 an irrecoverable error or might be the result of a user at a terminal
80 typing the `interrupt' character.
81 Signals are used when a process is stopped because it wishes to access
82 its control terminal while in the background (see
84 Signals are optionally generated
85 when a process resumes after being stopped,
86 when the status of child processes changes,
87 or when input is ready at the control terminal.
88 Most signals result in the termination of the process receiving them
90 is taken; some signals instead cause the process receiving them
91 to be stopped, or are simply discarded if the process has not
99 function allows for a signal to be caught, to be ignored, or to generate
101 These signals are defined in the file
103 .Bl -column No ".Dv SIGVTALRM" "create core image"
104 .It Sy "No Name Default Action Description"
105 .It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
106 .It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program"
107 .It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program"
108 .It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction"
109 .It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap"
110 .It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program"
113 .It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
114 .It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception"
115 .It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program"
116 .It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error"
117 .It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
118 .It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
119 .It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
120 .It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
121 .It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal"
122 .It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
123 .It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
124 .It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
125 .It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop"
126 .It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
127 .It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from"
129 .It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to"
131 .It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O"
132 is possible on a descriptor (see
134 .It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
136 .It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
138 .It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
140 .It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
142 .It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change"
143 .It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
144 .It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
145 .It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
146 .It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt"
151 argument specifies which signal was received.
154 procedure allows a user to choose the action upon receipt of a signal.
155 To set the default action of the signal to occur as listed above,
161 resets the default action.
166 This will cause subsequent instances of the signal to be ignored
167 and pending instances to be discarded.
171 further occurrences of the signal are
172 automatically blocked and
176 The handled signal is unblocked when the
178 the process continues from where it left off when the signal occurred.
180 Unlike previous signal facilities, the handler
181 func() remains installed after a signal has been delivered.
184 For some system calls, if a signal is caught while the call is
185 executing and the call is prematurely terminated,
186 the call is automatically restarted.
187 (The handler is installed using the
191 The affected system calls include
199 on a communications channel or a low speed device
204 However, calls that have already committed are not restarted,
205 but instead return a partial success (for example, a short read count).
206 These semantics could be changed with
209 When a process which has installed signal handlers forks,
210 the child process inherits the signals.
211 All caught signals may be reset to their default action by a call
215 ignored signals remain ignored.
217 If a process explicitly specifies
219 as the action for the signal
221 the system will not create zombie processes when children
222 of the calling process exit.
223 As a consequence, the system will discard the exit status
224 from the child processes.
225 If the calling process subsequently issues a call to
227 or equivalent, it will block until all of the calling process's
228 children terminate, and then return a value of \-1 with
235 for a list of functions
236 that are considered safe for use in signal handlers.
238 The previous action is returned on a successful call.
239 Otherwise, SIG_ERR is returned and the global variable
241 is set to indicate the error.
246 will fail and no action will take place if one of the
253 is not a valid signal number.
255 An attempt is made to ignore or supply a handler for
278 The option to avoid the creation of child zombies through ignoring