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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"
139 argument specifies which signal was received.
142 procedure allows a user to choose the action upon receipt of a signal.
143 To set the default action of the signal to occur as listed above,
149 resets the default action.
154 This will cause subsequent instances of the signal to be ignored
155 and pending instances to be discarded.
159 further occurrences of the signal are
160 automatically blocked and
164 The handled signal is unblocked when the
166 the process continues from where it left off when the signal occurred.
168 Unlike previous signal facilities, the handler
169 func() remains installed after a signal has been delivered.
172 For some system calls, if a signal is caught while the call is
173 executing and the call is prematurely terminated,
174 the call is automatically restarted.
175 Any handler installed with
179 flag set, meaning that any restartable system call will not return on
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.
207 If a process explicitly specifies
209 as the action for the signal
211 the system will not create zombie processes when children
212 of the calling process exit.
213 As a consequence, the system will discard the exit status
214 from the child processes.
215 If the calling process subsequently issues a call to
217 or equivalent, it will block until all of the calling process's
218 children terminate, and then return a value of \-1 with
225 for a list of functions
226 that are considered safe for use in signal handlers.
228 The previous action is returned on a successful call.
229 Otherwise, SIG_ERR is returned and the global variable
231 is set to indicate the error.
236 will fail and no action will take place if one of the
243 is not a valid signal number.
245 An attempt is made to ignore or supply a handler for
268 The option to avoid the creation of child zombies through ignoring