1SIGNAL(3C) SIGNAL(3C)
2
6 signal - simplified software signal facilities
7
9 #include <signal.h>
10 (*signal(sig, func))()
12 int (*func)();
13
15 Signal is a simplified interface to the more general sigvec(2) facil‐
16 ity.
17 A signal is generated by some abnormal event, initiated by a user at a
19 terminal (quit, interrupt, stop), by a program error (bus error, etc.),
20 by request of another program (kill), or when a process is stopped
21 because it wishes to access its control terminal while in the back‐
22 ground (see tty(4)). Signals are optionally generated when a process
23 resumes after being stopped, when the status of child processes
24 changes, or when input is ready at the control terminal. Most signals
25 cause termination of the receiving process if no action is taken; some
26 signals instead cause the process receiving them to be stopped, or are
27 simply discarded if the process has not requested otherwise. Except
28 for the SIGKILL and SIGSTOP signals, the signal call allows signals
29 either to be ignored or to cause an interrupt to a specified location.
30 The following is a list of all signals with names as in the include
31 file <signal.h>:
32 SIGHUP 1 hangup
34 SIGINT 2 interrupt
35 SIGQUIT 3* quit
36 SIGILL 4* illegal instruction
37 SIGTRAP 5* trace trap
38 SIGIOT 6* IOT instruction
39 SIGEMT 7* EMT instruction
40 SIGFPE 8* floating point exception
41 SIGKILL 9 kill (cannot be caught or ignored)
42 SIGBUS 10* bus error
43 SIGSEGV 11* segmentation violation
44 SIGSYS 12* bad argument to system call
45 SIGPIPE 13 write on a pipe with no one to read it
46 SIGALRM 14 alarm clock
47 SIGTERM 15 software termination signal
48 SIGURG 16@ urgent condition present on socket
49 SIGSTOP 17'|+'stop (cannot be caught or ignored)
50 SIGTSTP 18'|+'stop signal generated from keyboard
51 SIGCONT 19@ continue after stop
52 SIGCHLD 20@ child status has changed
53 SIGTTIN 21'|+'background read attempted from control terminal
54 SIGTTOU 22'|+'background write attempted to control terminal
55 SIGIO 23@ i/o is possible on a descriptor (see fcntl(2))
56 SIGXCPU 24 cpu time limit exceeded (see setrlimit(2))
57 SIGXFSZ 25 file size limit exceeded (see setrlimit(2))
58 SIGVTALRM 26 virtual time alarm (see setitimer(2))
59 SIGPROF 27 profiling timer alarm (see setitimer(2))
60 SIGWINCH 28@ Window size change
61 SIGUSR1 30 User defined signal 1
62 SIGUSR2 31 User defined signal 2
63 The starred signals in the list above cause a core image if not caught
65 or ignored.
66 If func is SIG_DFL, the default action for signal sig is reinstated;
68 this default is termination (with a core image for starred signals)
69 except for signals marked with @ or '|+'. Signals marked with @ are
70 discarded if the action is SIG_DFL; signals marked with '|+' cause the
71 process to stop. If func is SIG_IGN the signal is subsequently ignored
72 and pending instances of the signal are discarded. Otherwise, when the
73 signal occurs further occurrences of the signal are automatically
74 blocked and func is called.
75 A return from the function unblocks the handled signal and continues
77 the process at the point it was interrupted. Unlike previous signal
78 facilities, the handler func remains installed after a signal has been
79 delivered.
80 If a caught signal occurs during certain system calls, causing the call
82 to terminate prematurely, the call is automatically restarted. In par‐
83 ticular this can occur during a read or write(2) on a slow device (such
84 as a terminal; but not a file) and during a wait(2).
85 The value of signal is the previous (or initial) value of func for the
87 particular signal.
88 After a fork(2) or vfork(2) the child inherits all signals. Execve(2)
90 resets all caught signals to the default action; ignored signals remain
91 ignored.
92
94 The previous action is returned on a successful call. Otherwise, -1 is
95 returned and errno is set to indicate the error.
96
98 Signal will fail and no action will take place if one of the following
99 occur:
100 [EINVAL] Sig is not a valid signal number.
102 [EINVAL] An attempt is made to ignore or supply a handler for
104 SIGKILL or SIGSTOP.
105 [EINVAL] An attempt is made to ignore SIGCONT (by default SIGCONT
107 is ignored).
108
110 kill(1), ptrace(2), kill(2), sigvec(2), sigblock(2), sigsetmask(2),
111 sigpause(2), sigstack(2), setjmp(3), tty(4)
112
114 The handler routine can be declared:
115 handler(sig, code, scp)
117 Here sig is the signal number, into which the hardware faults and traps
119 are mapped as defined below. Code is a parameter which is either a
120 constant as given below or, for compatibility mode faults, the code
121 provided by the hardware. Scp is a pointer to the struct sigcontext
122 used by the system to restore the process context from before the sig‐
123 nal. Compatibility mode faults are distinguished from the other SIGILL
124 traps by having PSL_CM set in the psl.
125 The following defines the mapping of hardware traps to signals and
127 codes. All of these symbols are defined in <signal.h>:
128 Hardware condition Signal Code
130 Arithmetic traps:
132 Integer overflow SIGFPE FPE_INTOVF_TRAP
133 Integer division by zero SIGFPE FPE_INTDIV_TRAP
134 Floating overflow trap SIGFPE FPE_FLTOVF_TRAP
135 Floating/decimal division by zero SIGFPE FPE_FLTDIV_TRAP
136 Floating underflow trap SIGFPE FPE_FLTUND_TRAP
137 Decimal overflow trap SIGFPE FPE_DECOVF_TRAP
138 Subscript-range SIGFPE FPE_SUBRNG_TRAP
139 Floating overflow fault SIGFPE FPE_FLTOVF_FAULT
140 Floating divide by zero fault SIGFPE FPE_FLTDIV_FAULT
141 Floating underflow fault SIGFPE FPE_FLTUND_FAULT
142 Length access control SIGSEGV
143 Protection violation SIGBUS
144 Reserved instruction SIGILL ILL_RESAD_FAULT
145 Customer-reserved instr. SIGEMT
146 Reserved operand SIGILL ILL_PRIVIN_FAULT
147 Reserved addressing SIGILL ILL_RESOP_FAULT
148 Trace pending SIGTRAP
149 Bpt instruction SIGTRAP
150 Compatibility-mode SIGILL hardware supplied code
151 Chme SIGSEGV
152 Chms SIGSEGV
153 Chmu SIGSEGV
154
156 The handler routine can be declared:
157 handler(sig, code, scp)
159 int sig, code;
160 struct sigcontext *scp;
161 Here sig is the signal number, into which the hardware faults and traps
163 are mapped as defined below. Code is a parameter that is a constant as
164 given below. Scp is a pointer to the sigcontext structure (defined in
165 <signal.h>), used to restore the context from before the signal.
166 The following defines the mapping of hardware traps to signals and
168 codes. All of these symbols are defined in <signal.h>:
169 Hardware condition Signal Code
171 Arithmetic traps:
173 Floating overflow trap SIGFPE FPE_FLTOVF_TRAP
174 Floating/decimal division by zero SIGFPE FPE_FLTDIV_TRAP
175 Floating underflow trap SIGFPE FPE_FLTUND_TRAP
176 Decimal overflow trap SIGFPE FPE_DECOVF_TRAP
177 Illegal return code SIGFPE FPE_CRAZY
178 Bad op code SIGFPE FPE_OPCODE_TRAP
179 Bad operand SIGFPE FPE_OPERAND_TRAP
180 Maintenance trap SIGFPE FPE_MAINT_TRAP
181 Length access control SIGSEGV
182 Protection violation (odd address) SIGBUS
183 Reserved instruction SIGILL ILL_RESAD_FAULT
184 Customer-reserved instr. SIGEMT
185 Trace pending SIGTRAP
186 Bpt instruction SIGTRAP
187 The handler routine must save any registers it uses and restore them
189 before returning. On the PDP-11, the kernel saves r0 and r1 before
190 calling the handler routine, but expect the handler to save any other
191 registers it uses. The standard entry code generated by the C compiler
192 for handler routines written in C automatically saves the remaining
193 general registers, but floating point registers are not saved. As a
194 result there is currently no [standard] method for a handler routine
195 written in C to perform floating point operations without blowing the
196 interrupted program out of the water.
1974th Berkeley Distribution May 20, 1986 SIGNAL(3C)