SIGNAL(3C)

NAME
signal - simplified software signal facilities

SYNOPSIS
#include <signal.h>

(*signal(sig, func))()
int (*func)();

DESCRIPTION
Signal is a simplified interface to the more general sigvec(2) facility.

A signal is generated by some abnormal event, initiated by a user at a terminal (quit, interrupt, stop), by a program error (bus error, etc.), by request of another program (kill), or when a process is stopped because it wishes to access its control terminal while in the background (see tty(4)). Signals are optionally generated when a process resumes after being stopped, when the status of child processes changes, or when input is ready at the control terminal. Most signals cause termination of the receiving process if no action is taken; some signals instead cause the process receiving them to be stopped, or are simply discarded if the process has not requested otherwise. Except for the SIGKILL and SIGSTOP signals, the signal call allows signals either to be ignored or to cause an interrupt to a specified location. The following is a list of all signals with names as in the include file <signal.h>:

SIGHUP	1	hangup
SIGINT	2	interrupt
SIGQUIT	3*	quit
SIGILL	4*	illegal instruction
SIGTRAP	5*	trace trap
SIGIOT	6*	IOT instruction
SIGEMT	7*	EMT instruction
SIGFPE	8*	floating point exception
SIGKILL	9	kill (cannot be caught or ignored)
SIGBUS	10*	bus error
SIGSEGV	11*	segmentation violation
SIGSYS	12*	bad argument to system call
SIGPIPE	13	write on a pipe with no one to read it
SIGALRM	14	alarm clock
SIGTERM	15	software termination signal
SIGURG	16@	urgent condition present on socket
SIGSTOP	17'|+'	stop (cannot be caught or ignored)
SIGTSTP	18'|+'	stop signal generated from keyboard
SIGCONT	19@	continue after stop
SIGCHLD	20@	child status has changed
SIGTTIN	21'|+'	background read attempted from control terminal
SIGTTOU	22'|+'	background write attempted to control terminal
SIGIO	23@	i/o is possible on a descriptor (see fcntl(2))
SIGXCPU	24	cpu time limit exceeded (see setrlimit(2))
SIGXFSZ	25	file size limit exceeded (see setrlimit(2))
SIGVTALRM	26	virtual time alarm (see setitimer(2))
SIGPROF	27	profiling timer alarm (see setitimer(2))
SIGWINCH	28@	Window size change
SIGUSR1	30	User defined signal 1
SIGUSR2	31	User defined signal 2

The starred signals in the list above cause a core image if not caught or ignored.

If func is SIG_DFL, the default action for signal sig is reinstated; this default is termination (with a core image for starred signals) except for signals marked with @ or '|+'. Signals marked with @ are discarded if the action is SIG_DFL; signals marked with '|+' cause the process to stop. If func is SIG_IGN the signal is subsequently ignored and pending instances of the signal are discarded. Otherwise, when the signal occurs further occurrences of the signal are automatically blocked and func is called.

A return from the function unblocks the handled signal and continues the process at the point it was interrupted. Unlike previous signal facilities, the handler func remains installed after a signal has been delivered.

If a caught signal occurs during certain system calls, causing the call to terminate prematurely, the call is automatically restarted. In particular this can occur during a read or write(2) on a slow device (such as a terminal; but not a file) and during a wait(2).

The value of signal is the previous (or initial) value of func for the particular signal.

After a fork(2) or vfork(2) the child inherits all signals. Execve(2) resets all caught signals to the default action; ignored signals remain ignored.

RETURN VALUE
The previous action is returned on a successful call. Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS
Signal will fail and no action will take place if one of the following occur:

[EINVAL]
    Sig is not a valid signal number.

[EINVAL]
    An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

[EINVAL]
    An attempt is made to ignore SIGCONT (by default SIGCONT is ignored).

SEE ALSO
kill(1), ptrace(2), kill(2), sigvec(2), sigblock(2), sigsetmask(2), sigpause(2), sigstack(2), setjmp(3), tty(4)

NOTES (VAX-11)
The handler routine can be declared:

    handler(sig, code, scp)

Here sig is the signal number, into which the hardware faults and traps are mapped as defined below. Code is a parameter which is either a constant as given below or, for compatibility mode faults, the code provided by the hardware. Scp is a pointer to the struct sigcontext used by the system to restore the process context from before the signal. Compatibility mode faults are distinguished from the other SIGILL traps by having PSL_CM set in the psl.

The following defines the mapping of hardware traps to signals and codes. All of these symbols are defined in <signal.h>:

   Hardware condition	Signal	Code
Arithmetic traps:
   Integer overflow	SIGFPE	FPE_INTOVF_TRAP
   Integer division by zero	SIGFPE	FPE_INTDIV_TRAP
   Floating overflow trap	SIGFPE	FPE_FLTOVF_TRAP
   Floating/decimal division by zero	SIGFPE	FPE_FLTDIV_TRAP
   Floating underflow trap	SIGFPE	FPE_FLTUND_TRAP
   Decimal overflow trap	SIGFPE	FPE_DECOVF_TRAP
   Subscript-range	SIGFPE	FPE_SUBRNG_TRAP
   Floating overflow fault	SIGFPE	FPE_FLTOVF_FAULT
   Floating divide by zero fault	SIGFPE	FPE_FLTDIV_FAULT
   Floating underflow fault	SIGFPE	FPE_FLTUND_FAULT
Length access control	SIGSEGV
Protection violation	SIGBUS
Reserved instruction	SIGILL	ILL_RESAD_FAULT
Customer-reserved instr.	SIGEMT
Reserved operand	SIGILL	ILL_PRIVIN_FAULT
Reserved addressing	SIGILL	ILL_RESOP_FAULT
Trace pending	SIGTRAP
Bpt instruction	SIGTRAP
Compatibility-mode	SIGILL	hardware supplied code
Chme	SIGSEGV
Chms	SIGSEGV
Chmu	SIGSEGV

NOTES (PDP-11)
The handler routine can be declared:

    handler(sig, code, scp)
    int sig, code;
    struct sigcontext *scp;

Here sig is the signal number, into which the hardware faults and traps are mapped as defined below. Code is a parameter that is a constant as given below. Scp is a pointer to the sigcontext structure (defined in <signal.h>), used to restore the context from before the signal.

The following defines the mapping of hardware traps to signals and codes. All of these symbols are defined in <signal.h>:

   Hardware condition	Signal	Code
Arithmetic traps:
   Floating overflow trap	SIGFPE	FPE_FLTOVF_TRAP
   Floating/decimal division by zero	SIGFPE	FPE_FLTDIV_TRAP
   Floating underflow trap	SIGFPE	FPE_FLTUND_TRAP
   Decimal overflow trap	SIGFPE	FPE_DECOVF_TRAP
   Illegal return code	SIGFPE	FPE_CRAZY
   Bad op code	SIGFPE	FPE_OPCODE_TRAP
   Bad operand	SIGFPE	FPE_OPERAND_TRAP
   Maintenance trap	SIGFPE	FPE_MAINT_TRAP
Length access control	SIGSEGV
Protection violation (odd address)	SIGBUS
Reserved instruction	SIGILL	ILL_RESAD_FAULT
Customer-reserved instr.	SIGEMT
Trace pending	SIGTRAP
Bpt instruction	SIGTRAP

The handler routine must save any registers it uses and restore them before returning. On the PDP-11, the kernel saves r0 and r1 before calling the handler routine, but expect the handler to save any other registers it uses. The standard entry code generated by the C compiler for handler routines written in C automatically saves the remaining general registers, but floating point registers are not saved. As a result there is currently no [standard] method for a handler routine written in C to perform floating point operations without blowing the interrupted program out of the water.