SIGNAL(3C)

NAME
signal - simplified software signal facilities (obsolete)

SYNOPSIS
#include <signal.h>

/* BSD interface */
int (*signal(sig, func))()
int (*func)();

/* X/Open interface */
void (*signal(sig, func))()
void (*func)();

DESCRIPTION
Signal is a simplified interface to the more general sigvec(2) (BSD) or sigaction(3) (X/Open) signal control facilities.

The underlying implementation of signals now conforms to the POSIX and X/Open specifications; these vary in a few small details from the original BSD signal scheme, but for most programs there is no need to be aware of the differences.

A signal is generated by some abnormal event, initiated by a user at a terminal (quit, interrupt, stop), by a program error (illegal memory access, 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
SIGABRT	6*	process abort
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
SIGLOST	29	resource lost (eg, record-lock lost)
SIGUSR1	30	User defined signal 1
SIGUSR2	31	User defined signal 2

SIGABRT is generated by the X/Open version of abort(3), although the BSD version uses SIGILL, for backwards compatibility. Also for backwards compatibility, SIGIOT is defined in <signal.h>, as a synonym for SIGABRT.

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 signal which is caught occurs during certain ``slow'' system calls, causing the call to terminate prematurely, the subsequent treatment of the call is determined by whether the BSD or X/Open version of signal was used to set up the handler. This is of relevance for a read or write(2) on a slow device (such as a terminal; but not a file) and during a wait(2). The call will be restarted automatically if the BSD version of signal is used. Otherwise the call will not be restarted: in some cases it will return -1 and set errno to EINTR, otherwise (read or write on a ``slow'' device when some data has already been transferred) it will return the count of bytes transferred up to the time of the signal.

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.

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

NOTES
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 a constant as given below. Scp is a pointer to the struct sigcontext used by the system to restore the process context from before the signal.

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

   Hardware condition	Signal	Code
Arithmetic traps:
   Inexact result	SIGFPE	FPE_INEXACT
   Floating underflow	SIGFPE	FPE_FLTUND_TRAP
   Floating overflow 	SIGFPE	FPE_FLTOVF_TRAP
   Floating divide by zero	SIGFPE	FPE_FLTDIV_TRAP
   Floating invalid operand	SIGFPE	FPE_INVALID_OP
Prefetch abort	SIGSEGV
Data abort	SIGSEGV
Address exception	SIGBUS
Undefined instruction	SIGILL
Trace pending	SIGTRAP
Bpt instruction	SIGTRAP

Note that not all occurences of traps are seen by the user process. Notably, prefetch and data aborts happen in the normal course of program execution in virtual memory, but only where such aborts are the result of an inappropriate access to a piece of virtual address space will they be passed through as signals. Most traps occur synchronously with instruction execution: in such cases the value of the program counter seen by the trap handler is the address of the instruction which caused the problem. Only with hardware floating point implementations can some (floating point) instructions cause traps to be generated an arbitary time after the main cpu has moved on, since the FPU can execute in parallel with the ARM.

The handler should take the greatest care over the functions which it calls and the method which it uses to return to the interrupted code - see the NOTES in sigvec(2).