gno/usr.man/man2/signal.2

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.\"     @(#)signal.3	8.3 (Berkeley) 4/19/94
.\"
.TH SIGNAL 3 "19 January 1997" GNO "System Calls"
.SH NAME
.BR signal
\- simplified software signal facilities
.SH SYNOPSIS
.br
#include <signal.h>
.sp 1
void (*\fBsignal\fR (int \fIsig\fR, void (*\fIfunc\fR)(int, int)))(int, int);
.sp 1
or in GNO's equivalent but easier to read typedef'd version:
.sp 1
typedef void (*sig_t) (int, int)
.sp 1
sig_t
\fBsignal\fR(int \fIsig\fR, sig_t \fIfunc\fR);
.SH DESCRIPTION
Signals allow the manipulation of a process from outside its
domain as well as allowing the process to manipulate itself or
copies of itself (children). There are two general types of signals:
those that cause termination of a process and those that do not.
Signals which cause termination of a program might result from
an irrecoverable error or might be the result of a user at a terminal
typing the `interrupt' character.
Signals are used when a process is stopped because it wishes to access
its control terminal while in the background (see
.BR 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.
.LP
Most signals result in the termination of the process receiving them
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.
The
.BR signal 
function allows for a signal to be caught, to be ignored, or to generate
an interrupt, except for
.BR SIGCONT
(which cannot be blocked), and
.BR SIGKILL
and
.BR SIGSTOP 
(which cannot be caught, blocked, or ignored).
.LP
These signals are defined in the file <signal.h>:
.RS
.nf

Name		Default Action	Description

SIGHUP	termination	terminal line hangup
SIGINT	termination	interrupt program
SIGQUIT	termination	quit program
SIGILL	termination	illegal instruction
SIGTRAP	termination	trace trap
SIGABRT	termination	abort (generated by \fBabort\fR(3))
SIGEMT	termination	emulator trap
SIGFPE	termination	arithmetic exception
SIGKILL	termination	kill program
SIGBUS	termination	bus error
SIGSEGV	termination	segmentation fault
SIGSYS	termination	bad argument to system call
SIGPIPE	termination	write on a socket with no readers
SIGALRM	termination	real-time timer expired
SIGTERM	termination	software termination
SIGURG	discarded		urgent condition present on socket
SIGSTOP	stop			stop
SIGTSTP	stop			stop signal from keyboard
SIGCONT	discarded		continue after stop
SIGCHLD	discarded		child status has changed
SIGCLD	discarded		SYSV name for SIGCHLD
SIGTTIN	stop			background read attempted
SIGTTOU	stop			background write attempted
SIGIO	discarded		input/output possible on a file descriptor
SIGPOLL	discarded		SYSV name for SIGIO
SIGXCPU	termination	exceeded CPU time limit
SIGUSR1	termination	user defined signal 1
SIGUSR2	termination	user defined signal 2
.fi
.RE
.LP
The
.I func
procedure allows a user to choose the action upon receipt of a signal.
To set the default action of the signal to occur as listed above,
.I func
should be
.BR SIG_DFL .
A
.BR SIG_DFL
resets the default action.
To ignore the signal
.I func
should be
.BR SIG_IGN .
This will cause subsequent instances of the signal to be ignored
and pending instances to be discarded. If
.BR SIG_IGN
is not used,
further occurrences of the signal are
automatically blocked and
.I func
is called.
.LP
The handled signal is unblocked with the
function returns and
the process continues from where it left off when the signal occurred.
\fIUnlike previous Unix signal facilities, the handler
func() remains installed after a signal has been delivered.\fR
This behavior remains unchanged from GNO v2.0.4.
.LP
For some system calls, if a signal is caught while the call is
executing and the call is prematurely terminated,
the call is automatically restarted.
The affected system calls include
.BR read (2),
.BR write (2),
.BR sendto (2),
.BR recvfrom (2),
.BR sendmsg (2)
and
.BR recvmsg (2)
on a communications channel or a low speed device
and during a
.BR ioctl (2)
or
.BR wait (2).
However, calls that have already committed are not restarted,
but instead return a partial success (for example, a short read count).
.LP
When a process which has installed signal handlers forks,
the child process inherits the signals.
All caught signals will be reset to their default action by a call
to one of the
.BR execve (2)
family of functions;
ignored signals remain ignored.
.SH NOTES
As can be surmised from the prototype above, 
.IR func
should be defined as follows:
.RS
.sp 1
void \fIfunc\fR(int \fIsig\fR, int \fIcode\fR)
.sp 1
.RE
.I sig
is the signal that will invoke the handler, and 
.I code
is additional information about the interrupt condition.  Currently,
.I code
is always zero.  The handler should probably also be compiled using the
.B "#pragma databank 1"
directive, in the event 
.I func
is not in the same bank as the C global data segment
.RI ( func
is called with the data bank equal to the program bank).
.LP
Orca/C already provides a
.BR signal
function, but it doesn't do very much.  GNO's <signal.h> file replaces
the one that comes with Orca/C.
.SH RETURN VALUES
The previous action is returned on a successful call.
Otherwise, \-1 is returned and  the global variable
.IR errno
is set to indicate the error.
.SH ERRORS
.BR Signal 
will fail and no action will take place if one of the
following occur:
.RS
.IP \fBEINVAL\fR
.IR Sig 
is not a valid signal number.
.IP \fBEINVAL\fR
An attempt is made to ignore or supply a handler for
.BR SIGKILL
or
.BR SIGSTOP .
.IP \fBEINVAL\fR
An attempt is made to ignore
.BR SIGCONT
(by default 
.BR SIGCONT
is ignored).
.SH SEE ALSO
.BR kill (1),
.BR execve (2),
.BR fork (2),
.BR kill (2),
.BR sigblock (2),
.BR sigsetmask (2),
.BR wait (2),
.BR tty (4)
.SH HISTORY
A
.BR signal
facility appeared in 4.0BSD.