signal(3B)

simplified software signal facilities

From the original Amiga UNIX 2.1 manual. Some things it mentions may not be in this release.

Also by this name: signal(2), System calls · signal(5), Character sets, environment, package files

NAME

signal - simplified software signal facilities

SYNOPSIS

cc [ flag... ] file ... -lucb
#include <signal.h>
void (*signal(sig, func))()
void (*func)();

DESCRIPTION

signal  is  a  simplified  interface  to  the  more  general
sigvec(2)  facility.  Programs that use signal in preference
to sigvec are more likely to be portable to all systems.   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
termio(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 dis-
carded 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 interrupt to a  specified
location.  The following is a list of all signals with names
as in the include file <signal.h>:
     SIGHUP         hangup
     SIGINT         interrupt
     SIGQUIT   *    quit
     SIGILL    *    illegal instruction
     SIGTRAP   *    trace trap
     SIGABRT   *    abort (generated by abort(3) routine)
     SIGEMT    *    emulator trap
     SIGFPE    *    arithmetic exception
     SIGKILL        kill (cannot be caught, blocked, or ignored)
     SIGBUS    *    bus error
     SIGSEGV   *    segmentation violation
     SIGSYS    *    bad argument to system call
     SIGPIPE        write on a pipe or other socket with no one to read it
     SIGALRM        alarm clock
     SIGTERM        software termination signal
     SIGURG    @    urgent condition present on socket
     SIGSTOP   +    stop (cannot be caught, blocked, or ignored)
     SIGTSTP   +    stop signal generated from keyboard
     SIGCONT   @    continue after stop (cannot be blocked)
     SIGCHLD   @    child status has changed
     SIGTTIN   +    background read attempted from control terminal
     SIGTTOU   +    background write attempted to control terminal
     SIGIO     @    I/O is possible on a descriptor [see fcntl(2)]

     SIGXCPU   *    cpu time limit exceeded [see getrlimit(2)
     SIGXFSZ   *    file size limit exceeded [see getrlimit(2)
     SIGVTALRM      virtual time alarm [see getitimer(2)
     SIGPROF        profiling timer alarm [see getitimer(2)]
     SIGWINCH  @    window changed [see termio(4)]
     SIGUSR1        user-defined signal 1
     SIGUSR2        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  termi-
nation  (with  a  core image for starred signals) except for
signals marked with @ or +.  Signals marked with @ are  dis-
carded if the action is SIG_DFL; signals marked with + cause
the process to stop.  If func is SIG_IGN the signal is  sub-
sequently  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 inter-
rupted.  If a caught signal  occurs  during  certain  system
calls,   terminating  the  call  prematurely,  the  call  is
automatically restarted.  In particular this can occur  dur-
ing a read(2) or write(2) on a slow device (such as a termi-
nal; 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.  An execve(2) resets all caught sig-
nals to the default action; ignored signals remain ignored.

NOTES

The handler routine can be declared:
     void handler(sig, code, scp, addr)
     int sig, code;
     struct sigcontext *scp;
     char *addr;
Here sig is the signal number; code is a parameter  of  cer-
tain  signals  that  provides  additional  detail;  scp is a
pointer to the sigcontext structure (defined in <signal.h>),
used to restore the context from before the signal; and addr
is additional address information.  See sigvec(2)  for  more
details.

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, or  is

                    SIGKILL or SIGSTOP.