signal(3) (BSD Compatibility Package) signal(3)
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 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 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
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signal(3) (BSD Compatibility Package) signal(3)
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 SIGDFL, 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 SIGDFL; signals marked with † cause the
process to stop. If func is SIGIGN 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.
If a caught signal occurs during certain system calls, terminating
the call prematurely, the call is automatically restarted. In
particular this can occur during a read(2) 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. An
execve(2) resets all caught signals 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 certain 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.
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signal(3) (BSD Compatibility Package) signal(3)
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.
SEE ALSO
setjmp(3), sigblock(3), sigpause(3), sigsetmask(3), sigstack(3),
sigvec(3), wait(3)
execve(2), fork(2), getitimer(2), getrlimit(2), kill(2), ptrace(2),
read(2), sigaction(2) wait(2), write(2), setjmp(3C), in the
Programmer's Reference Manual.
kill(1), in the User's Reference Manual.
termio(7) in the System Administrator's Reference Manual.
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