NAME

signal − simplified software signal facilities

SYNOPSIS

/usr/ucb/cc [ flag. . . ] file . . .

#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, and so on), 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>:

SIGHUPhangup
SIGINTinterrupt
SIGQUIT∗quit
SIGILL∗illegal instruction
SIGTRAP∗trace trap
SIGABRT∗abort (generated by abort(3) routine)
SIGEMT∗emulator trap
SIGFPE∗arithmetic exception
SIGKILLkill (cannot be caught, blocked, or ignored)
SIGBUS∗bus error
SIGSEGV∗segmentation violation
SIGSYS∗bad argument to system call
SIGPIPEwrite on a pipe or other socket with no one to read it
SIGALRMalarm clock
SIGTERMsoftware 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)
SIGVTALRMvirtual time alarm [see getitimer(2)
SIGPROFprofiling timer alarm [see getitimer(2)]
SIGWINCH•window changed [see termio(4)]
SIGUSR1user-defined signal 1
SIGUSR2user-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. 

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. 

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

kill(1), execve(2), fork(2), getitimer(2), getrlimit(2), kill(2), ptrace(2), read(2), sigaction(2) wait(2), write(2), setjmp(3), sigblock(3), sigpause(3), sigsetmask(3), sigstack(3), sigvec(3), wait(3), setjmp(3C), termio(7). 

  —  BSD Compatibility Package