signal(3) - NetBSD Manual Pages

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SIGNAL(3)               NetBSD Library Functions Manual              SIGNAL(3)


NAME

     signal -- simplified software signal facilities


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <signal.h>

     void (*
     signal(int sig, void (*func)(int)))(int);


DESCRIPTION

     This signal() facility is a simplified interface to the more general
     sigaction(2) facility.

     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 ter-
     mination 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 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 result in the termination of the process receiv-
     ing 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.  Except for the SIGKILL and SIGSTOP signals, the
     signal() function allows for a signal to be caught, to be ignored, or to
     generate an interrupt.  See signal(7) for comprehensive list of supported
     signals.

     The 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, func should be SIG_DFL.  A SIG_DFL resets the default action.  To
     ignore the signal func should be SIG_IGN.  This will cause subsequent
     instances of the signal to be ignored and pending instances to be dis-
     carded.  If SIG_IGN is not used, further occurrences of the signal are
     automatically blocked and func is called.

     The handled signal is unblocked when the function returns and the process
     continues from where it left off when the signal occurred.  Unlike previ-
     ous signal facilities, the handler func() remains installed after a sig-
     nal has been delivered.

     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 handler is installed using the SA_RESTART flag with
     sigaction(2)).  The affected system calls include read(2), write(2),
     sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications
     channel or a low speed device and during a ioctl(2) or wait(2).  However,
     calls that have already committed are not restarted, but instead return a
     partial success (for example, a short read count).

     When a process which has installed signal handlers forks, the child
     process inherits the signals.  All caught signals may be reset to their
     default action by a call to the execve(2) function; ignored signals
     remain ignored.

     Only functions that are async-signal-safe can safely be used in signal
     handlers, see signal(7) for a complete list.


RETURN VALUES

     The previous action is returned on a successful call.  Otherwise, SIG_ERR
     is returned and the global variable errno is set to indicate the error.


ERRORS

     signal() will fail and no action will take place if one of the following
     occur:

     [EINVAL]           Specified 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), kill(2), ptrace(2), sigaction(2), sigaltstack(2),
     sigprocmask(2), sigsuspend(2), psignal(3), setjmp(3), strsignal(3),
     tty(4), signal(7)


HISTORY

     This signal() facility appeared in 4.0BSD.

NetBSD 5.2                       June 11, 2004                      NetBSD 5.2