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NAME    [Toc]    [Back]

     signal - simplified software signal facilities

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <signal.h>

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

DESCRIPTION    [Toc]    [Back]

     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 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 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 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.  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

     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 discarded.
  If SIG_IGN is not used, further occurrences of the signal are
     automatically blocked and func is called.

     The handled signal is unblocked with 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.

RETURN VALUES    [Toc]    [Back]

     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    [Toc]    [Back]

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

     [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    [Toc]    [Back]

     kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2),
     sigprocmask(2), sigsuspend(2), setjmp(3), tty(4), signal(7)

HISTORY    [Toc]    [Back]

     This signal() facility appeared in 4.0BSD.

BSD                             April 19, 1994                             BSD
[ Back ]
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