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`signal'--specify handler subroutine for a signal

     #include <signal.h>
     void ( * signal(int SIG, void(*FUNC)(int)) )(int);
     void ( * _signal_r(void *REENT,
         int SIG, void(*FUNC)(int)) )(int);
     int raise (int SIG);
     int _raise_r (void *REENT, int SIG);
`signal, raise' provide a simple signal/raise implementation for
embedded targets.
   `signal' allows you to request changed treatment for a particular
signal SIG.  You can use one of the predefined macros `SIG_DFL' (select
system default handling) or `SIG_IGN' (ignore this signal) as the value
of FUNC; otherwise, FUNC is a function pointer that identifies a
subroutine in your program as the handler for this signal.
   Some of the execution environment for signal handlers is
unpredictable; notably, the only library function required to work
correctly from within a signal handler is `signal' itself, and only
when used to redefine the handler for the current signal value.
   Static storage is likewise unreliable for signal handlers, with one
exception: if you declare a static storage location as ``volatile
sig_atomic_t'', then you may use that location in a signal handler to
store signal values.
   If your signal handler terminates using `return' (or implicit
return), your program's execution continues at the point where it was
when the signal was raised (whether by your program itself, or by an
external event).  Signal handlers can also use functions such as `exit'
and `abort' to avoid returning.
   `raise' sends the signal sig to the executing program.  It returns
zero if successful, non-zero if unsuccessful.
   The alternate functions `_signal_r, _raise_r' are the reentrant
versions.  The extra argument REENT is a pointer to a reentrancy
If your request for a signal handler cannot be honored, the result is
`SIG_ERR'; a specific error number is also recorded in `errno'.
   Otherwise, the result is the previous handler (a function pointer or
one of the predefined macros).
ANSI C requires `raise', `signal'.
   No supporting OS subroutines are required to link with `signal', but
it will not have any useful effects, except for software generated
signals, without an operating system that can actually raise exceptions.