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(libc.info)Signal and Sigaction

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Interaction of `signal' and `sigaction'
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   It's possible to use both the `signal' and `sigaction' functions
within a single program, but you have to be careful because they can
interact in slightly strange ways.

   The `sigaction' function specifies more information than the
`signal' function, so the return value from `signal' cannot express the
full range of `sigaction' possibilities.  Therefore, if you use
`signal' to save and later reestablish an action, it may not be able to
reestablish properly a handler that was established with `sigaction'.

   To avoid having problems as a result, always use `sigaction' to save
and restore a handler if your program uses `sigaction' at all.  Since
`sigaction' is more general, it can properly save and reestablish any
action, regardless of whether it was established originally with
`signal' or `sigaction'.

   On some systems if you establish an action with `signal' and then
examine it with `sigaction', the handler address that you get may not
be the same as what you specified with `signal'.  It may not even be
suitable for use as an action argument with `signal'.  But you can rely
on using it as an argument to `sigaction'.  This problem never happens
on the GNU system.

   So, you're better off using one or the other of the mechanisms
consistently within a single program.

   *Portability Note:* The basic `signal' function is a feature of
ISO C, while `sigaction' is part of the POSIX.1 standard.  If you are
concerned about portability to non-POSIX systems, then you should use
the `signal' function instead.