Getting the value of Lua's io.output() from the C API

[Peter Aronoff <telemachus@...>]

Hi,

As a way of exploring the C API, I've been working on a C module to give
Lua an isatty(3) function. My thought was to allow users to call the method
in two ways:

    isatty(filehandle) -- Returns true or false depending on whether the
                          filehandle refers to a tty
    isatty()           -- Checks whether the current value of io.output()
                          refers to a tty

Within Lua, io.output() with no explicit parameter returns the default
output file. I would like to have my isatty with no parameters *check* the
current output filehandle (whatever that is).

I thought of a way to call Lua from the C - get the io table and then call
the io.output() function with no parameters and push that return value back
onto the C stack. But it seems odd to write a C module that calls back to
Lua in this way.

However, I wasn't able to see a clear way to get at the value of "current
output file" from the C API. So, finally, my two questions:

    1) Did I miss something? Does the C API expose the value of io.output()
       somewhere?
    2) As a general rule, is it ok (common, not bad form) to write
       a C module that simultaneously provides a new function to Lua and
       calls out to Lua to get information that the C API doesn't make easy
       to get?

Sorry if this was long, and thanks in advance.

Peter
-- 
We have not been faced with the need to satisfy someone else's
requirements, and for this freedom we are grateful.
    Dennis Ritchie and Ken Thompson, The UNIX Time-Sharing System