Re: errno is not reset after require

[Sean Conner <sean@...>]

It was thus said that the Great Iurii Belobeev once stated:
> Hi,
> 
> I have the code:
> 
> printf("errno: %d\n", errno);
> lua_getglobal(L, "require");
> lua_pushliteral(L, "mymodule");
> lua_call(L, 1, 1);
> printf("errno: %d\n", errno);
> 
> package.cpath:
> /mydir/?/?.so
> 
> mymodule exists:
> /mydir/mymodule/mymodule.so
> 
> It is being required successfully.
> 
> The wrong thing here for me is that errno is set to 2 (No such file or
> directory) after the successful require.
> It gives me some trouble later.
> Is such behavior expected here?
> Or I'm doing something wrong?

  Yes, such behavior is expected, as it is an interaction between the C
standard and Lua's behavior.  First, the C standand, section 7.5.3: errno.h

	The value of errno is zero at program startup, but is never set to
	zero by any library function. The value of errno may be set to
	nonzero by a library function call whether or not there is an error,
	provided the use of errno is not documented in the description of
	the function in this International Standard.

Section 7.19.5.3.8: fopen() states:

	The fopen function returns a pointer to the object controlling the
	stream.  If the open operation fails, fopen returns a null pointer.

  There is no mention of errno in this function, so fopen() can set errno. 
The Lua manual, in the section about package.searchers[] (an array), states
that the default order of module searching is:

	1	search package.preload[]
	2	search for a file specified in package.path
	3	search for a file specified in package.cpath
	4	search for a file specified in package.cpath (different criteria)

  The second searcher will call fopen() on each segment in package.path,
causing errno to be set to the error seen.  No C library (per 7.5.3) will
ever reset errno to 0, and Lua only sets errno to 0 in two places (as I can
see): once in io_pclose() (when closing a pipe), and os_execute() (before
running the standard C call system()).

  I would not expect errno to be of much use after calling into Lua.

  -spc