cmp(1) as a builtin

[Roman Neuhauser <neuhauser@sigpipe.cz>]

hello,

i need to run cmp(1) on a heap of mostly small files, and the forking
overhead is killing me.  i added a PoC bit to zsh/files to expose b:cmp
and C:qf (eQual Files).  it seems to work, but i'm pretty sure it has
issues as i don't know what i'm doing here, on several levels.

questions:

the cmp builtin currently always reads the file data.  it should stat
both operands to see if that can be skipped.  what api do i use for
that?  does zsh have some kind of stat cache i should hook into?

if either operand is "-", it should read from stdin.  how do i get hold
of standard input in the builtin?  can i use the standard FILE* handle
(stdin), do i need fdopen(SHIN, "r") (or use SHIN directly), or...?

the condexpr operator ([[ x -qf y ]]) is implemented like this:

    static int
    cond_cmp(char **args, UNUSED(int func))
    {
        return 0 == do_cmp(
            "cond_qf", 1,
            cond_str(args, 0, 0),
            cond_str(args, 1, 0)
        );
    }

do_cmp() passes the cond_str() results to fopen().  am i using
cond_str() correctly?

-- 
roman