cmp(1) as a builtin

cmp(1) as a builtin

[Roman Neuhauser]

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

Re: cmp(1) as a builtin

[Bart Schaefer]

On Jun 2,  6:34pm, Roman Neuhauser wrote:
}
} 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?

Zsh does not have any special API or cache for stat data.  The closest
thing is cond.c:getstat() which has some special handling for /dev/fd/
files, but is currently declared static.

} 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...?

You can use stdin.  SHIN is the command input stream (e.g., terminal or
script file), so don't use that.

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

I believe so, yes.  Compare to Src/Modules/example.c cond_i_ex().

You may want to put these in your own module rather than modifying the
zsh/files module. Editing copies of Src/Modules/example.* works well
for this.