Re: request for review: Doing direct file I/O in Emacs Lisp

[storm@cua.dk (Kim F. Storm)]

John Wiegley <johnw@gnu.org> writes:

> The following patch implements a file-handle interface for Emacs Lisp,
> which allows files to be directly opened and read/written to without
> an intervening buffer.  Eshell can now use this, for example, to
> greatly speed up output redirection (by several orders of magnitude).
> 
> It is a simple interface that reads in strings, given a length, and
> writes strings by examining their length:
> 
>   (let ((handle (file-handle-open "/tmp/some-file" "w")))
>     (file-handle-write handle "Test data\n")
>     (file-handle-close handle)
> 
>     (setq handle (file-handle-open "/tmp/some-file" "r"))
>     (message (file-handle-read handle 128))
>     (file-handle-close handle))
> 


This seems like a great idea, but it is not up to me to decide.

Some comments:

The doc string for -open need improvement.  I'm not sure about using
"r", "w", etc for the mode; using things like 'read 'write and 'append
seems more lisp like.  But I don't prefer either.

The doc strings for -read and -write are bad.


Instead of an explicit CONS as the handle, I would rather declare
the file handle like this:

struct Lisp_File_Handle
  {
    EMACS_INT size;
    struct Lisp_Vector *v_next;
    Lisp_Object handle_hi;
    Lisp_Object handle_lo;
    Lisp_Object file_name;
    Lisp_Object open_mode;
};

and use handle_hi and handle_lo directly instead of all the CAR and
CDR'ing.

When you close the file handle, open_mode is set to nil.

The file_name and open_mode can be used to improve the printing of
the file handle as in

      else if (FILE_HANDLEP (obj))
	{
	  strout ("#<file-handle ", -1, -1, printcharfun, 0);
          if (!NILP (XFILE_HANDLE (obj)->open_mode))
            {
	      strout (XFILE_HANDLE (obj)->file_name, -1, -1, printcharfun, 0);
	      strout (" ", -1, -1, printcharfun, 0);
	      strout (XFILE_HANDLE (obj)->open_mode, -1, -1, printcharfun, 0);
            }
          else
            strout (" closed", -1, -1, printcharfun, 0);
          PRINTCHAR (')>');
	}


You don't address the issue of multibyte in -read and -write.
It seems that you always assume things to be unibyte.

I don't know what the right thing to do is, but maybe you should
at least signal an error if things are not unibyte ?  

Otherwise, you should associate a coding system with the file-handle
and use that for read and write.  As a first shot at this, you could
add the coding system as an optional third arg to -open, and assume
unibyte/binary if no coding system was specified.

A -seek operation would be nice I guess.
And a -position (aka ftell) would be nice too.


-- 
Kim F. Storm <storm@cua.dk> http://www.cua.dk