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null-terminated strings vs. strings with length



Dear authors of SRFI-50,

I would like to raise an issue concerning the actual 
interface for strings:

'Native C-strings' (ha ha) are null-terminated whereas

Scheme-strings have length. Unfortunately, SRFI-50 
tries to bridge the mismatch by conversion, instead 
of pushing the problem to either side (preferably to
C)
where it can be dealt with in an application-specific
manner as need arises.

For example, assume a printer escape sequence 
including several '\0's is constructed in C and should

be passed on to Scheme for printing. How can I do it?

Fortunately, there is a simple and effective
alternative:

*** 
a) The C-side of the FFI does not deal with
null-terminated 
native C-strings but rather with buffers of the form 
(const char *) pointer together with a (size_t) size. 
b) The content of the buffer is the internal
representation
of the Scheme-string, whatever that may be.
c) The FFI deals with Scheme-substrings, i.e., 
(STRING, START, END) in the sense of R5RS 6.3.5.,
rather than with entire Scheme-strings.
***

The rationale for this is as follows:

Ad a) Strings in Scheme are sometimes used for storing

vectors of bytes. Although this is clearly a misuse of
STRING,
it is common practice due to a lack of alternatives.
The FFI
should not rule out this option by defining the C-side
as
null-terminated strings.

The only inconvenience introduced by defining buffers
with 
explicit length instead of null-terminated strings is
an occasional
explicit call to strlen() when creating a
Scheme-string from
a native C-string.

Ad b) There is no way Scheme's character
representation can 
be hidden from the C-side, anyway. So there is no
point in
trying to convert CHAR to (char) in an undefined way. 
Prescribing a certain conversion on the other hand
turns
the FFI into a source of limitation, which should be
avoided.

The downside of this approach is that there is no
portable
way in C to produce a Scheme-string, even if it is
straight
no-bells-and-whistles printable ASCII. This can be
fixed
easily, though. (See below.)

Ad c) Depending on the GC, it is dangerous to deal
with 
pointers pointing directly into the internal
representation of
Scheme objects, in particular if concurrent threads
could 
initiate garbage collection at any moment. By using 
substrings it becomes affordable to always copy
between 
C and Scheme.

The proposal would require the following
modifications:

1. SCHEME_EXTRACT_STRING copies a specified substring
(with START, END as in R5RS 6.3.5) of the
Scheme-string into 
a buffer prepared by the caller (taking into account
the size of 
the buffer ;-) and returns the number of (char)s
actually copied.

An additional C-function can be called for computing
the number of 
(char)s of the internal representation of a given
Scheme-substring.
(Depending on character encoding this could be linear
time, but
copying is linear time anyhow.)

2. SCHEME_ENTER_STRING copies a C-buffer ((char *)s,
(size_t) n) 
into a specified substring of an already existing
Scheme-string (or #f
to allocate a new one) and returns the resulting
(scheme_value).
It should be defined what happens when the
Scheme-string is too
short to receive the C-buffer.

3. SCHEME_EXTRACT/ENTER_CHAR do not assume that a
Scheme-character is just a (char); like the
STRING-procedures
they are called with caller-prepared buffers of
sufficient length.

4. There is a C-function mapping (char) into the
internal rep. of
a Scheme-character, at least preserving ASCII
{0..127}. There
is also a C-function projecting (the internal rep. of)
a Scheme-
character into ASCII {0..127}, indicating if there was
a loss.
(These two conversions should make sure there is some
way
to say "Hello, world!" without diving into the muddy
waters of
Unicode.)

This proposal separates the issue of character
encoding  from the issue of null-terminated strings
vs. buffers with length.

Sebastian.


P.S. Good SRFI, please continue!

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