Public Comment Number PC-UK0088

ISO/IEC CD 9899 (SC22N2620) Public Comment

=========================================== 

Date: 1998-02-25
Author: N.M Maclaren
Author Affiliation: Self
Postal Address:
    University of Cambridge,
    Computer Laboratory,
    New Museums Site,
    Pembroke Street,
    Cambridge CB3 3QG,
    United Kingdom
E-mail Address: <nmm1@cam.ac.uk>
Telephone Number: +44 1223 334761
Fax Number:       +44 1223 334679
Number of individual comments: 1


Comment 1. 

Category: Normative change to existing feature retaining the original intent

Committee Draft subsection: 7.13.8

Title: Direct input/output functions


Detailed description:

These have caused considerable confusion to both users and implementors
since they were introduced.  What DO they do on text streams on non-Unix
systems?  Experience shows that they often do something unhelpful - such as
fwrite writing files that cannot be read back in again by fread without
loss of information.  Note that the problem is not in the actual data
transfer, but in the problem of how to convert an unspecified block of
storage to characters and back again.

In particular, 7.13.2 Streams states that only some sequences of characters
can be written out and read back in again - actually, there are even more
constraints than that (e.g. record lengths).  Some non-Unix implementations
have converted the data to unsigned characters and then to hexadecimal, but
most just write the raw characters and let the chaos ensue.  And chaos is
precisely what the programmer gets, in general.

This ambiguity has caused trouble for long enough.  I suggest adding one or
other of the paragraphs:

    An implementation may define that these functions are not available for
    text streams.  If that is the case, calling one of these functions on
    a text stream shall have no effect except to set errno to an
    implementation-defined positive value and return EOF.

or:

    It is implementation-defined how the storage is converted to and from
    characters for I/O to and from text streams.  The implementation shall
    ensure that one or more objects of any type can be written out using
    the fwrite function and read back in to objects of the same type using
    the fread function with the same value of size without loss of
    information.