[Nexus-developers] Handling of C-string null characters

[Ray Osborn]

There is one urgent thing that we need to clear up before we release NAPI
v3.0, and that concerns how we handle string lengths.  Following problems
with the XML API, Mark has now changed NXgetinfo so that it returns the
length of the string in the Fortran API but adds one to the length in the C
API to accommodate the NULL character.  I think this is the wrong way to
approach this problem, and I think Freddie agreed with me when he wrote to
confirm what the API now does.  We need to resolve this quickly so other
opinions are welcomed.

So I'm raising the old question - how long is a string?

Current Behaviour:

NXgetinfo and NXmalloc adds the extra byte to the length of character
strings, when called in C, but it is removed in the Fortran API.  The length
of "neutron" is 8 in C but 7 in Fortran (and presumably other APIs such as
Python).  NXgetdata will return "neutron\0" in C, but "neutron" in Fortran.

Proposal (my view, and I believe Freddie's):

The length of a character string returned by NXgetinfo should be the number
of characters excluding the NULL character, and NXgetdata should return
exactly those characters.  The documentation should warn the C-programmer to
add one byte to the allocation, if they use malloc directly, and to add the
NULL character to the string returned by NXgetdata to make a C-string.
NXmalloc will automatically add the extra byte when allocating memory.

This ensures that the length does not depend on the language used to read
the NeXus file.   C-programmers are used to dealing with this issue and
don't need to be spoon-fed.  The average non-programming user will, however,
be confused why "neutron" is 8 characters long according to NXbrowse and
most other generic file readers, but only seven according to the Fortran
API.  This will prevent such confusion in a well-documented way.

We may need to put this to a vote, but we should settle it before Friday if
Nick's timetable is to be kept.

Regards,
Ray 
-- 
Dr Ray Osborn                Tel: +1 (630) 252-9011
Materials Science Division   Fax: +1 (630) 252-7777
Argonne National Laboratory  E-mail: ROsborn at anl.gov
Argonne, IL 60439-4845