[Expat-discuss] XML_LARGE_SIZE binary compatibility problem

[Karl Waclawek]

Bruno Haible wrote:
> Hello,
>
> In expat-2.0.0, a compilation option XML_LARGE_SIZE has been introduced that,
> on 32-bit systems, changes the size of the return type of the functions
>   XML_GetCurrentLineNumber
>   XML_GetCurrentColumnNumber
>   XML_GetCurrentByteIndex
> from 32-bit ([unsigned] long) to 64-bit ([unsigned] long long).
>
> The ABI is not the same: on some CPUs 64-bit results are returned in memory,
> and on the other CPUs, where 64-bit results are returned in registers, the
> registers for 32-bit return and 64-bit return may not be the same.
>
> Question 1:
> How should a program that wants to use <expat.h> and -lexpat proceed?
> The installed expat.h and expat_external.h are the same in both cases.
> In other words, someone using the expat.m4 macro in his package and
> doing #include <expat.h> will be assuming that the return types are
> 32-bit large, while in the libexpat.so.1 they are actually 64-bit large.
>
> Question 2:
> How should a program that wants to use libexpat.so through dynamic loading
> (dlopen ("libexpat.so.1")) proceed? The list of exported symbols that are
> visible through dlsym() is the same in both cases. How can it know whether
> the library was built with XML_LARGE_SIZE or not?
>
> If these issues are not resolved, there is no way to make my program (GNU gettext)
> link against libexpat without risking crashes, and I will have to rewrite my
> code to use libxml2 instead of libexpat.
>
>   
The standard build is without XML_LARGE_SIZE defined. Therefore your 
program should
rely on that ABI. We introduced this compile option only for special 
needs, where one needs
to get the line/column number when processing extremely large files. In 
this case you would
create a custom build of libexpat and you would know that you need to 
define XML_LARGE_SIZE
when compiling your application to get the correct header files.

We could add a new return value to XML_GetFeatureList(), so that you can 
dynamically
detect if XML_LARGE_SIZE was used. Would that help you?

Karl