Allow interprocess to work with big files on x32 POSIX systems

[Antony Polukhin]

This patch adds O_LARGEFILE to file open mode (if it is supported).

Some info about defining -D_FILE_OFFSET_BITS=64 on x32 POSIX systems shall be added to official documentation and maybe to mapped_region doxygen documentation.

(patch was tested on x32 Linux on boost 1.53 and adopted for trunk version)

[Antony Polukhin]

#7295 will be partially fixed by this

[Ion Gaztañaga]

I have some questions about the patch. If I understand LFS correctly:

If _LARGEFILE64_SOURCE is defined, then we have new functions and types like open64/off64_t...

If _FILE_OFFSET_BITS=64 then "open"/"off_t"... are 64 bit ready.

Using O_LARGEFILE does not seem to guarantee compatibility as _FILE_OFFSET_BITS can be 32 and off_t would be 32 bits. We could open files bigger than 2GB but no seek or map them.

Maybe the solution is to use xxx64 functions and types if _LARGEFILE64_SOURCE is defined and _FILE_OFFSET_BITS is not defined or it's equal to 32. Or am I missing something?

[Antony Polukhin]

Replying to igaztanaga:

I have some questions about the patch. If I understand LFS correctly:

If _LARGEFILE64_SOURCE is defined, then we have new functions and types like open64/off64_t...

If _FILE_OFFSET_BITS=64 then "open"/"off_t"... are 64 bit ready.

Using O_LARGEFILE does not seem to guarantee compatibility as _FILE_OFFSET_BITS can be 32 and off_t would be 32 bits. We could open files bigger than 2GB but no seek or map them.

Maybe the solution is to use xxx64 functions and types if _LARGEFILE64_SOURCE is defined and _FILE_OFFSET_BITS is not defined or it's equal to 32. Or am I missing something?

After two more rereads of LSF I've finally got it right: To work with large files on x32 user only needs to define _FILE_OFFSET_BITS=64, so patch is incorrect.

But maybe forcing x32 users to define _FILE_OFFSET_BITS=64 will be a good idea:

#if !defined(__x86_64__) && !defined(__ppc64__) && \
 (!defined(_FILE_OFFSET_BITS) || _FILE_OFFSET_BITS==32)
#error "To avoid errors while using big files define _FILE_OFFSET_BITS macro to 64 for your project."
#endif

, otherwise this may lead to hard detectable errors.

[t.glaser@…]

x32 does define x86_64 though, you need to check for ILP32 which is defined on x32 but not on i386 or amd64.