Re: [RFC] Make _FILE_OFFSET_BITS=64 default.

[Paul Eggert <eggert at cs dot ucla dot edu>]

On 03/14/2014 11:18 AM, Rich Felker wrote:
> > >zlib's zlib.h attempts to work around the glibc problems with
> > >_FILE_OFFSET_BITS, and its workaround would need adjusting if glibc
> > >changed the default.
> Can you explain in more detail?

It's pretty obscure, and I expect we don't need to worry about it.  As I 
understand it, the problem affects only applications that use 
-D_FILE_OFFSET_BITS=32.  I don't know of any (who would want to do 
that?!), but there it is.  zconf.h does this:

#if defined(_LFS64_LARGEFILE) && _LFS64_LARGEFILE-0
#  define Z_LFS64
#endif
...
#if defined(_FILE_OFFSET_BITS) && _FILE_OFFSET_BITS-0 == 64 && 
defined(Z_LFS64)
#  define Z_WANT64
#endif

and Z_WANT64's only use is in zlib.h, in the code at the end of this 
email.  This causes zlib to use a workaroundsomething like glibc's when 
_FILE_OFFSET_BITSis explicitly defined to be 64.  zlib itself is 
compiled with no _FILE_OFFSET_BITS setting (it uses z_off64_t, lseek64 
etc. internally), so if glibc changes the _FILE_OFFSET_BITS default to 
64, zlib would then support only 64-bit off_t, and zlib's workaround 
would break programs compiled with -D_FILE_OFFSET_BITS=32. If zlib 
wanted to continue to support 32-bit-only off_t, it'd need to compile 
itself with -D_FILE_OFFSET_BITS=32, and tweak zconf.h and zlib.h a bit.  
My guess is that zlib wouldn't bother.  I know zlib's maintainer and so 
could ask him, if there's interest.

#if !defined(ZLIB_INTERNAL) && defined(Z_WANT64)
#  ifdef Z_PREFIX_SET
#    define z_gzopen z_gzopen64
#    define z_gzseek z_gzseek64
#    define z_gztell z_gztell64
#    define z_gzoffset z_gzoffset64
#    define z_adler32_combine z_adler32_combine64
#    define z_crc32_combine z_crc32_combine64
#  else
#    define gzopen gzopen64
#    define gzseek gzseek64
#    define gztell gztell64
#    define gzoffset gzoffset64
#    define adler32_combine adler32_combine64
#    define crc32_combine crc32_combine64
#  endif
#  ifndef Z_LARGE64
     ZEXTERN gzFile ZEXPORT gzopen64 OF((const char *, const char *));
     ZEXTERN z_off_t ZEXPORT gzseek64 OF((gzFile, z_off_t, int));
     ZEXTERN z_off_t ZEXPORT gztell64 OF((gzFile));
     ZEXTERN z_off_t ZEXPORT gzoffset64 OF((gzFile));
     ZEXTERN uLong ZEXPORT adler32_combine64 OF((uLong, uLong, z_off_t));
     ZEXTERN uLong ZEXPORT crc32_combine64 OF((uLong, uLong, z_off_t));
#  endif
#else
   ZEXTERN gzFile ZEXPORT gzopen OF((const char *, const char *));
   ZEXTERN z_off_t ZEXPORT gzseek OF((gzFile, z_off_t, int));
   ZEXTERN z_off_t ZEXPORT gztell OF((gzFile));
   ZEXTERN z_off_t ZEXPORT gzoffset OF((gzFile));
   ZEXTERN uLong ZEXPORT adler32_combine OF((uLong, uLong, z_off_t));
   ZEXTERN uLong ZEXPORT crc32_combine OF((uLong, uLong, z_off_t));
#endif