If you still have some usage of off_t in the header files of your library then it is a good thing to mark that somewhere. Just compile your library with 64bit-off_t - and reject when software gets compiled with the wrong model than what your library was compiled as:
# if _FILE_OFFSET_BITS+0 != 64 && MYLIB_FILE_OFFSET_BITS+0 == 64 # error need to compile as largefile when using MYLIB # endif
Another variant makes use of the AC_SYS_LARGEFILE_SENSITIVE macro - which is also a sanity check for the compilation of your library itself. That would simply use something like:
# in configure.ac :
AM_CONFIG_HEADER(config.h)
AX_PREFIX_CONFIG_H(mylibconf.h,mylib)
AC_SYS_LARGEFILE_SENSITIVE
/* in mylibstruct.h */
#include "mylibconf.h"
#if defined MYLIB_LARGEFILE_SENSITIVE && _FILE_OFFSET_BITS+0 != 64
#if defined __GNUC__
#warning mylib requires largefile mode - use AC_SYS_LARGEFILE
#else
# error mylib requires largefile mode - use AC_SYS_LARGEFILE
#endif
typedef struct {
off_t off;
} offstruct;
This scheme will keep your headers and sources `lean and mean` ;-) with your headers to actually take advantage of "off_t" as a native type that matches clean with standard filesystem functions. Just ensure to include "mylibconf.h" in every header file of your project that requires a check for a lib-create-time to lib-usage-time feature difference like off_t size - in other words, the library that uses an off_t and another program that takes advantage of that library.
Note also that the usage of AC_TYPE_OFF_T can be used
to make "off_t" exist - and with the prefix macro this will also
exist when the third-party software did forget to invoke this macro
in its `configure.ac`. So just make all your api-calls dependent on
that configured name, like `_mylib_off_t` everywhere.
#include <mylibconf.h> #ifndef _mylib_off_t #define _mylib_off_t off_t #endif _mylib_off_t mylib_telldir(MYLIB_DIR * dir); void mylib_seekdir(MYLIB_DIR * dir, _mylib_off_t offset);
That code is not just theory: have a look at http://zziplib.sf.net for a real-world and functional example.
One thing that I do not want to announce all too loud: if you find
an improper usage of off_t in an older project, then you can add a
quick hack by redefining those functions/structures to use int64_t
instead of off_t. This will make structure-mismatch and callframe-mismatch
problems to go away - but the seek problems will still exist and
they might even hit you harder later since you've covered the usage
of seek-offsets in a generic type instead of its dedicated off_t type,
and basically forgot about to fix the problem for real. Therefore,
if you did not add proper ifdefs to defend against improper usage in
largefile-sensitive systems then better make a library typedef like
`typedef int64_t mylib_off_t`. A later generation of
that software may add proper check routines around this symbol.
(remember: UNIX98 systems are required to provide 64bit LFS).