When your software is portable to systems using any variant of off_t then you can put in the next step: to enable largefile by default. This is easy for software which is not a library and atleast safe with libraries that do not export a file seek-value or file descriptor.
With autoconf, include additionally AC_SYS_LARGEFILE
into your autoconf script. In conjunction with AC_FUNC_FSEEKO
and AC_TYPE_OFF_T it will ensure that any system with
a largefile kernel will export a 64bit off_t to applications. As good
rule of thumb all systems since the late 1990ies have
a largefile kernel, and systems later than 1998 guarantee that in the
product description.
Without autoconf you can place the two #define's
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 somewhere in
your build environment. Those defines come from the LFS standard
included in the Unix98 specification of required features. In
64on32 systems they will magically make
source code with "ftell(x)" link to "ftell64" for real. On
win32 system you need to add that magic
manually in the application headers.
After that, your application uses largefile whereever available and portably. If you make up a library however that exports a file seek-value or a file descriptor then you need to heed on to the next steps of conversion. Otherwise you'd need to be ready to face problems when bringing your library into a production cycle.
If you converted an executable then you are done. If you converted a library and no binary-compatibility is needed and no plugins are ever bound late then you are done as well. Otherwise you need to seek binary-compatibility and in the course to decide about dualmode or twinlibs. Using twinlibs is recommended strongly but dualmode is about the only option when mixing multiple libraries of a different off_t format.