[GRASS-dev] Re: vector large file support

[Ivan Shmakov]

>>>>> Glynn Clements <glynn at gclements.plus.com> writes:

 >> Do I understand right that fseeko and ftello are only needed on
 >> 32-bit systems that want D_FILE_OFFSET_BITS=64? fseek e.g. returns
 >> long which is on my 64bit Linux 64bit, I guess that's why I can
 >> write coor files > 2GB with the current vector libs.

 > Yes. There's no point in using them unless off_t is larger than long
 > (i.e. 64-bit off_t versus 32-bit long).

	I think there is.  Per POSIX, off_t is /the/ type to hold file
	offsets and lengths.  The intention is for the program to retain
	portability whatever numeric types exists on a target.  I guess,
	it will even work if none of the standard C `int' types match
	`off_t' (although it's probably quite unlikely to become an
	issue for GRASS in a foreseeable future.)

 >>> It's not worth using "raw" I/O just to avoid this issue. Apart from
 >>> anything else, there's a potentially huge performance hit, as the
 >>> vector library tends to use many small read/write operations. Using
 >>> low-level I/O requires a system call for each operation, while the
 >>> stdio interface will coalesce these, reading/writing whole blocks.

 >> Interesting and good to know. So we do need G_fseek() and G_ftell()

 > Yes. Those would be useful regardless of anything related to the
 > vector format.

	And, as it seems, Gnulib already has them:

http://www.gnu.org/software/gnulib/MODULES.html#module=ftello
http://www.gnu.org/software/gnulib/MODULES.html#module=fseeko

[...]

PS.  ... And please forgive me for mentioning Gnulib with such a
	persistence without investing a bit of time to further its
	adoption into GRASS.  Unfortunately, I'm somewhat overloaded
	with my work lately.