Large File Support

The need

Standard C <stdio.h> file functions return file sizes as long integer. On 32-bit systems this overflows at 2 gigabytes. For support of files bigger than this, you need LFS. Currently only implimented in GRASS in libgis. (i.e. there is support for reading+writing raster maps, but not many import/export modules or vector functions have it)

The issues

The problem is that ftell() returns the result as a (signed) long. If the result won't fit into a long, it returns -1 (and sets errno to EOVERFLOW).

This can only happen if you also set _FILE_OFFSET_BITS to 64 so that fopen() is redirected to fopen64(), otherwise fopen() will simply refuse to open files larger than 2GiB (apparently, this isn't true on some versions of MacOSX, which open the file anyhow then fail on fseek/ftell once you've passed the 2GiB mark).

If you want to obtain the current offset for a file whose size exceeds the range of a signed long, you instead have to use the (non-ANSI) ftello() function, which returns the offset as an off_t. TODO: But before we do that, we would need to add configure checks so that we don't try to use ftello() on systems which don't provide it.

Coding LFS in GRASS

Currently the --enable-largefile switch only enables LFS in libgis, not anywhere else.

[Although config.h includes definitions to enable LFS automatically, those definitions are currently inactive. This is probably a good thing; a lot of GRASS' code isn't LFS-aware, and explicit failure is preferable to silently corrupting data.]

To enable LFS elsewhere, you need to manually add -D_FILE_OFFSET_BITS=64 to the compilation flags. The simplest approach is to add to the module's Makefile:

ifneq ($(USE_LARGEFILES),)
	EXTRA_CFLAGS = -D_FILE_OFFSET_BITS=64
endif

and add include config.h before all other header files in the code.

#include <grass/config.h>
#include <stdio.h>
#include <string.h>
#include <grass/gis.h>
...

LFS-safe libs and module list

• libgis

LFS works in progress

• r.in.xyz
• r.terraflow (intregrate current LFS support into GRASS's --enable-largefile ./configure switch)
  (r.terraflow creates huge temporary files which can easily go over 2GB)

LFS wish list

High priority modules to get LFS

• r.in.*
• r.out.*
• GRASS GDAL plugin (??)
• v.surf.rst
• v.surf.idw(2)
• vector libs (limited by number of features)
• v.in.ascii -bt (without topology)
• DB libs