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| == The need ==
| | {{MovedToTrac|LargeFileSupport}} |
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| 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)
| | [[Category: Development]] |
| | | [[Category: massive data analysis]] |
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| == The issues ==
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| The problem is that ftell() returns the result as a (signed) long. If
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| the result won't fit into a long, it returns -1 (and sets errno to
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| EOVERFLOW).
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| This can only happen if you also set _FILE_OFFSET_BITS to 64 so that
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| fopen() is redirected to fopen64(), otherwise fopen() will simply
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| refuse to open files larger than 2GiB (apparently, this isn't true on
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| some versions of MacOSX, which open the file anyhow then fail on
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| fseek/ftell once you've passed the 2GiB mark).
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| If you want to obtain the current offset for a file whose size exceeds
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| the range of a signed long, you instead have to use the (non-ANSI)
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| 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.
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| == Coding LFS in GRASS ==
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| Currently the <tt>--enable-largefile</tt> switch only enables LFS in libgis, not anywhere else.
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| [Although config.h includes definitions to enable LFS automatically, | |
| those definitions are currently inactive. This is probably a good
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| thing; a lot of GRASS' code isn't LFS-aware, and explicit failure is
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| preferable to silently corrupting data.]
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| To enable LFS elsewhere, you need to manually add
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| -D_FILE_OFFSET_BITS=64 to the compilation flags. The simplest approach
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| is to add to the module's Makefile:
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| ifneq ($(USE_LARGEFILES),)
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| EXTRA_CFLAGS = -D_FILE_OFFSET_BITS=64
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| endif
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| and add include <tt>config.h</tt> before '''all''' other header files in the code.
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| #include <grass/config.h>
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| #include <stdio.h>
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| #include <string.h>
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| #include <grass/gis.h>
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| ...
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| == LFS-safe libs and module list ==
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| * libgis
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| == LFS works in progress ==
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| * r.in.xyz
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| * r.terraflow (intregrate current LFS support into GRASS's --enable-largefile ./configure switch)
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| == LFS wish list ==
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| '''High priority modules to get LFS'''
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| * r.in.*
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| * r.out.*
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| * GRASS GDAL plugin (??)
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| * v.surf.rst
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| * v.surf.idw(2)
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| * vector libs (limited by number of features)
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| * v.in.ascii -bt (without topology)
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| * DB libs
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