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| (largely based on comments by Glynn Clements on the GRASS-dev mailing list)
| | {{MovedToTrac|LargeFileSupport}} |
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| == The need ==
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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)
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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.
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| '''''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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| There isn't a truly portable solution. Some platforms might not even
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| have an integral type larger than 32 bits.
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| The most practicaly solution is to use ftello() if it's available.
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| This will require some configure checks. These are simple enough to
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| implement; it's the design which is problematic (as usual).
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| Unlike most HAVE_FOO checks, fseeko() isn't a simple have/don't-have
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| check. Rather, it's usually a case that the function is available only
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| when certain macros are defined (e.g. _LARGEFILE_SOURCE).
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| That gives rise to the question of what we check for, how we check for
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| it, how we pass that information to the code, and how we use it.
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| The trick is deciding what to test, and what to indicate. Do we want
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| to know:
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| 1. Whether ftello() exists with the default CFLAGS/CPPFLAGS?
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| 2. Whether ftello() exists with the default CFLAGS/CPPFLAGS plus a
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| fixed selection of additional switches (e.g. -D_LARGEFILE_SOURCE)?
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| 3. Whether ftello() exists with the default CFLAGS/CPPFLAGS plus a
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| variable selection of additional switches (e.g. -D_LARGEFILE_SOURCE)
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| with those switches being communicated via an additional variable?
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| The test would need to be:
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| #if defined(HAVE_FTELLO) && defined(HAVE_FSEEKO)
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| as you are using both of those.
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| Also, if the existence of FSEEKO/FTELLO is conditional upon certain
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| macros (e.g. _LARGEFILE_SOURCE), you have to ensure that those macros
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| are defined before the first inclusion of <stdio.h>, including any
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| "hidden" inclusion from other headers, which essentially means that
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| you have to ensure that the macros are defined before you include any
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| other headers (except for <grass/config.h>).
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| Rather than try to come up with some infrastructure which allows us to
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| use LFS in a piecemeal fashion, it would be preferable to clean up the
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| GRASS code so that we can enable LFS globally. Then, we can just add:
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| -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
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| to CPPFLAGS, and not have to worry about adding the necessary macros
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| to individual files. Any HAVE_* checks then become simple
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| have/don't-have checks.
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| My inclination would be to add:
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| extern off_t G_ftell(FILE *fp);
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| extern int G_fseek(FILE *stream, off_t offset, int whence);
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| These would be implemented using fseeko/ftello where available, and
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| fseek/ftell otherwise. That eliminates the need to perform checks in
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| individual source files. However, we would need to take care not to
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| use them in code which can't handle large offsets (i.e. code which
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| will truncate off_t values to int/long).
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| If you use -D_FILE_OFFSET_BITS=64, fopen() should be redirected to
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| fopen64(). There are reports that the MacOSX fopen() is equivalent to
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| fopen64() even without additional switches.
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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,
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| 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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| === int versus off_t ===
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| You may as well just use "<tt>off_t filesize</tt>" unconditionally. An
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| "off_t" will always be large enough to hold a "long".
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| When adding "<tt>off_t</tt>", be sure to use:
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| #include <sys/types.h>
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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)<BR>(r.terraflow creates huge temporary files which can easily go over 2GB)
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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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| [[Category: Development]] | | [[Category: Development]] |
| | [[Category: massive data analysis]] |