(gettext) MO Files

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 10.3 The Format of GNU MO Files
 The format of the generated MO files is best described by a picture,
 which appears below.
    The first two words serve the identification of the file.  The magic
 number will always signal GNU MO files.  The number is stored in the
 byte order of the generating machine, so the magic number really is two
 numbers: `0x950412de' and `0xde120495'.  The second word describes the
 current revision of the file format.  For now the revision is 0.  This
 might change in future versions, and ensures that the readers of MO
 files can distinguish new formats from old ones, so that both can be
 handled correctly.  The version is kept separate from the magic number,
 instead of using different magic numbers for different formats, mainly
 because `/etc/magic' is not updated often.  It might be better to have
 magic separated from internal format version identification.
    Follow a number of pointers to later tables in the file, allowing
 for the extension of the prefix part of MO files without having to
 recompile programs reading them.  This might become useful for later
 inserting a few flag bits, indication about the charset used, new
 tables, or other things.
    Then, at offset O and offset T in the picture, two tables of string
 descriptors can be found.  In both tables, each string descriptor uses
 two 32 bits integers, one for the string length, another for the offset
 of the string in the MO file, counting in bytes from the start of the
 file.  The first table contains descriptors for the original strings,
 and is sorted so the original strings are in increasing lexicographical
 order.  The second table contains descriptors for the translated
 strings, and is parallel to the first table: to find the corresponding
 translation one has to access the array slot in the second array with
 the same index.
    Having the original strings sorted enables the use of simple binary
 search, for when the MO file does not contain an hashing table, or for
 when it is not practical to use the hashing table provided in the MO
 file.  This also has another advantage, as the empty string in a PO
 file GNU `gettext' is usually _translated_ into some system information
 attached to that particular MO file, and the empty string necessarily
 becomes the first in both the original and translated tables, making
 the system information very easy to find.
    The size S of the hash table can be zero.  In this case, the hash
 table itself is not contained in the MO file.  Some people might prefer
 this because a precomputed hashing table takes disk space, and does not
 win _that_ much speed.  The hash table contains indices to the sorted
 array of strings in the MO file.  Conflict resolution is done by double
 hashing.  The precise hashing algorithm used is fairly dependent on GNU
 `gettext' code, and is not documented here.
    As for the strings themselves, they follow the hash file, and each
 is terminated with a <NUL>, and this <NUL> is not counted in the length
 which appears in the string descriptor.  The `msgfmt' program has an
 option selecting the alignment for MO file strings.  With this option,
 each string is separately aligned so it starts at an offset which is a
 multiple of the alignment value.  On some RISC machines, a correct
 alignment will speed things up.
    Contexts are stored by storing the concatenation of the context, a
 <EOT> byte, and the original string, instead of the original string.
    Plural forms are stored by letting the plural of the original string
 follow the singular of the original string, separated through a <NUL>
 byte.  The length which appears in the string descriptor includes both.
 However, only the singular of the original string takes part in the
 hash table lookup.  The plural variants of the translation are all
 stored consecutively, separated through a <NUL> byte.  Here also, the
 length in the string descriptor includes all of them.
    Nothing prevents a MO file from having embedded <NUL>s in strings.
 However, the program interface currently used already presumes that
 strings are <NUL> terminated, so embedded <NUL>s are somewhat useless.
 But the MO file format is general enough so other interfaces would be
 later possible, if for example, we ever want to implement wide
 characters right in MO files, where <NUL> bytes may accidentally
 appear.  (No, we don't want to have wide characters in MO files.  They
 would make the file unnecessarily large, and the `wchar_t' type being
 platform dependent, MO files would be platform dependent as well.)
    This particular issue has been strongly debated in the GNU `gettext'
 development forum, and it is expectable that MO file format will evolve
 or change over time.  It is even possible that many formats may later
 be supported concurrently.  But surely, we have to start somewhere, and
 the MO file format described here is a good start.  Nothing is cast in
 concrete, and the format may later evolve fairly easily, so we should
 feel comfortable with the current approach.
                0  | magic number = 0x950412de                |
                   |                                          |
                4  | file format revision = 0                 |
                   |                                          |
                8  | number of strings                        |  == N
                   |                                          |
               12  | offset of table with original strings    |  == O
                   |                                          |
               16  | offset of table with translation strings |  == T
                   |                                          |
               20  | size of hashing table                    |  == S
                   |                                          |
               24  | offset of hashing table                  |  == H
                   |                                          |
                   .                                          .
                   .    (possibly more entries later)         .
                   .                                          .
                   |                                          |
                O  | length & offset 0th string  ----------------.
            O + 8  | length & offset 1st string  ------------------.
                    ...                                    ...   | |
      O + ((N-1)*8)| length & offset (N-1)th string           |  | |
                   |                                          |  | |
                T  | length & offset 0th translation  ---------------.
            T + 8  | length & offset 1st translation  -----------------.
                    ...                                    ...   | | | |
      T + ((N-1)*8)| length & offset (N-1)th translation      |  | | | |
                   |                                          |  | | | |
                H  | start hash table                         |  | | | |
                    ...                                    ...   | | | |
        H + S * 4  | end hash table                           |  | | | |
                   |                                          |  | | | |
                   | NUL terminated 0th string  <----------------' | | |
                   |                                          |    | | |
                   | NUL terminated 1st string  <------------------' | |
                   |                                          |      | |
                    ...                                    ...       | |
                   |                                          |      | |
                   | NUL terminated 0th translation  <---------------' |
                   |                                          |        |
                   | NUL terminated 1st translation  <-----------------'
                   |                                          |
                    ...                                    ...
                   |                                          |
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