(gettext.info)MO Files
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 used when the MO file was generated, so the magic number
really is two numbers: ‘0x950412de’ and ‘0xde120495’.
The second word describes the current revision of the file format,
composed of a major and a minor revision number. The revision numbers
ensure that the readers of MO files can distinguish new formats from old
ones and handle their contents, as far as possible. For now the major
revision is 0 or 1, and the minor revision is also 0 or 1. More
revisions might be added in the future. A program seeing an unexpected
major revision number should stop reading the MO file entirely; whereas
an unexpected minor revision number means that the file can be read but
will not reveal its full contents, when parsed by a program that
supports only smaller minor revision numbers.
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.
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.
byte
+------------------------------------------+
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 <-----------------'
| |
... ...
| |
+------------------------------------------+
automatically generated by info2www version 1.2.2.9