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:
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
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.
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
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
NULs in strings.
However, the program interface currently used already presumes
that strings are
NUL terminated, so embedded
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 <-----------------' | | ... ... | | +------------------------------------------+