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<TITLE>GNU gettext utilities - 7 Producing Binary MO Files</TITLE>
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<H1><A NAME="SEC34" HREF="gettext_toc.html#TOC34">7 Producing Binary MO Files</A></H1>
<H2><A NAME="SEC35" HREF="gettext_toc.html#TOC35">7.1 Invoking the <CODE>msgfmt</CODE> Program</A></H2>
<PRE>
Usage: msgfmt [<VAR>option</VAR>] <VAR>filename</VAR>.po ...
</PRE>
<DL COMPACT>
<DT><SAMP>`-a <VAR>number</VAR>'</SAMP>
<DD>
<DT><SAMP>`--alignment=<VAR>number</VAR>'</SAMP>
<DD>
Align strings to <VAR>number</VAR> bytes (default: 1).
<DT><SAMP>`-h'</SAMP>
<DD>
<DT><SAMP>`--help'</SAMP>
<DD>
Display this help and exit.
<DT><SAMP>`--no-hash'</SAMP>
<DD>
Binary file will not include the hash table.
<DT><SAMP>`-o <VAR>file</VAR>'</SAMP>
<DD>
<DT><SAMP>`--output-file=<VAR>file</VAR>'</SAMP>
<DD>
Specify output file name as <VAR>file</VAR>.
<DT><SAMP>`--strict'</SAMP>
<DD>
Direct the program to work strictly following the Uniforum/Sun
implementation. Currently this only affects the naming of the output
file. If this option is not given the name of the output file is the
same as the domain name. If the strict Uniforum mode is enabled the
suffix <TT>`.mo'</TT> is added to the file name if it is not already
present.
We find this behaviour of Sun's implementation rather silly and so by
default this mode is <EM>not</EM> selected.
<DT><SAMP>`-v'</SAMP>
<DD>
<DT><SAMP>`--verbose'</SAMP>
<DD>
Detect and diagnose input file anomalies which might represent
translation errors. The <CODE>msgid</CODE> and <CODE>msgstr</CODE> strings are
studied and compared. It is considered abnormal that one string
starts or ends with a newline while the other does not.
Also, if the string represents a format string used in a
<CODE>printf</CODE>-like function both strings should have the same number of
<SAMP>`%'</SAMP> format specifiers, with matching types. If the flag
<CODE>c-format</CODE> or <CODE>possible-c-format</CODE> appears in the special
comment <KBD>#,</KBD> for this entry a check is performed. For example, the
check will diagnose using <SAMP>`%.*s'</SAMP> against <SAMP>`%s'</SAMP>, or <SAMP>`%d'</SAMP>
against <SAMP>`%s'</SAMP>, or <SAMP>`%d'</SAMP> against <SAMP>`%x'</SAMP>. It can even handle
positional parameters.
Normally the <CODE>xgettext</CODE> program automatically decides whether a
string is a format string or not. This algorithm is not perfect,
though. It might regard a string as a format string though it is not
used in a <CODE>printf</CODE>-like function and so <CODE>msgfmt</CODE> might report
errors where there are none. Or the other way round: a string is not
regarded as a format string but it is used in a <CODE>printf</CODE>-like
function.
So solve this problem the programmer can dictate the decision to the
<CODE>xgettext</CODE> program (see section <A HREF="gettext_3.html#SEC17">3.4 Special Comments preceding Keywords</A>). The translator should not
consider removing the flag from the <KBD>#,</KBD> line. This "fix" would be
reversed again as soon as <CODE>msgmerge</CODE> is called the next time.
<DT><SAMP>`-V'</SAMP>
<DD>
<DT><SAMP>`--version'</SAMP>
<DD>
Output version information and exit.
</DL>
<P>
If input file is <SAMP>`-'</SAMP>, standard input is read. If output file
is <SAMP>`-'</SAMP>, output is written to standard output.
</P>
<H2><A NAME="SEC36" HREF="gettext_toc.html#TOC36">7.2 The Format of GNU MO Files</A></H2>
<P>
The format of the generated MO files is best described by a picture,
which appears below.
</P>
<P>
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: <CODE>0x950412de</CODE> and <CODE>0xde120495</CODE>. 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 <TT>`/etc/magic'</TT> is
not updated often. It might be better to have magic separated from
internal format version identification.
</P>
<P>
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.
</P>
<P>
Then, at offset <VAR>O</VAR> and offset <VAR>T</VAR> 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.
</P>
<P>
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 <CODE>gettext</CODE> is usually <EM>translated</EM> 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.
</P>
<P>
The size <VAR>S</VAR> 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 <EM>that</EM> 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 of GNU <CODE>gettext</CODE> code, and is not documented here.
</P>
<P>
As for the strings themselves, they follow the hash file, and each
is terminated with a <KBD>NUL</KBD>, and this <KBD>NUL</KBD> is not counted in
the length which appears in the string descriptor. The <CODE>msgfmt</CODE>
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.
</P>
<P>
Plural forms are stored by letting the plural of the original string
follow the singular of the original string, separated through a
<KBD>NUL</KBD> 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
<KBD>NUL</KBD> byte. Here also, the length in the string descriptor
includes all of them.
</P>
<P>
Nothing prevents a MO file from having embedded <KBD>NUL</KBD>s in strings.
However, the program interface currently used already presumes
that strings are <KBD>NUL</KBD> terminated, so embedded <KBD>NUL</KBD>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 <KBD>NUL</KBD> bytes may
accidently appear. (No, we don't want to have wide characters in MO
files. They would make the file unnecessarily large, and the
<SAMP>`wchar_t'</SAMP> type being platform dependent, MO files would be
platform dependent as well.)
</P>
<P>
This particular issue has been strongly debated in the GNU
<CODE>gettext</CODE> 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.
</P>
<PRE>
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 <-----------------'
| |
... ...
| |
+------------------------------------------+
</PRE>
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