GNU gettext is a popular solution in free/open source software world to do i18n/l10n of software, by providing translated strings that will substitute strings in original language (typically, English).
gettext .mo is a binary database format which stores these string translation pairs in an efficient binary format, ready to be used by gettext-enabled software. .mo format is a result of compilation of text-based .po files using msgfmt utility. The reverse conversion (.mo -> .po) is also possible using msgunfmt decompiler utility.
This page hosts a formal specification of gettext binary database using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
digraph {
rankdir=LR;
node [shape=plaintext];
subgraph cluster__gettext_mo {
label="GettextMo";
graph[style=dotted];
gettext_mo__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="signature_pos">0</TD><TD PORT="signature_size">4</TD><TD></TD><TD PORT="signature_type">signature</TD></TR>
<TR><TD PORT="mo_pos">4</TD><TD PORT="mo_size">24</TD><TD>Mo</TD><TD PORT="mo_type">mo</TD></TR>
</TABLE>>];
subgraph cluster__hash_lookup_iteration {
label="GettextMo::HashLookupIteration";
graph[style=dotted];
hash_lookup_iteration__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
</TABLE>>];
hash_lookup_iteration__inst__original [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>original</TD><TD>_root.mo.originals[idx].str</TD></TR>
</TABLE>>];
hash_lookup_iteration__inst__translation [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>translation</TD><TD>_root.mo.translations[idx].str</TD></TR>
</TABLE>>];
hash_lookup_iteration__inst__next_idx [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>next_idx</TD><TD>((idx + collision_step) - (idx >= (_root.mo.num_hashtable_items - collision_step) ? _root.mo.num_hashtable_items : 0))</TD></TR>
</TABLE>>];
hash_lookup_iteration__inst__next [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="next_pos">0</TD><TD PORT="next_size">0</TD><TD>HashLookupIteration</TD><TD PORT="next_type">next</TD></TR>
</TABLE>>];
}
subgraph cluster__lookup_iteration {
label="GettextMo::LookupIteration";
graph[style=dotted];
lookup_iteration__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
</TABLE>>];
lookup_iteration__inst__found [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>found</TD><TD>query == current.original</TD></TR>
</TABLE>>];
lookup_iteration__inst__next [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="next_pos">0</TD><TD PORT="next_size">0</TD><TD>LookupIteration</TD><TD PORT="next_type">next</TD></TR>
</TABLE>>];
}
subgraph cluster__hashtable_lookup {
label="GettextMo::HashtableLookup";
graph[style=dotted];
hashtable_lookup__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
</TABLE>>];
hashtable_lookup__inst__collision_step [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>collision_step</TD><TD>((hash % (_root.mo.num_hashtable_items - 2)) + 1)</TD></TR>
</TABLE>>];
hashtable_lookup__inst__idx [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>idx</TD><TD>(hash % _root.mo.num_hashtable_items)</TD></TR>
</TABLE>>];
hashtable_lookup__inst__hash_lookup_iteration [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="hash_lookup_iteration_pos">0</TD><TD PORT="hash_lookup_iteration_size">0</TD><TD>HashLookupIteration</TD><TD PORT="hash_lookup_iteration_type">hash_lookup_iteration</TD></TR>
</TABLE>>];
hashtable_lookup__inst__entry [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="entry_pos">0</TD><TD PORT="entry_size">0</TD><TD>LookupIteration</TD><TD PORT="entry_type">entry</TD></TR>
</TABLE>>];
}
subgraph cluster__mo {
label="GettextMo::Mo";
graph[style=dotted];
mo__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="version_pos">0</TD><TD PORT="version_size">4</TD><TD>Version</TD><TD PORT="version_type">version</TD></TR>
<TR><TD PORT="num_translations_pos">4</TD><TD PORT="num_translations_size">4</TD><TD>u4</TD><TD PORT="num_translations_type">num_translations</TD></TR>
<TR><TD PORT="ofs_originals_pos">8</TD><TD PORT="ofs_originals_size">4</TD><TD>u4</TD><TD PORT="ofs_originals_type">ofs_originals</TD></TR>
<TR><TD PORT="ofs_translations_pos">12</TD><TD PORT="ofs_translations_size">4</TD><TD>u4</TD><TD PORT="ofs_translations_type">ofs_translations</TD></TR>
<TR><TD PORT="num_hashtable_items_pos">16</TD><TD PORT="num_hashtable_items_size">4</TD><TD>u4</TD><TD PORT="num_hashtable_items_type">num_hashtable_items</TD></TR>
<TR><TD PORT="ofs_hashtable_items_pos">20</TD><TD PORT="ofs_hashtable_items_size">4</TD><TD>u4</TD><TD PORT="ofs_hashtable_items_type">ofs_hashtable_items</TD></TR>
</TABLE>>];
mo__inst__originals [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="originals_pos">ofs_originals</TD><TD PORT="originals_size">8</TD><TD>Descriptor</TD><TD PORT="originals_type">originals</TD></TR>
<TR><TD COLSPAN="4" PORT="originals__repeat">repeat num_translations times</TD></TR>
</TABLE>>];
mo__inst__translations [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="translations_pos">ofs_translations</TD><TD PORT="translations_size">8</TD><TD>Descriptor</TD><TD PORT="translations_type">translations</TD></TR>
<TR><TD COLSPAN="4" PORT="translations__repeat">repeat num_translations times</TD></TR>
</TABLE>>];
mo__inst__hashtable_items [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="hashtable_items_pos">ofs_hashtable_items</TD><TD PORT="hashtable_items_size">4</TD><TD>HashtableItem</TD><TD PORT="hashtable_items_type">hashtable_items</TD></TR>
<TR><TD COLSPAN="4" PORT="hashtable_items__repeat">repeat num_hashtable_items times</TD></TR>
</TABLE>>];
subgraph cluster__version {
label="GettextMo::Mo::Version";
graph[style=dotted];
version__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="version_raw_pos">0</TD><TD PORT="version_raw_size">4</TD><TD>u4</TD><TD PORT="version_raw_type">version_raw</TD></TR>
</TABLE>>];
version__inst__major [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>major</TD><TD>(version_raw >> 16)</TD></TR>
</TABLE>>];
version__inst__minor [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>minor</TD><TD>(version_raw & 65535)</TD></TR>
</TABLE>>];
}
subgraph cluster__hashtable_item {
label="GettextMo::Mo::HashtableItem";
graph[style=dotted];
hashtable_item__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="raw_val_pos">0</TD><TD PORT="raw_val_size">4</TD><TD>u4</TD><TD PORT="raw_val_type">raw_val</TD></TR>
</TABLE>>];
hashtable_item__inst__mask [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>mask</TD><TD>2147483648</TD></TR>
</TABLE>>];
hashtable_item__inst__val_1 [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>val_1</TD><TD>(raw_val - 1)</TD></TR>
</TABLE>>];
hashtable_item__inst__is_system_dependent [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>is_system_dependent</TD><TD>(val_1 & mask) == 1</TD></TR>
</TABLE>>];
hashtable_item__inst__val [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>val</TD><TD>(val_1 & ~(mask))</TD></TR>
</TABLE>>];
}
subgraph cluster__descriptor {
label="GettextMo::Mo::Descriptor";
graph[style=dotted];
descriptor__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="len_str_pos">0</TD><TD PORT="len_str_size">4</TD><TD>u4</TD><TD PORT="len_str_type">len_str</TD></TR>
<TR><TD PORT="ofs_str_pos">4</TD><TD PORT="ofs_str_size">4</TD><TD>u4</TD><TD PORT="ofs_str_type">ofs_str</TD></TR>
</TABLE>>];
descriptor__inst__str [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="str_pos">ofs_str</TD><TD PORT="str_size">len_str</TD><TD>str(UTF-8)</TD><TD PORT="str_type">str</TD></TR>
</TABLE>>];
}
}
}
gettext_mo__seq:mo_type -> mo__seq [style=bold];
descriptor__inst__str:str_type -> hash_lookup_iteration__inst__original [color="#404040"];
descriptor__inst__str:str_type -> hash_lookup_iteration__inst__translation [color="#404040"];
hash_lookup_iteration__params:idx_type -> hash_lookup_iteration__inst__next_idx [color="#404040"];
hash_lookup_iteration__params:collision_step_type -> hash_lookup_iteration__inst__next_idx [color="#404040"];
hash_lookup_iteration__params:idx_type -> hash_lookup_iteration__inst__next_idx [color="#404040"];
mo__seq:num_hashtable_items_type -> hash_lookup_iteration__inst__next_idx [color="#404040"];
hash_lookup_iteration__params:collision_step_type -> hash_lookup_iteration__inst__next_idx [color="#404040"];
mo__seq:num_hashtable_items_type -> hash_lookup_iteration__inst__next_idx [color="#404040"];
hash_lookup_iteration__inst__next:next_type -> hash_lookup_iteration__seq [style=bold];
lookup_iteration__params:query_type -> lookup_iteration__inst__found [color="#404040"];
hash_lookup_iteration__inst__original:original_type -> lookup_iteration__inst__found [color="#404040"];
lookup_iteration__inst__next:next_type -> lookup_iteration__seq [style=bold];
hashtable_lookup__params:hash_type -> hashtable_lookup__inst__collision_step [color="#404040"];
mo__seq:num_hashtable_items_type -> hashtable_lookup__inst__collision_step [color="#404040"];
hashtable_lookup__params:hash_type -> hashtable_lookup__inst__idx [color="#404040"];
mo__seq:num_hashtable_items_type -> hashtable_lookup__inst__idx [color="#404040"];
hashtable_lookup__inst__hash_lookup_iteration:hash_lookup_iteration_type -> hash_lookup_iteration__seq [style=bold];
hashtable_lookup__inst__entry:entry_type -> lookup_iteration__seq [style=bold];
mo__seq:version_type -> version__seq [style=bold];
mo__seq:ofs_originals_type -> mo__inst__originals:originals_pos [color="#404040"];
mo__inst__originals:originals_type -> descriptor__seq [style=bold];
mo__seq:num_translations_type -> mo__inst__originals:originals__repeat [color="#404040"];
mo__seq:ofs_translations_type -> mo__inst__translations:translations_pos [color="#404040"];
mo__inst__translations:translations_type -> descriptor__seq [style=bold];
mo__seq:num_translations_type -> mo__inst__translations:translations__repeat [color="#404040"];
mo__seq:ofs_hashtable_items_type -> mo__inst__hashtable_items:hashtable_items_pos [color="#404040"];
mo__inst__hashtable_items:hashtable_items_type -> hashtable_item__seq [style=bold];
mo__seq:num_hashtable_items_type -> mo__inst__hashtable_items:hashtable_items__repeat [color="#404040"];
version__seq:version_raw_type -> version__inst__major [color="#404040"];
version__seq:version_raw_type -> version__inst__minor [color="#404040"];
hashtable_item__seq:raw_val_type -> hashtable_item__inst__val_1 [color="#404040"];
hashtable_item__inst__val_1:val_1_type -> hashtable_item__inst__is_system_dependent [color="#404040"];
hashtable_item__inst__mask:mask_type -> hashtable_item__inst__is_system_dependent [color="#404040"];
hashtable_item__inst__val_1:val_1_type -> hashtable_item__inst__val [color="#404040"];
hashtable_item__inst__mask:mask_type -> hashtable_item__inst__val [color="#404040"];
descriptor__seq:ofs_str_type -> descriptor__inst__str:str_pos [color="#404040"];
descriptor__seq:len_str_type -> descriptor__inst__str:str_size [color="#404040"];
}