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   6 <TITLE>GNU gettext utilities - Producing Binary MO Files</TITLE>
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  15
  16
  17 <H1><A NAME="SEC32" HREF="gettext_toc.html#TOC32">Producing Binary MO Files</A></H1>
  18
  19
  20
  21 <H2><A NAME="SEC33" HREF="gettext_toc.html#TOC33">Invoking the <CODE>msgfmt</CODE> Program</A></H2>
  22
  23
  24 <PRE>
  25 Usage: msgfmt [<VAR>option</VAR>] <VAR>filename</VAR>.po ...
  26 </PRE>
  27
  28 <DL COMPACT>
  29
  30 <DT><SAMP>`-a <VAR>number</VAR>'</SAMP>
  31 <DD>
  32 <DT><SAMP>`--alignment=<VAR>number</VAR>'</SAMP>
  33 <DD>
  34 Align strings to <VAR>number</VAR> bytes (default: 1).
  35
  36 <DT><SAMP>`-h'</SAMP>
  37 <DD>
  38 <DT><SAMP>`--help'</SAMP>
  39 <DD>
  40 Display this help and exit.
  41
  42 <DT><SAMP>`--no-hash'</SAMP>
  43 <DD>
  44 Binary file will not include the hash table.
  45
  46 <DT><SAMP>`-o <VAR>file</VAR>'</SAMP>
  47 <DD>
  48 <DT><SAMP>`--output-file=<VAR>file</VAR>'</SAMP>
  49 <DD>
  50 Specify output file name as <VAR>file</VAR>.
  51
  52 <DT><SAMP>`--strict'</SAMP>
  53 <DD>
  54 Direct the program to work strictly following the Uniforum/Sun
  55 implementation.  Currently this only affects the naming of the output
  56 file.  If this option is not given the name of the output file is the
  57 same as the domain name.  If the strict Uniforum mode is enable the
  58 suffix <TT>`.mo'</TT> is added to the file name if it is not already
  59 present.
  60
  61 We find this behaviour of Sun's implementation rather silly and so by
  62 default this mode is <EM>not</EM> selected.
  63
  64 <DT><SAMP>`-v'</SAMP>
  65 <DD>
  66 <DT><SAMP>`--verbose'</SAMP>
  67 <DD>
  68 Detect and diagnose input file anomalies which might represent
  69 translation errors.  The <CODE>msgid</CODE> and <CODE>msgstr</CODE> strings are
  70 studied and compared.  It is considered abnormal that one string
  71 starts or ends with a newline while the other does not.
  72
  73 Also, if the string represents a format string used in a
  74 <CODE>printf</CODE>-like function both strings should have the same number of
  75 <SAMP>`%'</SAMP> format specifiers, with matching types.  If the flag
  76 <CODE>c-format</CODE> or <CODE>possible-c-format</CODE> appears in the special
  77 comment <KBD>#,</KBD> for this entry a check is performed.  For example, the
  78 check will diagnose using <SAMP>`%.*s'</SAMP> against <SAMP>`%s'</SAMP>, or <SAMP>`%d'</SAMP>
  79 against <SAMP>`%s'</SAMP>, or <SAMP>`%d'</SAMP> against <SAMP>`%x'</SAMP>.  It can even handle
  80 positional parameters.
  81
  82 Normally the <CODE>xgettext</CODE> program automatically decides whether a
  83 string is a format string or not.  This algorithm is not perfect,
  84 though.  It might regard a string as a format string though it is not
  85 used in a <CODE>printf</CODE>-like function and so <CODE>msgfmt</CODE> might report
  86 errors where there are none.  Or the other way round: a string is not
  87 regarded as a format string but it is used in a <CODE>printf</CODE>-like
  88 function.
  89
  90 So solve this problem the programmer can dictate the decision to the
  91 <CODE>xgettext</CODE> program (see section <A HREF="gettext_3.html#SEC17">Special Comments preceding Keywords</A>).  The translator should not
  92 consider removing the flag from the <KBD>#,</KBD> line.  This "fix" would be
  93 reversed again as soon as <CODE>msgmerge</CODE> is called the next time.
  94
  95 <DT><SAMP>`-V'</SAMP>
  96 <DD>
  97 <DT><SAMP>`--version'</SAMP>
  98 <DD>
  99 Output version information and exit.
 100
 101 </DL>
 102
 103 <P>
 104 If input file is <SAMP>`-'</SAMP>, standard input is read.  If output file
 105 is <SAMP>`-'</SAMP>, output is written to standard output.
 106
 107 </P>
 108
 109
 110 <H2><A NAME="SEC34" HREF="gettext_toc.html#TOC34">The Format of GNU MO Files</A></H2>
 111
 112 <P>
 113 The format of the generated MO files is best described by a picture,
 114 which appears below.
 115
 116 </P>
 117 <P>
 118 The first two words serve the identification of the file.  The magic
 119 number will always signal GNU MO files.  The number is stored in the
 120 byte order of the generating machine, so the magic number really is
 121 two numbers: <CODE>0x950412de</CODE> and <CODE>0xde120495</CODE>.  The second
 122 word describes the current revision of the file format.  For now the
 123 revision is 0.  This might change in future versions, and ensures
 124 that the readers of MO files can distinguish new formats from old
 125 ones, so that both can be handled correctly.  The version is kept
 126 separate from the magic number, instead of using different magic
 127 numbers for different formats, mainly because <TT>`/etc/magic'</TT> is
 128 not updated often.  It might be better to have magic separated from
 129 internal format version identification.
 130
 131 </P>
 132 <P>
 133 Follow a number of pointers to later tables in the file, allowing
 134 for the extension of the prefix part of MO files without having to
 135 recompile programs reading them.  This might become useful for later
 136 inserting a few flag bits, indication about the charset used, new
 137 tables, or other things.
 138
 139 </P>
 140 <P>
 141 Then, at offset <VAR>O</VAR> and offset <VAR>T</VAR> in the picture, two tables
 142 of string descriptors can be found.  In both tables, each string
 143 descriptor uses two 32 bits integers, one for the string length,
 144 another for the offset of the string in the MO file, counting in bytes
 145 from the start of the file.  The first table contains descriptors
 146 for the original strings, and is sorted so the original strings
 147 are in increasing lexicographical order.  The second table contains
 148 descriptors for the translated strings, and is parallel to the first
 149 table: to find the corresponding translation one has to access the
 150 array slot in the second array with the same index.
 151
 152 </P>
 153 <P>
 154 Having the original strings sorted enables the use of simple binary
 155 search, for when the MO file does not contain an hashing table, or
 156 for when it is not practical to use the hashing table provided in
 157 the MO file.  This also has another advantage, as the empty string
 158 in a PO file GNU <CODE>gettext</CODE> is usually <EM>translated</EM> into
 159 some system information attached to that particular MO file, and the
 160 empty string necessarily becomes the first in both the original and
 161 translated tables, making the system information very easy to find.
 162
 163 </P>
 164 <P>
 165 The size <VAR>S</VAR> of the hash table can be zero.  In this case, the
 166 hash table itself is not contained in the MO file.  Some people might
 167 prefer this because a precomputed hashing table takes disk space, and
 168 does not win <EM>that</EM> much speed.  The hash table contains indices
 169 to the sorted array of strings in the MO file.  Conflict resolution is
 170 done by double hashing.  The precise hashing algorithm used is fairly
 171 dependent of GNU <CODE>gettext</CODE> code, and is not documented here.
 172
 173 </P>
 174 <P>
 175 As for the strings themselves, they follow the hash file, and each
 176 is terminated with a <KBD>NUL</KBD>, and this <KBD>NUL</KBD> is not counted in
 177 the length which appears in the string descriptor.  The <CODE>msgfmt</CODE>
 178 program has an option selecting the alignment for MO file strings.
 179 With this option, each string is separately aligned so it starts at
 180 an offset which is a multiple of the alignment value.  On some RISC
 181 machines, a correct alignment will speed things up.
 182
 183 </P>
 184 <P>
 185 Nothing prevents a MO file from having embedded <KBD>NUL</KBD>s in strings.
 186 However, the program interface currently used already presumes
 187 that strings are <KBD>NUL</KBD> terminated, so embedded <KBD>NUL</KBD>s are
 188 somewhat useless.  But MO file format is general enough so other
 189 interfaces would be later possible, if for example, we ever want to
 190 implement wide characters right in MO files, where <KBD>NUL</KBD> bytes may
 191 accidently appear.
 192
 193 </P>
 194 <P>
 195 This particular issue has been strongly debated in the GNU
 196 <CODE>gettext</CODE> development forum, and it is expectable that MO file
 197 format will evolve or change over time.  It is even possible that many
 198 formats may later be supported concurrently.  But surely, we have to
 199 start somewhere, and the MO file format described here is a good start.
 200 Nothing is cast in concrete, and the format may later evolve fairly
 201 easily, so we should feel comfortable with the current approach.
 202
 203 </P>
 204
 205 <PRE>
 206         byte
 207              +------------------------------------------+
 208           0  | magic number = 0x950412de                |
 209              |                                          |
 210           4  | file format revision = 0                 |
 211              |                                          |
 212           8  | number of strings                        |  == N
 213              |                                          |
 214          12  | offset of table with original strings    |  == O
 215              |                                          |
 216          16  | offset of table with translation strings |  == T
 217              |                                          |
 218          20  | size of hashing table                    |  == S
 219              |                                          |
 220          24  | offset of hashing table                  |  == H
 221              |                                          |
 222              .                                          .
 223              .    (possibly more entries later)         .
 224              .                                          .
 225              |                                          |
 226           O  | length &#38; offset 0th string  ----------------.
 227       O + 8  | length &#38; offset 1st string  ------------------.
 228               ...                                    ...   | |
 229 O + ((N-1)*8)| length &#38; offset (N-1)th string           |  | |
 230              |                                          |  | |
 231           T  | length &#38; offset 0th translation  ---------------.
 232       T + 8  | length &#38; offset 1st translation  -----------------.
 233               ...                                    ...   | | | |
 234 T + ((N-1)*8)| length &#38; offset (N-1)th translation      |  | | | |
 235              |                                          |  | | | |
 236           H  | start hash table                         |  | | | |
 237               ...                                    ...   | | | |
 238   H + S * 4  | end hash table                           |  | | | |
 239              |                                          |  | | | |
 240              | NUL terminated 0th string  &#60;----------------' | | |
 241              |                                          |    | | |
 242              | NUL terminated 1st string  &#60;------------------' | |
 243              |                                          |      | |
 244               ...                                    ...       | |
 245              |                                          |      | |
 246              | NUL terminated 0th translation  &#60;---------------' |
 247              |                                          |        |
 248              | NUL terminated 1st translation  &#60;-----------------'
 249              |                                          |
 250               ...                                    ...
 251              |                                          |
 252              +------------------------------------------+
 253 </PRE>
 254
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