Define the fixed and string-like types a bit more formally

Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=38252

1 files changed, 164 insertions, 4 deletions

diff --git a/doc/dbus-specification.xml b/doc/dbus-specification.xml
index 9907fa3b..eea9ef34 100644
--- a/doc/dbus-specification.xml
+++ b/doc/dbus-specification.xml
@@ -343,6 +343,20 @@
       <title>Basic types</title>
 
       <para>
+        The simplest type codes are the <firstterm id="term-basic-type">basic
+          types</firstterm>, which are the types whose structure is entirely
+        defined by their 1-character type code. Basic types consist of
+        fixed types and string-like types.
+      </para>
+
+      <para>
+        The <firstterm id="term-fixed-type">fixed types</firstterm>
+        are basic types whose values have a fixed length, namely BYTE,
+        BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
+        16, 32 or 64 bits.
+      </para>
+
+      <para>
         As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
         the ASCII character 'i'. So the signature for a block of values 
         containing a single <literal>INT32</literal> would be:
@@ -356,11 +370,130 @@
       </para>
 
       <para>
-        All fixed types work like
-        <literal>INT32</literal> in this example: to marshal and unmarshal
-        fixed types, you simply read one value from the data
-        block corresponding to each type code in the signature.
+        The characteristics of the fixed types are listed in this table.
+
+        <informaltable>
+          <tgroup cols="3">
+            <thead>
+              <row>
+                <entry>Conventional name</entry>
+                <entry>ASCII type-code</entry>
+                <entry>Encoding</entry>
+              </row>
+            </thead>
+            <tbody>
+              <row>
+                <entry><literal>BYTE</literal></entry>
+                <entry><literal>y</literal> (121)</entry>
+                <entry>Unsigned 8-bit integer</entry>
+              </row>
+              <row>
+                <entry><literal>BOOLEAN</literal></entry>
+                <entry><literal>b</literal> (98)</entry>
+                <entry>Boolean value: 0 is false, 1 is true, any other value
+                  allowed by the marshalling format is invalid</entry>
+              </row>
+              <row>
+                <entry><literal>INT16</literal></entry>
+                <entry><literal>n</literal> (110)</entry>
+                <entry>Signed (two's complement) 16-bit integer</entry>
+              </row>
+              <row>
+                <entry><literal>UINT16</literal></entry>
+                <entry><literal>q</literal> (113)</entry>
+                <entry>Unsigned 16-bit integer</entry>
+              </row>
+              <row>
+                <entry><literal>INT32</literal></entry>
+                <entry><literal>i</literal> (105)</entry>
+                <entry>Signed (two's complement) 32-bit integer</entry>
+              </row>
+              <row>
+                <entry><literal>UINT32</literal></entry>
+                <entry><literal>u</literal> (117)</entry>
+                <entry>Unsigned 32-bit integer</entry>
+              </row>
+              <row>
+                <entry><literal>INT64</literal></entry>
+                <entry><literal>x</literal> (120)</entry>
+                <entry>Signed (two's complement) 64-bit integer
+                  (mnemonic: x and t are the first characters in "sixty" not
+                  already used for something more common)</entry>
+              </row>
+              <row>
+                <entry><literal>UINT64</literal></entry>
+                <entry><literal>t</literal> (116)</entry>
+                <entry>Unsigned 64-bit integer</entry>
+              </row>
+              <row>
+                <entry><literal>DOUBLE</literal></entry>
+                <entry><literal>d</literal> (100)</entry>
+                <entry>IEEE 754 double-precision floating point</entry>
+              </row>
+              <row>
+                <entry><literal>UNIX_FD</literal></entry>
+                <entry><literal>h</literal> (104)</entry>
+                <entry>Unsigned 32-bit integer representing an index into an
+                  out-of-band array of file descriptors, transferred via some
+                  platform-specific mechanism (mnemonic: h for handle)</entry>
+              </row>
+            </tbody>
+          </tgroup>
+        </informaltable>
+      </para>
+
+      <para>
+        The <firstterm id="term-string-like-type">string-like types</firstterm>
+        are basic types with a variable length. The value of any string-like
+        type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
+        none of which may be U+0000. The UTF-8 text must be validated
+        strictly: in particular, it must not contain overlong sequences,
+        noncharacters such as U+FFFE, or codepoints above U+10FFFF.
+      </para>
+
+      <para>
+        The marshalling formats for the string-like types all end with a
+        single zero (NUL) byte, but that byte is not considered to be part of
+        the text.
+      </para>
+
+      <para>
+        The characteristics of the string-like types are listed in this table.
+
+        <informaltable>
+          <tgroup cols="3">
+            <thead>
+              <row>
+                <entry>Conventional name</entry>
+                <entry>ASCII type-code</entry>
+                <entry>Validity constraints</entry>
+              </row>
+            </thead>
+            <tbody>
+              <row>
+                <entry><literal>STRING</literal></entry>
+                <entry><literal>s</literal> (115)</entry>
+                <entry>No extra constraints</entry>
+              </row>
+              <row>
+                <entry><literal>OBJECT_PATH</literal></entry>
+                <entry><literal>o</literal> (111)</entry>
+                <entry>Must be
+                  <link linkend="message-protocol-marshaling-object-path">a
+                    syntactically valid object path</link></entry>
+              </row>
+              <row>
+                <entry><literal>SIGNATURE</literal></entry>
+                <entry><literal>g</literal> (103)</entry>
+                <entry>Zero or more
+                  <firstterm linkend="term-single-complete-type">single
+                    complete types</firstterm></entry>
+              </row>
+            </tbody>
+          </tgroup>
+        </informaltable>
       </para>
+
     </sect2>
 
     <sect2 id="container-types">
@@ -617,6 +750,33 @@
       </para>
 
       <para>
+        To marshal and unmarshal fixed types, you simply read one value
+        from the data block corresponding to each type code in the signature.
+        All signed integer values are encoded in two's complement, DOUBLE
+        values are IEEE 754 double-precision floating-point, and BOOLEAN
+        values are encoded in 32 bits (of which only the least significant
+        bit is used).
+      </para>
+
+      <para>
+        The string-like types are all marshalled as a
+        fixed-length unsigned integer <varname>n</varname> giving the
+        length of the variable part, followed by <varname>n</varname>
+        nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
+        which is not considered to be part of the text. The alignment
+        of the string-like type is the same as the alignment of
+        <varname>n</varname>.
+      </para>
+
+      <para>
+        For the STRING and OBJECT_PATH types, <varname>n</varname> is
+        encoded in 4 bytes, leading to 4-byte alignment.
+        For the SIGNATURE type, <varname>n</varname> is encoded as a single
+        byte. As a result, alignment padding is never required before a
+        SIGNATURE.
+      </para>
+
+      <para>
         Given all this, the types are marshaled on the wire as follows:
         <informaltable>
           <tgroup cols="3">