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+<a name="Stab-Section-Basics"></a>
+<div class="header">
+<p>
+Next: <a href="ELF-Linker-Relocation.html#ELF-Linker-Relocation" accesskey="n" rel="next">ELF Linker Relocation</a>, Up: <a href="Stab-Sections.html#Stab-Sections" accesskey="u" rel="up">Stab Sections</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>
+</div>
+<hr>
+<a name="How-to-Embed-Stabs-in-Sections"></a>
+<h3 class="appendixsec">F.1 How to Embed Stabs in Sections</h3>
+
+<p>The assembler creates two custom sections, a section named <code>.stab</code>
+which contains an array of fixed length structures, one struct per stab,
+and a section named <code>.stabstr</code> containing all the variable length
+strings that are referenced by stabs in the <code>.stab</code> section.  The
+byte order of the stabs binary data depends on the object file format.
+For ELF, it matches the byte order of the ELF file itself, as determined
+from the <code>EI_DATA</code> field in the <code>e_ident</code> member of the ELF
+header.  For SOM, it is always big-endian (is this true??? FIXME).  For
+COFF, it matches the byte order of the COFF headers.  The meaning of the
+fields is the same as for a.out (see <a href="Symbol-Table-Format.html#Symbol-Table-Format">Symbol Table Format</a>), except
+that the <code>n_strx</code> field is relative to the strings for the current
+compilation unit (which can be found using the synthetic N_UNDF stab
+described below), rather than the entire string table.
+</p>
+<p>The first stab in the <code>.stab</code> section for each compilation unit is
+synthetic, generated entirely by the assembler, with no corresponding
+<code>.stab</code> directive as input to the assembler.  This stab contains
+the following fields:
+</p>
+<dl compact="compact">
+<dt><code>n_strx</code></dt>
+<dd><p>Offset in the <code>.stabstr</code> section to the source filename.
+</p>
+</dd>
+<dt><code>n_type</code></dt>
+<dd><p><code>N_UNDF</code>.
+</p>
+</dd>
+<dt><code>n_other</code></dt>
+<dd><p>Unused field, always zero.
+This may eventually be used to hold overflows from the count in
+the <code>n_desc</code> field.
+</p>
+</dd>
+<dt><code>n_desc</code></dt>
+<dd><p>Count of upcoming symbols, i.e., the number of remaining stabs for this
+source file.
+</p>
+</dd>
+<dt><code>n_value</code></dt>
+<dd><p>Size of the string table fragment associated with this source file, in
+bytes.
+</p></dd>
+</dl>
+
+<p>The <code>.stabstr</code> section always starts with a null byte (so that string
+offsets of zero reference a null string), followed by random length strings,
+each of which is null byte terminated.
+</p>
+<p>The ELF section header for the <code>.stab</code> section has its
+<code>sh_link</code> member set to the section number of the <code>.stabstr</code>
+section, and the <code>.stabstr</code> section has its ELF section
+header <code>sh_type</code> member set to <code>SHT_STRTAB</code> to mark it as a
+string table.  SOM and COFF have no way of linking the sections together
+or marking them as string tables.
+</p>
+<p>For COFF, the <code>.stab</code> and <code>.stabstr</code> sections may be simply
+concatenated by the linker.  GDB then uses the <code>n_desc</code> fields to
+figure out the extent of the original sections.  Similarly, the
+<code>n_value</code> fields of the header symbols are added together in order
+to get the actual position of the strings in a desired <code>.stabstr</code>
+section.  Although this design obviates any need for the linker to
+relocate or otherwise manipulate <code>.stab</code> and <code>.stabstr</code>
+sections, it also requires some care to ensure that the offsets are
+calculated correctly.  For instance, if the linker were to pad in
+between the <code>.stabstr</code> sections before concatenating, then the
+offsets to strings in the middle of the executable&rsquo;s <code>.stabstr</code>
+section would be wrong.
+</p>
+<p>The GNU linker is able to optimize stabs information by merging
+duplicate strings and removing duplicate header file information
+(see <a href="Include-Files.html#Include-Files">Include Files</a>).  When some versions of the GNU linker optimize
+stabs in sections, they remove the leading <code>N_UNDF</code> symbol and
+arranges for all the <code>n_strx</code> fields to be relative to the start of
+the <code>.stabstr</code> section.
+</p>
+<hr>
+<div class="header">
+<p>
+Next: <a href="ELF-Linker-Relocation.html#ELF-Linker-Relocation" accesskey="n" rel="next">ELF Linker Relocation</a>, Up: <a href="Stab-Sections.html#Stab-Sections" accesskey="u" rel="up">Stab Sections</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>
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