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