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| <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>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: |
| |
| <dl> |
| <dt><code>n_strx</code><dd>Offset in the <code>.stabstr</code> section to the source filename. |
| |
| <br><dt><code>n_type</code><dd><code>N_UNDF</code>. |
| |
| <br><dt><code>n_other</code><dd>Unused field, always zero. |
| This may eventually be used to hold overflows from the count in |
| the <code>n_desc</code> field. |
| |
| <br><dt><code>n_desc</code><dd>Count of upcoming symbols, i.e., the number of remaining stabs for this |
| source file. |
| |
| <br><dt><code>n_value</code><dd>Size of the string table fragment associated with this source file, in |
| bytes. |
| </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>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>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's <code>.stabstr</code> |
| section would be wrong. |
| |
| <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. |
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