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| <title>LD: Canonical format</title> |
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| <a name="Canonical-format"></a> |
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| <p> |
| Previous: <a href="BFD-information-loss.html#BFD-information-loss" accesskey="p" rel="prev">BFD information loss</a>, Up: <a href="BFD-outline.html#BFD-outline" accesskey="u" rel="up">BFD outline</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="The-BFD-canonical-object_002dfile-format"></a> |
| <h4 class="subsection">5.1.2 The BFD canonical object-file format</h4> |
| |
| <p>The greatest potential for loss of information occurs when there is the least |
| overlap between the information provided by the source format, that |
| stored by the canonical format, and that needed by the |
| destination format. A brief description of the canonical form may help |
| you understand which kinds of data you can count on preserving across |
| conversions. |
| <a name="index-BFD-canonical-format"></a> |
| <a name="index-internal-object_002dfile-format"></a> |
| </p> |
| <dl compact="compact"> |
| <dt><em>files</em></dt> |
| <dd><p>Information stored on a per-file basis includes target machine |
| architecture, particular implementation format type, a demand pageable |
| bit, and a write protected bit. Information like Unix magic numbers is |
| not stored here—only the magic numbers’ meaning, so a <code>ZMAGIC</code> |
| file would have both the demand pageable bit and the write protected |
| text bit set. The byte order of the target is stored on a per-file |
| basis, so that big- and little-endian object files may be used with one |
| another. |
| </p> |
| </dd> |
| <dt><em>sections</em></dt> |
| <dd><p>Each section in the input file contains the name of the section, the |
| section’s original address in the object file, size and alignment |
| information, various flags, and pointers into other BFD data |
| structures. |
| </p> |
| </dd> |
| <dt><em>symbols</em></dt> |
| <dd><p>Each symbol contains a pointer to the information for the object file |
| which originally defined it, its name, its value, and various flag |
| bits. When a BFD back end reads in a symbol table, it relocates all |
| symbols to make them relative to the base of the section where they were |
| defined. Doing this ensures that each symbol points to its containing |
| section. Each symbol also has a varying amount of hidden private data |
| for the BFD back end. Since the symbol points to the original file, the |
| private data format for that symbol is accessible. <code>ld</code> can |
| operate on a collection of symbols of wildly different formats without |
| problems. |
| </p> |
| <p>Normal global and simple local symbols are maintained on output, so an |
| output file (no matter its format) will retain symbols pointing to |
| functions and to global, static, and common variables. Some symbol |
| information is not worth retaining; in <code>a.out</code>, type information is |
| stored in the symbol table as long symbol names. This information would |
| be useless to most COFF debuggers; the linker has command line switches |
| to allow users to throw it away. |
| </p> |
| <p>There is one word of type information within the symbol, so if the |
| format supports symbol type information within symbols (for example, COFF, |
| IEEE, Oasys) and the type is simple enough to fit within one word |
| (nearly everything but aggregates), the information will be preserved. |
| </p> |
| </dd> |
| <dt><em>relocation level</em></dt> |
| <dd><p>Each canonical BFD relocation record contains a pointer to the symbol to |
| relocate to, the offset of the data to relocate, the section the data |
| is in, and a pointer to a relocation type descriptor. Relocation is |
| performed by passing messages through the relocation type |
| descriptor and the symbol pointer. Therefore, relocations can be performed |
| on output data using a relocation method that is only available in one of the |
| input formats. For instance, Oasys provides a byte relocation format. |
| A relocation record requesting this relocation type would point |
| indirectly to a routine to perform this, so the relocation may be |
| performed on a byte being written to a 68k COFF file, even though 68k COFF |
| has no such relocation type. |
| </p> |
| </dd> |
| <dt><em>line numbers</em></dt> |
| <dd><p>Object formats can contain, for debugging purposes, some form of mapping |
| between symbols, source line numbers, and addresses in the output file. |
| These addresses have to be relocated along with the symbol information. |
| Each symbol with an associated list of line number records points to the |
| first record of the list. The head of a line number list consists of a |
| pointer to the symbol, which allows finding out the address of the |
| function whose line number is being described. The rest of the list is |
| made up of pairs: offsets into the section and line numbers. Any format |
| which can simply derive this information can pass it successfully |
| between formats (COFF, IEEE and Oasys). |
| </p></dd> |
| </dl> |
| |
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| Previous: <a href="BFD-information-loss.html#BFD-information-loss" accesskey="p" rel="prev">BFD information loss</a>, Up: <a href="BFD-outline.html#BFD-outline" accesskey="u" rel="up">BFD outline</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p> |
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