118 lines
6 KiB
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118 lines
6 KiB
HTML
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<title>Canonical format - Untitled</title>
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<!--
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This file documents the GNU linker LD
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(GNU Binutils)
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version 2.19.
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000,
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2001, 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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<div class="node">
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<p>
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<a name="Canonical-format"></a>Previous: <a rel="previous" accesskey="p" href="BFD-information-loss.html#BFD-information-loss">BFD information loss</a>,
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Up: <a rel="up" accesskey="u" href="BFD-outline.html#BFD-outline">BFD outline</a>
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<hr><br>
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</div>
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<h4 class="subsection">5.1.2 The BFD canonical object-file format</h4>
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<p>The greatest potential for loss of information occurs when there is the least
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overlap between the information provided by the source format, that
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stored by the canonical format, and that needed by the
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destination format. A brief description of the canonical form may help
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you understand which kinds of data you can count on preserving across
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conversions.
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<a name="index-BFD-canonical-format-636"></a><a name="index-internal-object_002dfile-format-637"></a>
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<dl>
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<dt><em>files</em><dd>Information stored on a per-file basis includes target machine
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architecture, particular implementation format type, a demand pageable
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bit, and a write protected bit. Information like Unix magic numbers is
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not stored here—only the magic numbers' meaning, so a <code>ZMAGIC</code>
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file would have both the demand pageable bit and the write protected
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text bit set. The byte order of the target is stored on a per-file
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basis, so that big- and little-endian object files may be used with one
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another.
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<br><dt><em>sections</em><dd>Each section in the input file contains the name of the section, the
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section's original address in the object file, size and alignment
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information, various flags, and pointers into other BFD data
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structures.
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<br><dt><em>symbols</em><dd>Each symbol contains a pointer to the information for the object file
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which originally defined it, its name, its value, and various flag
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bits. When a BFD back end reads in a symbol table, it relocates all
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symbols to make them relative to the base of the section where they were
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defined. Doing this ensures that each symbol points to its containing
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section. Each symbol also has a varying amount of hidden private data
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for the BFD back end. Since the symbol points to the original file, the
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private data format for that symbol is accessible. <code>ld</code> can
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operate on a collection of symbols of wildly different formats without
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problems.
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<p>Normal global and simple local symbols are maintained on output, so an
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output file (no matter its format) will retain symbols pointing to
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functions and to global, static, and common variables. Some symbol
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information is not worth retaining; in <code>a.out</code>, type information is
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stored in the symbol table as long symbol names. This information would
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be useless to most COFF debuggers; the linker has command line switches
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to allow users to throw it away.
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<p>There is one word of type information within the symbol, so if the
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format supports symbol type information within symbols (for example, COFF,
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IEEE, Oasys) and the type is simple enough to fit within one word
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(nearly everything but aggregates), the information will be preserved.
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<br><dt><em>relocation level</em><dd>Each canonical BFD relocation record contains a pointer to the symbol to
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relocate to, the offset of the data to relocate, the section the data
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is in, and a pointer to a relocation type descriptor. Relocation is
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performed by passing messages through the relocation type
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descriptor and the symbol pointer. Therefore, relocations can be performed
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on output data using a relocation method that is only available in one of the
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input formats. For instance, Oasys provides a byte relocation format.
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A relocation record requesting this relocation type would point
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indirectly to a routine to perform this, so the relocation may be
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performed on a byte being written to a 68k COFF file, even though 68k COFF
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has no such relocation type.
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<br><dt><em>line numbers</em><dd>Object formats can contain, for debugging purposes, some form of mapping
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between symbols, source line numbers, and addresses in the output file.
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These addresses have to be relocated along with the symbol information.
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Each symbol with an associated list of line number records points to the
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first record of the list. The head of a line number list consists of a
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pointer to the symbol, which allows finding out the address of the
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function whose line number is being described. The rest of the list is
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made up of pairs: offsets into the section and line numbers. Any format
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which can simply derive this information can pass it successfully
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between formats (COFF, IEEE and Oasys).
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</dl>
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</body></html>
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