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164 lines
7.6 KiB
HTML
164 lines
7.6 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU linker LD
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(SiFive Binutils 2.32.0-2020.04.1)
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version 2.32.
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Copyright (C) 1991-2019 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 1.3
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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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section entitled "GNU Free Documentation License". -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Canonical format (LD)</title>
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<meta name="description" content="Canonical format (LD)">
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<meta name="keywords" content="Canonical format (LD)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="LD-Index.html#LD-Index" rel="index" title="LD Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="BFD-outline.html#BFD-outline" rel="up" title="BFD outline">
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<link href="Reporting-Bugs.html#Reporting-Bugs" rel="next" title="Reporting Bugs">
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<link href="BFD-information-loss.html#BFD-information-loss" rel="prev" title="BFD information loss">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Canonical-format"></a>
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<div class="header">
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<p>
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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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</div>
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<hr>
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<a name="The-BFD-canonical-object_002dfile-format"></a>
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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"></a>
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<a name="index-internal-object_002dfile-format"></a>
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</p>
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<dl compact="compact">
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<dt><em>files</em></dt>
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<dd><p>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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</p>
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</dd>
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<dt><em>sections</em></dt>
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<dd><p>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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</p>
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</dd>
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<dt><em>symbols</em></dt>
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<dd><p>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>
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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>
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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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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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</p>
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</dd>
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<dt><em>relocation level</em></dt>
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<dd><p>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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</p>
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</dd>
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<dt><em>line numbers</em></dt>
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<dd><p>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.
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</p></dd>
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</dl>
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<hr>
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|
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<div class="header">
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<p>
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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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</div>
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</body>
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</html>
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