183 lines
8.6 KiB
HTML
183 lines
8.6 KiB
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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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 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Object File Format (Guile Reference Manual)</title>
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<meta name="description" content="Object File Format (Guile Reference Manual)">
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<meta name="keywords" content="Object File Format (Guile Reference Manual)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="A-Virtual-Machine-for-Guile.html" rel="up" title="A Virtual Machine for Guile">
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<link href="Instruction-Set.html" rel="next" title="Instruction Set">
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<link href="VM-Programs.html" rel="prev" title="VM Programs">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Object-File-Format">
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<div class="nav-panel">
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<p>
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Next: <a href="Instruction-Set.html" accesskey="n" rel="next">Instruction Set</a>, Previous: <a href="VM-Programs.html" accesskey="p" rel="prev">Compiled Procedures are VM Programs</a>, Up: <a href="A-Virtual-Machine-for-Guile.html" accesskey="u" rel="up">A Virtual Machine for Guile</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsection" id="Object-File-Format-1"><span>9.3.6 Object File Format<a class="copiable-link" href="#Object-File-Format-1"> ¶</a></span></h4>
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<p>To compile a file to disk, we need a format in which to write the
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compiled code to disk, and later load it into Guile. A good <em class="dfn">object
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file format</em> has a number of characteristics:
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</p>
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<ul class="itemize mark-bullet">
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<li>Above all else, it should be very cheap to load a compiled file.
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</li><li>It should be possible to statically allocate constants in the
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file. For example, a bytevector literal in source code can be emitted
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directly into the object file.
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</li><li>The compiled file should enable maximum code and data sharing
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between different processes.
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</li><li>The compiled file should contain debugging information, such as
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line numbers, but that information should be separated from the code
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itself. It should be possible to strip debugging information if space
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is tight.
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</li></ul>
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<p>These characteristics are not specific to Scheme. Indeed, mainstream
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languages like C and C++ have solved this issue many times in the past.
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Guile builds on their work by adopting ELF, the object file format of
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GNU and other Unix-like systems, as its object file format. Although
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Guile uses ELF on all platforms, we do not use platform support for ELF.
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Guile implements its own linker and loader. The advantage of using ELF
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is not sharing code, but sharing ideas. ELF is simply a well-designed
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object file format.
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</p>
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<p>An ELF file has two meta-tables describing its contents. The first
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meta-table is for the loader, and is called the <em class="dfn">program table</em> or
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sometimes the <em class="dfn">segment table</em>. The program table divides the file
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into big chunks that should be treated differently by the loader.
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Mostly the difference between these <em class="dfn">segments</em> is their
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permissions.
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</p>
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<p>Typically all segments of an ELF file are marked as read-only, except
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that part that represents modifiable static data or static data that
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needs load-time initialization. Loading an ELF file is as simple as
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mmapping the thing into memory with read-only permissions, then using
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the segment table to mark a small sub-region of the file as writable.
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This writable section is typically added to the root set of the garbage
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collector as well.
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</p>
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<p>One ELF segment is marked as “dynamic”, meaning that it has data of
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interest to the loader. Guile uses this segment to record the Guile
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version corresponding to this file. There is also an entry in the
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dynamic segment that points to the address of an initialization thunk
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that is run to perform any needed link-time initialization. (This is
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like dynamic relocations for normal ELF shared objects, except that we
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compile the relocations as a procedure instead of having the loader
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interpret a table of relocations.) Finally, the dynamic segment marks
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the location of the “entry thunk” of the object file. This thunk is
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returned to the caller of <code class="code">load-thunk-from-memory</code> or
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<code class="code">load-thunk-from-file</code>. When called, it will execute the “body”
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of the compiled expression.
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</p>
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<p>The other meta-table in an ELF file is the <em class="dfn">section table</em>. Whereas
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the program table divides an ELF file into big chunks for the loader,
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the section table specifies small sections for use by introspective
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tools like debuggers or the like. One segment (program table entry)
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typically contains many sections. There may be sections outside of any
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segment, as well.
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</p>
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<p>Typical sections in a Guile <code class="code">.go</code> file include:
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</p>
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<dl class="table">
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<dt><code class="code">.rtl-text</code></dt>
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<dd><p>Bytecode.
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</p></dd>
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<dt><code class="code">.data</code></dt>
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<dd><p>Data that needs initialization, or which may be modified at runtime.
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</p></dd>
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<dt><code class="code">.rodata</code></dt>
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<dd><p>Statically allocated data that needs no run-time initialization, and
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which therefore can be shared between processes.
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</p></dd>
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<dt><code class="code">.dynamic</code></dt>
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<dd><p>The dynamic section, discussed above.
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</p></dd>
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<dt><code class="code">.symtab</code></dt>
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<dt><code class="code">.strtab</code></dt>
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<dd><p>A table mapping addresses in the <code class="code">.rtl-text</code> to procedure names.
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<code class="code">.strtab</code> is used by <code class="code">.symtab</code>.
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</p></dd>
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<dt><code class="code">.guile.procprops</code></dt>
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<dt><code class="code">.guile.arities</code></dt>
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<dt><code class="code">.guile.arities.strtab</code></dt>
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<dt><code class="code">.guile.docstrs</code></dt>
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<dt><code class="code">.guile.docstrs.strtab</code></dt>
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<dd><p>Side tables of procedure properties, arities, and docstrings.
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</p></dd>
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<dt><code class="code">.guile.docstrs.strtab</code></dt>
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<dd><p>Side table of frame maps, describing the set of live slots for ever
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return point in the program text, and whether those slots are pointers
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are not. Used by the garbage collector.
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</p></dd>
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<dt><code class="code">.debug_info</code></dt>
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<dt><code class="code">.debug_abbrev</code></dt>
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<dt><code class="code">.debug_str</code></dt>
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<dt><code class="code">.debug_loc</code></dt>
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<dt><code class="code">.debug_line</code></dt>
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<dd><p>Debugging information, in DWARF format. See the DWARF specification,
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for more information.
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</p></dd>
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<dt><code class="code">.shstrtab</code></dt>
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<dd><p>Section name string table.
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</p></dd>
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</dl>
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<p>For more information, see <a class="uref" href="http://linux.die.net/man/5/elf">the
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elf(5) man page</a>. See <a class="uref" href="http://dwarfstd.org/">the DWARF
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specification</a> for more on the DWARF debugging format. Or if you are an
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adventurous explorer, try running <code class="code">readelf</code> or <code class="code">objdump</code> on
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compiled <code class="code">.go</code> files. It’s good times!
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Instruction-Set.html">Instruction Set</a>, Previous: <a href="VM-Programs.html">Compiled Procedures are VM Programs</a>, Up: <a href="A-Virtual-Machine-for-Guile.html">A Virtual Machine for Guile</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" 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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