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172 lines
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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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<div class="node">
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<p>
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<a name="PHDRS"></a>Next: <a rel="next" accesskey="n" href="VERSION.html#VERSION">VERSION</a>,
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Previous: <a rel="previous" accesskey="p" href="MEMORY.html#MEMORY">MEMORY</a>,
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Up: <a rel="up" accesskey="u" href="Scripts.html#Scripts">Scripts</a>
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<hr><br>
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</div>
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<h3 class="section">3.8 PHDRS Command</h3>
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<p><a name="index-PHDRS-443"></a><a name="index-program-headers-444"></a><a name="index-ELF-program-headers-445"></a><a name="index-program-segments-446"></a><a name="index-segments_002c-ELF-447"></a>The ELF object file format uses <dfn>program headers</dfn>, also knows as
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<dfn>segments</dfn>. The program headers describe how the program should be
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loaded into memory. You can print them out by using the <code>objdump</code>
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program with the <span class="samp">-p</span> option.
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<p>When you run an ELF program on a native ELF system, the system loader
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reads the program headers in order to figure out how to load the
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program. This will only work if the program headers are set correctly.
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This manual does not describe the details of how the system loader
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interprets program headers; for more information, see the ELF ABI.
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<p>The linker will create reasonable program headers by default. However,
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in some cases, you may need to specify the program headers more
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precisely. You may use the <code>PHDRS</code> command for this purpose. When
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the linker sees the <code>PHDRS</code> command in the linker script, it will
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not create any program headers other than the ones specified.
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<p>The linker only pays attention to the <code>PHDRS</code> command when
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generating an ELF output file. In other cases, the linker will simply
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ignore <code>PHDRS</code>.
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<p>This is the syntax of the <code>PHDRS</code> command. The words <code>PHDRS</code>,
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<code>FILEHDR</code>, <code>AT</code>, and <code>FLAGS</code> are keywords.
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<pre class="smallexample"> PHDRS
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{
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<var>name</var> <var>type</var> [ FILEHDR ] [ PHDRS ] [ AT ( <var>address</var> ) ]
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[ FLAGS ( <var>flags</var> ) ] ;
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}
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</pre>
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<p>The <var>name</var> is used only for reference in the <code>SECTIONS</code> command
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of the linker script. It is not put into the output file. Program
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header names are stored in a separate name space, and will not conflict
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with symbol names, file names, or section names. Each program header
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must have a distinct name.
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<p>Certain program header types describe segments of memory which the
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system loader will load from the file. In the linker script, you
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specify the contents of these segments by placing allocatable output
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sections in the segments. You use the <span class="samp">:</span><var>phdr</var> output section
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attribute to place a section in a particular segment. See <a href="Output-Section-Phdr.html#Output-Section-Phdr">Output Section Phdr</a>.
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<p>It is normal to put certain sections in more than one segment. This
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merely implies that one segment of memory contains another. You may
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repeat <span class="samp">:</span><var>phdr</var>, using it once for each segment which should
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contain the section.
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<p>If you place a section in one or more segments using <span class="samp">:</span><var>phdr</var>,
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then the linker will place all subsequent allocatable sections which do
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not specify <span class="samp">:</span><var>phdr</var> in the same segments. This is for
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convenience, since generally a whole set of contiguous sections will be
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placed in a single segment. You can use <code>:NONE</code> to override the
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default segment and tell the linker to not put the section in any
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segment at all.
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<p><a name="index-FILEHDR-448"></a><a name="index-PHDRS-449"></a>You may use the <code>FILEHDR</code> and <code>PHDRS</code> keywords appear after
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the program header type to further describe the contents of the segment.
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The <code>FILEHDR</code> keyword means that the segment should include the ELF
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file header. The <code>PHDRS</code> keyword means that the segment should
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include the ELF program headers themselves.
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<p>The <var>type</var> may be one of the following. The numbers indicate the
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value of the keyword.
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<dl>
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<dt><code>PT_NULL</code> (0)<dd>Indicates an unused program header.
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<br><dt><code>PT_LOAD</code> (1)<dd>Indicates that this program header describes a segment to be loaded from
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the file.
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<br><dt><code>PT_DYNAMIC</code> (2)<dd>Indicates a segment where dynamic linking information can be found.
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<br><dt><code>PT_INTERP</code> (3)<dd>Indicates a segment where the name of the program interpreter may be
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found.
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<br><dt><code>PT_NOTE</code> (4)<dd>Indicates a segment holding note information.
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<br><dt><code>PT_SHLIB</code> (5)<dd>A reserved program header type, defined but not specified by the ELF
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ABI.
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<br><dt><code>PT_PHDR</code> (6)<dd>Indicates a segment where the program headers may be found.
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<br><dt><var>expression</var><dd>An expression giving the numeric type of the program header. This may
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be used for types not defined above.
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</dl>
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<p>You can specify that a segment should be loaded at a particular address
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in memory by using an <code>AT</code> expression. This is identical to the
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<code>AT</code> command used as an output section attribute (see <a href="Output-Section-LMA.html#Output-Section-LMA">Output Section LMA</a>). The <code>AT</code> command for a program header overrides the
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output section attribute.
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<p>The linker will normally set the segment flags based on the sections
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which comprise the segment. You may use the <code>FLAGS</code> keyword to
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explicitly specify the segment flags. The value of <var>flags</var> must be
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an integer. It is used to set the <code>p_flags</code> field of the program
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header.
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<p>Here is an example of <code>PHDRS</code>. This shows a typical set of program
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headers used on a native ELF system.
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<pre class="example"> PHDRS
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{
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headers PT_PHDR PHDRS ;
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interp PT_INTERP ;
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text PT_LOAD FILEHDR PHDRS ;
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data PT_LOAD ;
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dynamic PT_DYNAMIC ;
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}
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SECTIONS
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{
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. = SIZEOF_HEADERS;
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.interp : { *(.interp) } :text :interp
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.text : { *(.text) } :text
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.rodata : { *(.rodata) } /* defaults to :text */
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...
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. = . + 0x1000; /* move to a new page in memory */
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.data : { *(.data) } :data
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.dynamic : { *(.dynamic) } :data :dynamic
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...
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}
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</pre>
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</body></html>
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