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