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225 lines
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<title>Builtin Functions - Untitled</title>
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This file documents the GNU linker LD
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000,
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<p>
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<a name="Builtin-Functions"></a>Previous: <a rel="previous" accesskey="p" href="Expression-Section.html#Expression-Section">Expression Section</a>,
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Up: <a rel="up" accesskey="u" href="Expressions.html#Expressions">Expressions</a>
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<hr><br>
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</div>
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<h4 class="subsection">3.10.8 Builtin Functions</h4>
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<p><a name="index-functions-in-expressions-486"></a>The linker script language includes a number of builtin functions for
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use in linker script expressions.
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<dl>
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<dt><code>ABSOLUTE(</code><var>exp</var><code>)</code><dd><a name="index-ABSOLUTE_0028_0040var_007bexp_007d_0029-487"></a><a name="index-expression_002c-absolute-488"></a>Return the absolute (non-relocatable, as opposed to non-negative) value
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of the expression <var>exp</var>. Primarily useful to assign an absolute
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value to a symbol within a section definition, where symbol values are
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normally section relative. See <a href="Expression-Section.html#Expression-Section">Expression Section</a>.
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<br><dt><code>ADDR(</code><var>section</var><code>)</code><dd><a name="index-ADDR_0028_0040var_007bsection_007d_0029-489"></a><a name="index-section-address-in-expression-490"></a>Return the absolute address (the VMA) of the named <var>section</var>. Your
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script must previously have defined the location of that section. In
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the following example, <code>symbol_1</code> and <code>symbol_2</code> are assigned
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identical values:
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<pre class="smallexample"> SECTIONS { ...
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.output1 :
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{
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start_of_output_1 = ABSOLUTE(.);
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...
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}
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.output :
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{
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symbol_1 = ADDR(.output1);
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symbol_2 = start_of_output_1;
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}
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... }
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</pre>
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<br><dt><code>ALIGN(</code><var>align</var><code>)</code><dt><code>ALIGN(</code><var>exp</var><code>,</code><var>align</var><code>)</code><dd><a name="index-ALIGN_0028_0040var_007balign_007d_0029-491"></a><a name="index-ALIGN_0028_0040var_007bexp_007d_002c_0040var_007balign_007d_0029-492"></a><a name="index-round-up-location-counter-493"></a><a name="index-align-location-counter-494"></a><a name="index-round-up-expression-495"></a><a name="index-align-expression-496"></a>Return the location counter (<code>.</code>) or arbitrary expression aligned
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to the next <var>align</var> boundary. The single operand <code>ALIGN</code>
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doesn't change the value of the location counter—it just does
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arithmetic on it. The two operand <code>ALIGN</code> allows an arbitrary
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expression to be aligned upwards (<code>ALIGN(</code><var>align</var><code>)</code> is
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equivalent to <code>ALIGN(., </code><var>align</var><code>)</code>).
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<p>Here is an example which aligns the output <code>.data</code> section to the
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next <code>0x2000</code> byte boundary after the preceding section and sets a
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variable within the section to the next <code>0x8000</code> boundary after the
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input sections:
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<pre class="smallexample"> SECTIONS { ...
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.data ALIGN(0x2000): {
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*(.data)
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variable = ALIGN(0x8000);
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}
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... }
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</pre>
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<p class="noindent">The first use of <code>ALIGN</code> in this example specifies the location of
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a section because it is used as the optional <var>address</var> attribute of
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a section definition (see <a href="Output-Section-Address.html#Output-Section-Address">Output Section Address</a>). The second use
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of <code>ALIGN</code> is used to defines the value of a symbol.
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<p>The builtin function <code>NEXT</code> is closely related to <code>ALIGN</code>.
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<br><dt><code>ALIGNOF(</code><var>section</var><code>)</code><dd><a name="index-ALIGNOF_0028_0040var_007bsection_007d_0029-497"></a><a name="index-section-alignment-498"></a>Return the alignment in bytes of the named <var>section</var>, if that section has
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been allocated. If the section has not been allocated when this is
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evaluated, the linker will report an error. In the following example,
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the alignment of the <code>.output</code> section is stored as the first
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value in that section.
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<pre class="smallexample"> SECTIONS{ ...
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.output {
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LONG (ALIGNOF (.output))
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...
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}
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... }
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</pre>
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<br><dt><code>BLOCK(</code><var>exp</var><code>)</code><dd><a name="index-BLOCK_0028_0040var_007bexp_007d_0029-499"></a>This is a synonym for <code>ALIGN</code>, for compatibility with older linker
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scripts. It is most often seen when setting the address of an output
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section.
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<br><dt><code>DATA_SEGMENT_ALIGN(</code><var>maxpagesize</var><code>, </code><var>commonpagesize</var><code>)</code><dd><a name="index-DATA_005fSEGMENT_005fALIGN_0028_0040var_007bmaxpagesize_007d_002c-_0040var_007bcommonpagesize_007d_0029-500"></a>This is equivalent to either
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<pre class="smallexample"> (ALIGN(<var>maxpagesize</var>) + (. & (<var>maxpagesize</var> - 1)))
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</pre>
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<p>or
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<pre class="smallexample"> (ALIGN(<var>maxpagesize</var>) + (. & (<var>maxpagesize</var> - <var>commonpagesize</var>)))
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</pre>
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<p class="noindent">depending on whether the latter uses fewer <var>commonpagesize</var> sized pages
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for the data segment (area between the result of this expression and
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<code>DATA_SEGMENT_END</code>) than the former or not.
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If the latter form is used, it means <var>commonpagesize</var> bytes of runtime
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memory will be saved at the expense of up to <var>commonpagesize</var> wasted
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bytes in the on-disk file.
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<p>This expression can only be used directly in <code>SECTIONS</code> commands, not in
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any output section descriptions and only once in the linker script.
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<var>commonpagesize</var> should be less or equal to <var>maxpagesize</var> and should
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be the system page size the object wants to be optimized for (while still
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working on system page sizes up to <var>maxpagesize</var>).
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<p class="noindent">Example:
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<pre class="smallexample"> . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
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</pre>
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<br><dt><code>DATA_SEGMENT_END(</code><var>exp</var><code>)</code><dd><a name="index-DATA_005fSEGMENT_005fEND_0028_0040var_007bexp_007d_0029-501"></a>This defines the end of data segment for <code>DATA_SEGMENT_ALIGN</code>
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evaluation purposes.
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<pre class="smallexample"> . = DATA_SEGMENT_END(.);
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</pre>
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<br><dt><code>DATA_SEGMENT_RELRO_END(</code><var>offset</var><code>, </code><var>exp</var><code>)</code><dd><a name="index-DATA_005fSEGMENT_005fRELRO_005fEND_0028_0040var_007boffset_007d_002c-_0040var_007bexp_007d_0029-502"></a>This defines the end of the <code>PT_GNU_RELRO</code> segment when
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<span class="samp">-z relro</span> option is used. Second argument is returned.
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When <span class="samp">-z relro</span> option is not present, <code>DATA_SEGMENT_RELRO_END</code>
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does nothing, otherwise <code>DATA_SEGMENT_ALIGN</code> is padded so that
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<var>exp</var> + <var>offset</var> is aligned to the most commonly used page
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boundary for particular target. If present in the linker script,
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it must always come in between <code>DATA_SEGMENT_ALIGN</code> and
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<code>DATA_SEGMENT_END</code>.
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<pre class="smallexample"> . = DATA_SEGMENT_RELRO_END(24, .);
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</pre>
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<br><dt><code>DEFINED(</code><var>symbol</var><code>)</code><dd><a name="index-DEFINED_0028_0040var_007bsymbol_007d_0029-503"></a><a name="index-symbol-defaults-504"></a>Return 1 if <var>symbol</var> is in the linker global symbol table and is
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defined before the statement using DEFINED in the script, otherwise
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return 0. You can use this function to provide
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default values for symbols. For example, the following script fragment
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shows how to set a global symbol <span class="samp">begin</span> to the first location in
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the <span class="samp">.text</span> section—but if a symbol called <span class="samp">begin</span> already
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existed, its value is preserved:
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<pre class="smallexample"> SECTIONS { ...
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.text : {
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begin = DEFINED(begin) ? begin : . ;
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...
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}
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...
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}
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</pre>
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<br><dt><code>LENGTH(</code><var>memory</var><code>)</code><dd><a name="index-LENGTH_0028_0040var_007bmemory_007d_0029-505"></a>Return the length of the memory region named <var>memory</var>.
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<br><dt><code>LOADADDR(</code><var>section</var><code>)</code><dd><a name="index-LOADADDR_0028_0040var_007bsection_007d_0029-506"></a><a name="index-section-load-address-in-expression-507"></a>Return the absolute LMA of the named <var>section</var>. This is normally
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the same as <code>ADDR</code>, but it may be different if the <code>AT</code>
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attribute is used in the output section definition (see <a href="Output-Section-LMA.html#Output-Section-LMA">Output Section LMA</a>).
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<p><a name="index-MAX-508"></a><br><dt><code>MAX(</code><var>exp1</var><code>, </code><var>exp2</var><code>)</code><dd>Returns the maximum of <var>exp1</var> and <var>exp2</var>.
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<p><a name="index-MIN-509"></a><br><dt><code>MIN(</code><var>exp1</var><code>, </code><var>exp2</var><code>)</code><dd>Returns the minimum of <var>exp1</var> and <var>exp2</var>.
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<br><dt><code>NEXT(</code><var>exp</var><code>)</code><dd><a name="index-NEXT_0028_0040var_007bexp_007d_0029-510"></a><a name="index-unallocated-address_002c-next-511"></a>Return the next unallocated address that is a multiple of <var>exp</var>.
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This function is closely related to <code>ALIGN(</code><var>exp</var><code>)</code>; unless you
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use the <code>MEMORY</code> command to define discontinuous memory for the
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output file, the two functions are equivalent.
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<br><dt><code>ORIGIN(</code><var>memory</var><code>)</code><dd><a name="index-ORIGIN_0028_0040var_007bmemory_007d_0029-512"></a>Return the origin of the memory region named <var>memory</var>.
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<br><dt><code>SEGMENT_START(</code><var>segment</var><code>, </code><var>default</var><code>)</code><dd><a name="index-SEGMENT_005fSTART_0028_0040var_007bsegment_007d_002c-_0040var_007bdefault_007d_0029-513"></a>Return the base address of the named <var>segment</var>. If an explicit
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value has been given for this segment (with a command-line <span class="samp">-T</span>
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option) that value will be returned; otherwise the value will be
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<var>default</var>. At present, the <span class="samp">-T</span> command-line option can only
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be used to set the base address for the “text”, “data”, and
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“bss” sections, but you use <code>SEGMENT_START</code> with any segment
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name.
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<br><dt><code>SIZEOF(</code><var>section</var><code>)</code><dd><a name="index-SIZEOF_0028_0040var_007bsection_007d_0029-514"></a><a name="index-section-size-515"></a>Return the size in bytes of the named <var>section</var>, if that section has
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been allocated. If the section has not been allocated when this is
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evaluated, the linker will report an error. In the following example,
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<code>symbol_1</code> and <code>symbol_2</code> are assigned identical values:
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<pre class="smallexample"> SECTIONS{ ...
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.output {
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.start = . ;
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...
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.end = . ;
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}
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symbol_1 = .end - .start ;
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symbol_2 = SIZEOF(.output);
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... }
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</pre>
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<br><dt><code>SIZEOF_HEADERS</code><dt><code>sizeof_headers</code><dd><a name="index-SIZEOF_005fHEADERS-516"></a><a name="index-header-size-517"></a>Return the size in bytes of the output file's headers. This is
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information which appears at the start of the output file. You can use
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this number when setting the start address of the first section, if you
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choose, to facilitate paging.
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<p><a name="index-not-enough-room-for-program-headers-518"></a><a name="index-program-headers_002c-not-enough-room-519"></a>When producing an ELF output file, if the linker script uses the
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<code>SIZEOF_HEADERS</code> builtin function, the linker must compute the
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number of program headers before it has determined all the section
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addresses and sizes. If the linker later discovers that it needs
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additional program headers, it will report an error <span class="samp">not enough
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room for program headers</span>. To avoid this error, you must avoid using
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the <code>SIZEOF_HEADERS</code> function, or you must rework your linker
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script to avoid forcing the linker to use additional program headers, or
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you must define the program headers yourself using the <code>PHDRS</code>
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command (see <a href="PHDRS.html#PHDRS">PHDRS</a>).
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</dl>
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</body></html>
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