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<h4 class="subsection">3.4.5 Other Linker Script Commands</h4>

<p>There are a few other linker scripts commands.

     <dl>
<dt><code>ASSERT(</code><var>exp</var><code>, </code><var>message</var><code>)</code><dd><a name="index-ASSERT-422"></a><a name="index-assertion-in-linker-script-423"></a>Ensure that <var>exp</var> is non-zero.  If it is zero, then exit the linker
with an error code, and print <var>message</var>.

     <p>Note that assertions are checked before the final stages of linking
take place.  This means that expressions involving symbols PROVIDEd
inside section definitions will fail if the user has not set values
for those symbols.  The only exception to this rule is PROVIDEd
symbols that just reference dot.  Thus an assertion like this:

     <pre class="smallexample">            .stack :
            {
              PROVIDE (__stack = .);
              PROVIDE (__stack_size = 0x100);
              ASSERT ((__stack &gt; (_end + __stack_size)), "Error: No room left for the stack");
            }
</pre>
     <p>will fail if <code>__stack_size</code> is not defined elsewhere.  Symbols
PROVIDEd outside of section definitions are evaluated earlier, so they
can be used inside ASSERTions.  Thus:

     <pre class="smallexample">            PROVIDE (__stack_size = 0x100);
            .stack :
            {
              PROVIDE (__stack = .);
              ASSERT ((__stack &gt; (_end + __stack_size)), "Error: No room left for the stack");
            }
</pre>
     <p>will work.

     <br><dt><code>EXTERN(</code><var>symbol</var> <var>symbol</var><code> ...)</code><dd><a name="index-EXTERN-424"></a><a name="index-undefined-symbol-in-linker-script-425"></a>Force <var>symbol</var> to be entered in the output file as an undefined
symbol.  Doing this may, for example, trigger linking of additional
modules from standard libraries.  You may list several <var>symbol</var>s for
each <code>EXTERN</code>, and you may use <code>EXTERN</code> multiple times.  This
command has the same effect as the &lsquo;<samp><span class="samp">-u</span></samp>&rsquo; command-line option.

     <br><dt><code>FORCE_COMMON_ALLOCATION</code><dd><a name="index-FORCE_005fCOMMON_005fALLOCATION-426"></a><a name="index-common-allocation-in-linker-script-427"></a>This command has the same effect as the &lsquo;<samp><span class="samp">-d</span></samp>&rsquo; command-line option:
to make <samp><span class="command">ld</span></samp> assign space to common symbols even if a relocatable
output file is specified (&lsquo;<samp><span class="samp">-r</span></samp>&rsquo;).

     <br><dt><code>INHIBIT_COMMON_ALLOCATION</code><dd><a name="index-INHIBIT_005fCOMMON_005fALLOCATION-428"></a><a name="index-common-allocation-in-linker-script-429"></a>This command has the same effect as the &lsquo;<samp><span class="samp">--no-define-common</span></samp>&rsquo;
command-line option: to make <code>ld</code> omit the assignment of addresses
to common symbols even for a non-relocatable output file.

     <br><dt><code>FORCE_GROUP_ALLOCATION</code><dd><a name="index-FORCE_005fGROUP_005fALLOCATION-430"></a><a name="index-group-allocation-in-linker-script-431"></a><a name="index-section-groups-432"></a><a name="index-COMDAT-433"></a>This command has the same effect as the
&lsquo;<samp><span class="samp">--force-group-allocation</span></samp>&rsquo; command-line option: to make
<samp><span class="command">ld</span></samp> place section group members like normal input sections,
and to delete the section groups even if a relocatable output file is
specified (&lsquo;<samp><span class="samp">-r</span></samp>&rsquo;).

     <br><dt><code>INSERT [ AFTER | BEFORE ] </code><var>output_section</var><dd><a name="index-INSERT-434"></a><a name="index-insert-user-script-into-default-script-435"></a>This command is typically used in a script specified by &lsquo;<samp><span class="samp">-T</span></samp>&rsquo; to
augment the default <code>SECTIONS</code> with, for example, overlays.  It
inserts all prior linker script statements after (or before)
<var>output_section</var>, and also causes &lsquo;<samp><span class="samp">-T</span></samp>&rsquo; to not override the
default linker script.  The exact insertion point is as for orphan
sections.  See <a href="Location-Counter.html#Location-Counter">Location Counter</a>.  The insertion happens after the
linker has mapped input sections to output sections.  Prior to the
insertion, since &lsquo;<samp><span class="samp">-T</span></samp>&rsquo; scripts are parsed before the default
linker script, statements in the &lsquo;<samp><span class="samp">-T</span></samp>&rsquo; script occur before the
default linker script statements in the internal linker representation
of the script.  In particular, input section assignments will be made
to &lsquo;<samp><span class="samp">-T</span></samp>&rsquo; output sections before those in the default script.  Here
is an example of how a &lsquo;<samp><span class="samp">-T</span></samp>&rsquo; script using <code>INSERT</code> might look:

     <pre class="smallexample">          SECTIONS
          {
            OVERLAY :
            {
              .ov1 { ov1*(.text) }
              .ov2 { ov2*(.text) }
            }
          }
          INSERT AFTER .text;
</pre>
     <br><dt><code>NOCROSSREFS(</code><var>section</var> <var>section</var><code> ...)</code><dd><a name="index-NOCROSSREFS_0028_0040var_007bsections_007d_0029-436"></a><a name="index-cross-references-437"></a>This command may be used to tell <samp><span class="command">ld</span></samp> to issue an error about any
references among certain output sections.

     <p>In certain types of programs, particularly on embedded systems when
using overlays, when one section is loaded into memory, another section
will not be.  Any direct references between the two sections would be
errors.  For example, it would be an error if code in one section called
a function defined in the other section.

     <p>The <code>NOCROSSREFS</code> command takes a list of output section names.  If
<samp><span class="command">ld</span></samp> detects any cross references between the sections, it reports
an error and returns a non-zero exit status.  Note that the
<code>NOCROSSREFS</code> command uses output section names, not input section
names.

     <br><dt><code>NOCROSSREFS_TO(</code><var>tosection</var> <var>fromsection</var><code> ...)</code><dd><a name="index-NOCROSSREFS_005fTO_0028_0040var_007btosection_007d-_0040var_007bfromsections_007d_0029-438"></a><a name="index-cross-references-439"></a>This command may be used to tell <samp><span class="command">ld</span></samp> to issue an error about any
references to one section from a list of other sections.

     <p>The <code>NOCROSSREFS</code> command is useful when ensuring that two or more
output sections are entirely independent but there are situations where
a one-way dependency is needed. For example, in a multi-core application
there may be shared code that can be called from each core but for safety
must never call back.

     <p>The <code>NOCROSSREFS_TO</code> command takes a list of output section names. 
The first section can not be referenced from any of the other sections. 
If <samp><span class="command">ld</span></samp> detects any references to the first section from any of
the other sections, it reports an error and returns a non-zero exit
status.  Note that the <code>NOCROSSREFS_TO</code> command uses output section
names, not input section names.

     <br><dt><code>OUTPUT_ARCH(</code><var>bfdarch</var><code>)</code><dd><a name="index-OUTPUT_005fARCH_0028_0040var_007bbfdarch_007d_0029-440"></a><a name="index-machine-architecture-441"></a><a name="index-architecture-442"></a>Specify a particular output machine architecture.  The argument is one
of the names used by the BFD library (see <a href="BFD.html#BFD">BFD</a>).  You can see the
architecture of an object file by using the <code>objdump</code> program with
the &lsquo;<samp><span class="samp">-f</span></samp>&rsquo; option.

     <br><dt><code>LD_FEATURE(</code><var>string</var><code>)</code><dd><a name="index-LD_005fFEATURE_0028_0040var_007bstring_007d_0029-443"></a>This command may be used to modify <samp><span class="command">ld</span></samp> behavior.  If
<var>string</var> is <code>"SANE_EXPR"</code> then absolute symbols and numbers
in a script are simply treated as numbers everywhere. 
See <a href="Expression-Section.html#Expression-Section">Expression Section</a>. 
</dl>

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