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<h3 class="section">3.13 Options for Linking</h3>

<p><a name="index-link-options-933"></a><a name="index-options_002c-linking-934"></a>
These options come into play when the compiler links object files into
an executable output file.  They are meaningless if the compiler is
not doing a link step.

     
<a name="index-file-names-935"></a>
<dl><dt><var>object-file-name</var><dd>A file name that does not end in a special recognized suffix is
considered to name an object file or library.  (Object files are
distinguished from libraries by the linker according to the file
contents.)  If linking is done, these object files are used as input
to the linker.

     <br><dt><code>-c</code><dt><code>-S</code><dt><code>-E</code><dd><a name="index-c-936"></a><a name="index-S-937"></a><a name="index-E-938"></a>If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.  See <a href="Overall-Options.html#Overall-Options">Overall Options</a>.

     <p><a name="index-Libraries-939"></a><br><dt><code>-l</code><var>library</var><dt><code>-l </code><var>library</var><dd><a name="index-l-940"></a>Search the library named <var>library</var> when linking.  (The second
alternative with the library as a separate argument is only for
POSIX compliance and is not recommended.)

     <p>It makes a difference where in the command you write this option; the
linker searches and processes libraries and object files in the order they
are specified.  Thus, &lsquo;<samp><span class="samp">foo.o -lz bar.o</span></samp>&rsquo; searches library &lsquo;<samp><span class="samp">z</span></samp>&rsquo;
after file <samp><span class="file">foo.o</span></samp> but before <samp><span class="file">bar.o</span></samp>.  If <samp><span class="file">bar.o</span></samp> refers
to functions in &lsquo;<samp><span class="samp">z</span></samp>&rsquo;, those functions may not be loaded.

     <p>The linker searches a standard list of directories for the library,
which is actually a file named <samp><span class="file">lib</span><var>library</var><span class="file">.a</span></samp>.  The linker
then uses this file as if it had been specified precisely by name.

     <p>The directories searched include several standard system directories
plus any that you specify with <samp><span class="option">-L</span></samp>.

     <p>Normally the files found this way are library files&mdash;archive files
whose members are object files.  The linker handles an archive file by
scanning through it for members which define symbols that have so far
been referenced but not defined.  But if the file that is found is an
ordinary object file, it is linked in the usual fashion.  The only
difference between using an <samp><span class="option">-l</span></samp> option and specifying a file name
is that <samp><span class="option">-l</span></samp> surrounds <var>library</var> with &lsquo;<samp><span class="samp">lib</span></samp>&rsquo; and &lsquo;<samp><span class="samp">.a</span></samp>&rsquo;
and searches several directories.

     <br><dt><code>-lobjc</code><dd><a name="index-lobjc-941"></a>You need this special case of the <samp><span class="option">-l</span></samp> option in order to
link an Objective-C or Objective-C++ program.

     <br><dt><code>-nostartfiles</code><dd><a name="index-nostartfiles-942"></a>Do not use the standard system startup files when linking. 
The standard system libraries are used normally, unless <samp><span class="option">-nostdlib</span></samp>
or <samp><span class="option">-nodefaultlibs</span></samp> is used.

     <br><dt><code>-nodefaultlibs</code><dd><a name="index-nodefaultlibs-943"></a>Do not use the standard system libraries when linking. 
Only the libraries you specify will be passed to the linker, options
specifying linkage of the system libraries, such as <code>-static-libgcc</code>
or <code>-shared-libgcc</code>, will be ignored. 
The standard startup files are used normally, unless <samp><span class="option">-nostartfiles</span></samp>
is used.  The compiler may generate calls to <code>memcmp</code>,
<code>memset</code>, <code>memcpy</code> and <code>memmove</code>. 
These entries are usually resolved by entries in
libc.  These entry points should be supplied through some other
mechanism when this option is specified.

     <br><dt><code>-nostdlib</code><dd><a name="index-nostdlib-944"></a>Do not use the standard system startup files or libraries when linking. 
No startup files and only the libraries you specify will be passed to
the linker, options specifying linkage of the system libraries, such as
<code>-static-libgcc</code> or <code>-shared-libgcc</code>, will be ignored. 
The compiler may generate calls to <code>memcmp</code>, <code>memset</code>,
<code>memcpy</code> and <code>memmove</code>. 
These entries are usually resolved by entries in
libc.  These entry points should be supplied through some other
mechanism when this option is specified.

     <p><a name="index-g_t_0040option_007b_002dlgcc_007d_002c-use-with-_0040option_007b_002dnostdlib_007d-945"></a><a name="index-g_t_0040option_007b_002dnostdlib_007d-and-unresolved-references-946"></a><a name="index-unresolved-references-and-_0040option_007b_002dnostdlib_007d-947"></a><a name="index-g_t_0040option_007b_002dlgcc_007d_002c-use-with-_0040option_007b_002dnodefaultlibs_007d-948"></a><a name="index-g_t_0040option_007b_002dnodefaultlibs_007d-and-unresolved-references-949"></a><a name="index-unresolved-references-and-_0040option_007b_002dnodefaultlibs_007d-950"></a>One of the standard libraries bypassed by <samp><span class="option">-nostdlib</span></samp> and
<samp><span class="option">-nodefaultlibs</span></samp> is <samp><span class="file">libgcc.a</span></samp>, a library of internal subroutines
that GCC uses to overcome shortcomings of particular machines, or special
needs for some languages. 
(See <a href="../gccint/Interface.html#Interface">Interfacing to GCC Output</a>,
for more discussion of <samp><span class="file">libgcc.a</span></samp>.) 
In most cases, you need <samp><span class="file">libgcc.a</span></samp> even when you want to avoid
other standard libraries.  In other words, when you specify <samp><span class="option">-nostdlib</span></samp>
or <samp><span class="option">-nodefaultlibs</span></samp> you should usually specify <samp><span class="option">-lgcc</span></samp> as well. 
This ensures that you have no unresolved references to internal GCC
library subroutines.  (For example, &lsquo;<samp><span class="samp">__main</span></samp>&rsquo;, used to ensure C++
constructors will be called; see <a href="../gccint/Collect2.html#Collect2"><code>collect2</code></a>.)

     <br><dt><code>-pie</code><dd><a name="index-pie-951"></a>Produce a position independent executable on targets which support it. 
For predictable results, you must also specify the same set of options
that were used to generate code (<samp><span class="option">-fpie</span></samp>, <samp><span class="option">-fPIE</span></samp>,
or model suboptions) when you specify this option.

     <br><dt><code>-rdynamic</code><dd><a name="index-rdynamic-952"></a>Pass the flag <samp><span class="option">-export-dynamic</span></samp> to the ELF linker, on targets
that support it. This instructs the linker to add all symbols, not
only used ones, to the dynamic symbol table. This option is needed
for some uses of <code>dlopen</code> or to allow obtaining backtraces
from within a program.

     <br><dt><code>-s</code><dd><a name="index-s-953"></a>Remove all symbol table and relocation information from the executable.

     <br><dt><code>-static</code><dd><a name="index-static-954"></a>On systems that support dynamic linking, this prevents linking with the shared
libraries.  On other systems, this option has no effect.

     <br><dt><code>-shared</code><dd><a name="index-shared-955"></a>Produce a shared object which can then be linked with other objects to
form an executable.  Not all systems support this option.  For predictable
results, you must also specify the same set of options that were used to
generate code (<samp><span class="option">-fpic</span></samp>, <samp><span class="option">-fPIC</span></samp>, or model suboptions)
when you specify this option.<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>

     <br><dt><code>-shared-libgcc</code><dt><code>-static-libgcc</code><dd><a name="index-shared_002dlibgcc-956"></a><a name="index-static_002dlibgcc-957"></a>On systems that provide <samp><span class="file">libgcc</span></samp> as a shared library, these options
force the use of either the shared or static version respectively. 
If no shared version of <samp><span class="file">libgcc</span></samp> was built when the compiler was
configured, these options have no effect.

     <p>There are several situations in which an application should use the
shared <samp><span class="file">libgcc</span></samp> instead of the static version.  The most common
of these is when the application wishes to throw and catch exceptions
across different shared libraries.  In that case, each of the libraries
as well as the application itself should use the shared <samp><span class="file">libgcc</span></samp>.

     <p>Therefore, the G++ and GCJ drivers automatically add
<samp><span class="option">-shared-libgcc</span></samp> whenever you build a shared library or a main
executable, because C++ and Java programs typically use exceptions, so
this is the right thing to do.

     <p>If, instead, you use the GCC driver to create shared libraries, you may
find that they will not always be linked with the shared <samp><span class="file">libgcc</span></samp>. 
If GCC finds, at its configuration time, that you have a non-GNU linker
or a GNU linker that does not support option <samp><span class="option">--eh-frame-hdr</span></samp>,
it will link the shared version of <samp><span class="file">libgcc</span></samp> into shared libraries
by default.  Otherwise, it will take advantage of the linker and optimize
away the linking with the shared version of <samp><span class="file">libgcc</span></samp>, linking with
the static version of libgcc by default.  This allows exceptions to
propagate through such shared libraries, without incurring relocation
costs at library load time.

     <p>However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ or GCJ driver, as appropriate
for the languages used in the program, or using the option
<samp><span class="option">-shared-libgcc</span></samp>, such that it is linked with the shared
<samp><span class="file">libgcc</span></samp>.

     <br><dt><code>-static-libstdc++</code><dd>When the <samp><span class="command">g++</span></samp> program is used to link a C++ program, it will
normally automatically link against <samp><span class="option">libstdc++</span></samp>.  If
<samp><span class="file">libstdc++</span></samp> is available as a shared library, and the
<samp><span class="option">-static</span></samp> option is not used, then this will link against the
shared version of <samp><span class="file">libstdc++</span></samp>.  That is normally fine.  However, it
is sometimes useful to freeze the version of <samp><span class="file">libstdc++</span></samp> used by
the program without going all the way to a fully static link.  The
<samp><span class="option">-static-libstdc++</span></samp> option directs the <samp><span class="command">g++</span></samp> driver to
link <samp><span class="file">libstdc++</span></samp> statically, without necessarily linking other
libraries statically.

     <br><dt><code>-symbolic</code><dd><a name="index-symbolic-958"></a>Bind references to global symbols when building a shared object.  Warn
about any unresolved references (unless overridden by the link editor
option &lsquo;<samp><span class="samp">-Xlinker -z -Xlinker defs</span></samp>&rsquo;).  Only a few systems support
this option.

     <br><dt><code>-T </code><var>script</var><dd><a name="index-T-959"></a><a name="index-linker-script-960"></a>Use <var>script</var> as the linker script.  This option is supported by most
systems using the GNU linker.  On some targets, such as bare-board
targets without an operating system, the <samp><span class="option">-T</span></samp> option may be required
when linking to avoid references to undefined symbols.

     <br><dt><code>-Xlinker </code><var>option</var><dd><a name="index-Xlinker-961"></a>Pass <var>option</var> as an option to the linker.  You can use this to
supply system-specific linker options which GCC does not know how to
recognize.

     <p>If you want to pass an option that takes a separate argument, you must use
<samp><span class="option">-Xlinker</span></samp> twice, once for the option and once for the argument. 
For example, to pass <samp><span class="option">-assert definitions</span></samp>, you must write
&lsquo;<samp><span class="samp">-Xlinker -assert -Xlinker definitions</span></samp>&rsquo;.  It does not work to write
<samp><span class="option">-Xlinker "-assert definitions"</span></samp>, because this passes the entire
string as a single argument, which is not what the linker expects.

     <p>When using the GNU linker, it is usually more convenient to pass
arguments to linker options using the <samp><var>option</var><span class="option">=</span><var>value</var></samp>
syntax than as separate arguments.  For example, you can specify
&lsquo;<samp><span class="samp">-Xlinker -Map=output.map</span></samp>&rsquo; rather than
&lsquo;<samp><span class="samp">-Xlinker -Map -Xlinker output.map</span></samp>&rsquo;.  Other linkers may not support
this syntax for command-line options.

     <br><dt><code>-Wl,</code><var>option</var><dd><a name="index-Wl-962"></a>Pass <var>option</var> as an option to the linker.  If <var>option</var> contains
commas, it is split into multiple options at the commas.  You can use this
syntax to pass an argument to the option. 
For example, &lsquo;<samp><span class="samp">-Wl,-Map,output.map</span></samp>&rsquo; passes &lsquo;<samp><span class="samp">-Map output.map</span></samp>&rsquo; to the
linker.  When using the GNU linker, you can also get the same effect with
&lsquo;<samp><span class="samp">-Wl,-Map=output.map</span></samp>&rsquo;.

     <br><dt><code>-u </code><var>symbol</var><dd><a name="index-u-963"></a>Pretend the symbol <var>symbol</var> is undefined, to force linking of
library modules to define it.  You can use <samp><span class="option">-u</span></samp> multiple times with
different symbols to force loading of additional library modules. 
</dl>

 <div class="footnote">
<hr>
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> On some systems, &lsquo;<samp><span class="samp">gcc -shared</span></samp>&rsquo;
needs to build supplementary stub code for constructors to work.  On
multi-libbed systems, &lsquo;<samp><span class="samp">gcc -shared</span></samp>&rsquo; must select the correct support
libraries to link against.  Failing to supply the correct flags may lead
to subtle defects.  Supplying them in cases where they are not necessary
is innocuous.</p>

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