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<title>Using the GNU Compiler Collection (GCC): Link Options</title>

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Next: <a href="Directory-Options.html#Directory-Options" accesskey="n" rel="next">Directory Options</a>, Previous: <a href="Assembler-Options.html#Assembler-Options" accesskey="p" rel="prev">Assembler Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Options-for-Linking"></a>
<h3 class="section">3.14 Options for Linking</h3>
<a name="index-link-options"></a>
<a name="index-options_002c-linking"></a>

<p>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.
</p>
<dl compact="compact">
<dd><a name="index-file-names"></a>
</dd>
<dt><code><var>object-file-name</var></code></dt>
<dd><p>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.
</p>
</dd>
<dt><code>-c</code></dt>
<dt><code>-S</code></dt>
<dt><code>-E</code></dt>
<dd><a name="index-c-1"></a>
<a name="index-S-1"></a>
<a name="index-E-1"></a>
<p>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>
</dd>
<dt><code>-fuse-ld=bfd</code></dt>
<dd><a name="index-fuse_002dld_003dbfd"></a>
<p>Use the <code>bfd</code> linker instead of the default linker.
</p>
</dd>
<dt><code>-fuse-ld=gold</code></dt>
<dd><a name="index-fuse_002dld_003dgold"></a>
<p>Use the <code>gold</code> linker instead of the default linker.
</p>
<a name="index-Libraries"></a>
</dd>
<dt><code>-l<var>library</var></code></dt>
<dt><code>-l <var>library</var></code></dt>
<dd><a name="index-l"></a>
<p>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>
<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>foo.o -lz bar.o</samp>&rsquo; searches library &lsquo;<samp>z</samp>&rsquo;
after file <samp>foo.o</samp> but before <samp>bar.o</samp>.  If <samp>bar.o</samp> refers
to functions in &lsquo;<samp>z</samp>&rsquo;, those functions may not be loaded.
</p>
<p>The linker searches a standard list of directories for the library,
which is actually a file named <samp>lib<var>library</var>.a</samp>.  The linker
then uses this file as if it had been specified precisely by name.
</p>
<p>The directories searched include several standard system directories
plus any that you specify with <samp>-L</samp>.
</p>
<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>-l</samp> option and specifying a file name
is that <samp>-l</samp> surrounds <var>library</var> with &lsquo;<samp>lib</samp>&rsquo; and &lsquo;<samp>.a</samp>&rsquo;
and searches several directories.
</p>
</dd>
<dt><code>-lobjc</code></dt>
<dd><a name="index-lobjc"></a>
<p>You need this special case of the <samp>-l</samp> option in order to
link an Objective-C or Objective-C++ program.
</p>
</dd>
<dt><code>-nostartfiles</code></dt>
<dd><a name="index-nostartfiles"></a>
<p>Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless <samp>-nostdlib</samp>
or <samp>-nodefaultlibs</samp> is used.
</p>
</dd>
<dt><code>-nodefaultlibs</code></dt>
<dd><a name="index-nodefaultlibs"></a>
<p>Do not use the standard system libraries when linking.
Only the libraries you specify are passed to the linker, and options
specifying linkage of the system libraries, such as <samp>-static-libgcc</samp>
or <samp>-shared-libgcc</samp>, are ignored.  
The standard startup files are used normally, unless <samp>-nostartfiles</samp>
is used.  
</p>
<p>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>
</dd>
<dt><code>-nostdlib</code></dt>
<dd><a name="index-nostdlib"></a>
<p>Do not use the standard system startup files or libraries when linking.
No startup files and only the libraries you specify are passed to
the linker, and options specifying linkage of the system libraries, such as
<samp>-static-libgcc</samp> or <samp>-shared-libgcc</samp>, are ignored.
</p>
<p>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-_002dlgcc_002c-use-with-_002dnostdlib"></a>
<a name="index-_002dnostdlib-and-unresolved-references"></a>
<a name="index-unresolved-references-and-_002dnostdlib"></a>
<a name="index-_002dlgcc_002c-use-with-_002dnodefaultlibs"></a>
<a name="index-_002dnodefaultlibs-and-unresolved-references"></a>
<a name="index-unresolved-references-and-_002dnodefaultlibs"></a>
<p>One of the standard libraries bypassed by <samp>-nostdlib</samp> and
<samp>-nodefaultlibs</samp> is <samp>libgcc.a</samp>, a library of internal subroutines
which GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
(See <a href="http://gcc.gnu.org/onlinedocs/gccint/Interface.html#Interface">Interfacing to GCC Output</a> in <cite>GNU Compiler
Collection (GCC) Internals</cite>,
for more discussion of <samp>libgcc.a</samp>.)
In most cases, you need <samp>libgcc.a</samp> even when you want to avoid
other standard libraries.  In other words, when you specify <samp>-nostdlib</samp>
or <samp>-nodefaultlibs</samp> you should usually specify <samp>-lgcc</samp> as well.
This ensures that you have no unresolved references to internal GCC
library subroutines.
(An example of such an internal subroutine is <code>__main</code>, used to ensure C++
constructors are called; see <a href="http://gcc.gnu.org/onlinedocs/gccint/Collect2.html#Collect2"><code>collect2</code></a> in <cite>GNU Compiler Collection (GCC) Internals</cite>.)
</p>
</dd>
<dt><code>-pie</code></dt>
<dd><a name="index-pie"></a>
<p>Produce a position independent executable on targets that support it.
For predictable results, you must also specify the same set of options
used for compilation (<samp>-fpie</samp>, <samp>-fPIE</samp>,
or model suboptions) when you specify this linker option.
</p>
</dd>
<dt><code>-no-pie</code></dt>
<dd><a name="index-no_002dpie"></a>
<p>Don&rsquo;t produce a position independent executable.
</p>
</dd>
<dt><code>-pthread</code></dt>
<dd><a name="index-pthread-1"></a>
<p>Link with the POSIX threads library.  This option is supported on 
GNU/Linux targets, most other Unix derivatives, and also on 
x86 Cygwin and MinGW targets.  On some targets this option also sets 
flags for the preprocessor, so it should be used consistently for both
compilation and linking.
</p>
</dd>
<dt><code>-rdynamic</code></dt>
<dd><a name="index-rdynamic"></a>
<p>Pass the flag <samp>-export-dynamic</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.
</p>
</dd>
<dt><code>-s</code></dt>
<dd><a name="index-s"></a>
<p>Remove all symbol table and relocation information from the executable.
</p>
</dd>
<dt><code>-static</code></dt>
<dd><a name="index-static"></a>
<p>On systems that support dynamic linking, this prevents linking with the shared
libraries.  On other systems, this option has no effect.
</p>
</dd>
<dt><code>-shared</code></dt>
<dd><a name="index-shared"></a>
<p>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 used for compilation
(<samp>-fpic</samp>, <samp>-fPIC</samp>, or model suboptions) when
you specify this linker option.<a name="DOCF1" href="#FOOT1"><sup>1</sup></a>
</p>
</dd>
<dt><code>-shared-libgcc</code></dt>
<dt><code>-static-libgcc</code></dt>
<dd><a name="index-shared_002dlibgcc"></a>
<a name="index-static_002dlibgcc"></a>
<p>On systems that provide <samp>libgcc</samp> as a shared library, these options
force the use of either the shared or static version, respectively.
If no shared version of <samp>libgcc</samp> was built when the compiler was
configured, these options have no effect.
</p>
<p>There are several situations in which an application should use the
shared <samp>libgcc</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>libgcc</samp>.
</p>
<p>Therefore, the G++ driver automatically adds <samp>-shared-libgcc</samp>
whenever you build a shared library or a main executable, because C++
programs typically use exceptions, so this is the right thing to do.
</p>
<p>If, instead, you use the GCC driver to create shared libraries, you may
find that they are not always linked with the shared <samp>libgcc</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>--eh-frame-hdr</samp>,
it links the shared version of <samp>libgcc</samp> into shared libraries
by default.  Otherwise, it takes advantage of the linker and optimizes
away the linking with the shared version of <samp>libgcc</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>
<p>However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ driver, or using the option
<samp>-shared-libgcc</samp>, such that it is linked with the shared
<samp>libgcc</samp>.
</p>
</dd>
<dt><code>-static-libasan</code></dt>
<dd><a name="index-static_002dlibasan"></a>
<p>When the <samp>-fsanitize=address</samp> option is used to link a program,
the GCC driver automatically links against <samp>libasan</samp>.  If
<samp>libasan</samp> is available as a shared library, and the <samp>-static</samp>
option is not used, then this links against the shared version of
<samp>libasan</samp>.  The <samp>-static-libasan</samp> option directs the GCC
driver to link <samp>libasan</samp> statically, without necessarily linking
other libraries statically.
</p>
</dd>
<dt><code>-static-libtsan</code></dt>
<dd><a name="index-static_002dlibtsan"></a>
<p>When the <samp>-fsanitize=thread</samp> option is used to link a program,
the GCC driver automatically links against <samp>libtsan</samp>.  If
<samp>libtsan</samp> is available as a shared library, and the <samp>-static</samp>
option is not used, then this links against the shared version of
<samp>libtsan</samp>.  The <samp>-static-libtsan</samp> option directs the GCC
driver to link <samp>libtsan</samp> statically, without necessarily linking
other libraries statically.
</p>
</dd>
<dt><code>-static-liblsan</code></dt>
<dd><a name="index-static_002dliblsan"></a>
<p>When the <samp>-fsanitize=leak</samp> option is used to link a program,
the GCC driver automatically links against <samp>liblsan</samp>.  If
<samp>liblsan</samp> is available as a shared library, and the <samp>-static</samp>
option is not used, then this links against the shared version of
<samp>liblsan</samp>.  The <samp>-static-liblsan</samp> option directs the GCC
driver to link <samp>liblsan</samp> statically, without necessarily linking
other libraries statically.
</p>
</dd>
<dt><code>-static-libubsan</code></dt>
<dd><a name="index-static_002dlibubsan"></a>
<p>When the <samp>-fsanitize=undefined</samp> option is used to link a program,
the GCC driver automatically links against <samp>libubsan</samp>.  If
<samp>libubsan</samp> is available as a shared library, and the <samp>-static</samp>
option is not used, then this links against the shared version of
<samp>libubsan</samp>.  The <samp>-static-libubsan</samp> option directs the GCC
driver to link <samp>libubsan</samp> statically, without necessarily linking
other libraries statically.
</p>
</dd>
<dt><code>-static-libmpx</code></dt>
<dd><a name="index-static_002dlibmpx"></a>
<p>When the <samp>-fcheck-pointer bounds</samp> and <samp>-mmpx</samp> options are
used to link a program, the GCC driver automatically links against
<samp>libmpx</samp>.  If <samp>libmpx</samp> is available as a shared library,
and the <samp>-static</samp> option is not used, then this links against
the shared version of <samp>libmpx</samp>.  The <samp>-static-libmpx</samp>
option directs the GCC driver to link <samp>libmpx</samp> statically,
without necessarily linking other libraries statically.
</p>
</dd>
<dt><code>-static-libmpxwrappers</code></dt>
<dd><a name="index-static_002dlibmpxwrappers"></a>
<p>When the <samp>-fcheck-pointer bounds</samp> and <samp>-mmpx</samp> options are used
to link a program without also using <samp>-fno-chkp-use-wrappers</samp>, the
GCC driver automatically links against <samp>libmpxwrappers</samp>.  If
<samp>libmpxwrappers</samp> is available as a shared library, and the
<samp>-static</samp> option is not used, then this links against the shared
version of <samp>libmpxwrappers</samp>.  The <samp>-static-libmpxwrappers</samp>
option directs the GCC driver to link <samp>libmpxwrappers</samp> statically,
without necessarily linking other libraries statically.
</p>
</dd>
<dt><code>-static-libstdc++</code></dt>
<dd><a name="index-static_002dlibstdc_002b_002b"></a>
<p>When the <code>g++</code> program is used to link a C++ program, it
normally automatically links against <samp>libstdc++</samp>.  If
<samp>libstdc++</samp> is available as a shared library, and the
<samp>-static</samp> option is not used, then this links against the
shared version of <samp>libstdc++</samp>.  That is normally fine.  However, it
is sometimes useful to freeze the version of <samp>libstdc++</samp> used by
the program without going all the way to a fully static link.  The
<samp>-static-libstdc++</samp> option directs the <code>g++</code> driver to
link <samp>libstdc++</samp> statically, without necessarily linking other
libraries statically.
</p>
</dd>
<dt><code>-symbolic</code></dt>
<dd><a name="index-symbolic"></a>
<p>Bind references to global symbols when building a shared object.  Warn
about any unresolved references (unless overridden by the link editor
option <samp>-Xlinker -z -Xlinker defs</samp>).  Only a few systems support
this option.
</p>
</dd>
<dt><code>-T <var>script</var></code></dt>
<dd><a name="index-T"></a>
<a name="index-linker-script"></a>
<p>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>-T</samp> option may be required
when linking to avoid references to undefined symbols.
</p>
</dd>
<dt><code>-Xlinker <var>option</var></code></dt>
<dd><a name="index-Xlinker"></a>
<p>Pass <var>option</var> as an option to the linker.  You can use this to
supply system-specific linker options that GCC does not recognize.
</p>
<p>If you want to pass an option that takes a separate argument, you must use
<samp>-Xlinker</samp> twice, once for the option and once for the argument.
For example, to pass <samp>-assert definitions</samp>, you must write
<samp>-Xlinker -assert -Xlinker definitions</samp>.  It does not work to write
<samp>-Xlinker &quot;-assert definitions&quot;</samp>, because this passes the entire
string as a single argument, which is not what the linker expects.
</p>
<p>When using the GNU linker, it is usually more convenient to pass
arguments to linker options using the <samp><var>option</var>=<var>value</var></samp>
syntax than as separate arguments.  For example, you can specify
<samp>-Xlinker -Map=output.map</samp> rather than
<samp>-Xlinker -Map -Xlinker output.map</samp>.  Other linkers may not support
this syntax for command-line options.
</p>
</dd>
<dt><code>-Wl,<var>option</var></code></dt>
<dd><a name="index-Wl"></a>
<p>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, <samp>-Wl,-Map,output.map</samp> passes <samp>-Map output.map</samp> to the
linker.  When using the GNU linker, you can also get the same effect with
<samp>-Wl,-Map=output.map</samp>.
</p>
</dd>
<dt><code>-u <var>symbol</var></code></dt>
<dd><a name="index-u"></a>
<p>Pretend the symbol <var>symbol</var> is undefined, to force linking of
library modules to define it.  You can use <samp>-u</samp> multiple times with
different symbols to force loading of additional library modules.
</p>
</dd>
<dt><code>-z <var>keyword</var></code></dt>
<dd><a name="index-z"></a>
<p><samp>-z</samp> is passed directly on to the linker along with the keyword
<var>keyword</var>. See the section in the documentation of your linker for
permitted values and their meanings.
</p></dd>
</dl>

<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT1" href="#DOCF1">(1)</a></h3>
<p>On some systems, &lsquo;<samp>gcc -shared</samp>&rsquo;
needs to build supplementary stub code for constructors to work.  On
multi-libbed systems, &lsquo;<samp>gcc -shared</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>
</div>
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