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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="previous">Assembler Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Options-for-Linking"></a>
<h3 class="section">3.15 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>
<a name="index-c-1"></a>
<a name="index-S-1"></a>
<a name="index-E-1"></a>
</dd>
<dt><code>-c</code></dt>
<dt><code>-S</code></dt>
<dt><code>-E</code></dt>
<dd><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>
<a name="index-flinker_002doutput"></a>
</dd>
<dt><code>-flinker-output=<var>type</var></code></dt>
<dd><p>This option controls code generation of the link-time optimizer. By
default the linker output is automatically determined by the linker
plugin. For debugging the compiler and if incremental linking with a
non-LTO object file is desired, it may be useful to control the type
manually.
</p>
<p>If <var>type</var> is ‘<samp>exec</samp>’, code generation produces a static
binary. In this case <samp>-fpic</samp> and <samp>-fpie</samp> are both
disabled.
</p>
<p>If <var>type</var> is ‘<samp>dyn</samp>’, code generation produces a shared
library. In this case <samp>-fpic</samp> or <samp>-fPIC</samp> is preserved,
but not enabled automatically. This allows to build shared libraries
without position-independent code on architectures where this is
possible, i.e. on x86.
</p>
<p>If <var>type</var> is ‘<samp>pie</samp>’, code generation produces an <samp>-fpie</samp>
executable. This results in similar optimizations as ‘<samp>exec</samp>’
except that <samp>-fpie</samp> is not disabled if specified at compilation
time.
</p>
<p>If <var>type</var> is ‘<samp>rel</samp>’, the compiler assumes that incremental linking is
done. The sections containing intermediate code for link-time optimization are
merged, pre-optimized, and output to the resulting object file. In addition, if
<samp>-ffat-lto-objects</samp> is specified, binary code is produced for future
non-LTO linking. The object file produced by incremental linking is smaller
than a static library produced from the same object files. At link time the
result of incremental linking also loads faster than a static
library assuming that the majority of objects in the library are used.
</p>
<p>Finally ‘<samp>nolto-rel</samp>’ configures the compiler for incremental linking where
code generation is forced, a final binary is produced, and the intermediate
code for later link-time optimization is stripped. When multiple object files
are linked together the resulting code is better optimized than with
link-time optimizations disabled (for example, cross-module inlining
happens), but most of benefits of whole program optimizations are lost.
</p>
<p>During the incremental link (by <samp>-r</samp>) the linker plugin defaults to
<samp>rel</samp>. With current interfaces to GNU Binutils it is however not
possible to incrementally link LTO objects and non-LTO objects into a single
mixed object file. If any of object files in incremental link cannot
be used for link-time optimization, the linker plugin issues a warning and
uses ‘<samp>nolto-rel</samp>’. To maintain whole program optimization, it is
recommended to link such objects into static library instead. Alternatively it
is possible to use H.J. Lu’s binutils with support for mixed objects.
</p>
<a name="index-fuse_002dld_003dbfd"></a>
</dd>
<dt><code>-fuse-ld=bfd</code></dt>
<dd><p>Use the <code>bfd</code> linker instead of the default linker.
</p>
<a name="index-fuse_002dld_003dgold"></a>
</dd>
<dt><code>-fuse-ld=gold</code></dt>
<dd><p>Use the <code>gold</code> linker instead of the default linker.
</p>
<a name="index-fuse_002dld_003dlld"></a>
</dd>
<dt><code>-fuse-ld=lld</code></dt>
<dd><p>Use the LLVM <code>lld</code> linker instead of the default linker.
</p>
<a name="index-fuse_002dld_003dmold"></a>
</dd>
<dt><code>-fuse-ld=mold</code></dt>
<dd><p>Use the Modern Linker (<code>mold</code>) instead of the default linker.
</p>
<a name="index-Libraries"></a>
<a name="index-l"></a>
</dd>
<dt><code>-l<var>library</var></code></dt>
<dt><code>-l <var>library</var></code></dt>
<dd><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>The <samp>-l</samp> option is passed directly to the linker by GCC. Refer
to your linker documentation for exact details. The general
description below applies to the GNU linker.
</p>
<p>The linker searches a standard list of directories for the library.
The directories searched include several standard system directories
plus any that you specify with <samp>-L</samp>.
</p>
<p>Static libraries are archives of object files, and have file names
like <samp>lib<var>library</var>.a</samp>. Some targets also support shared
libraries, which typically have names like <samp>lib<var>library</var>.so</samp>.
If both static and shared libraries are found, the linker gives
preference to linking with the shared library unless the
<samp>-static</samp> option is used.
</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, ‘<samp>foo.o -lz bar.o</samp>’ searches library ‘<samp>z</samp>’
after file <samp>foo.o</samp> but before <samp>bar.o</samp>. If <samp>bar.o</samp> refers
to functions in ‘<samp>z</samp>’, those functions may not be loaded.
</p>
<a name="index-lobjc"></a>
</dd>
<dt><code>-lobjc</code></dt>
<dd><p>You need this special case of the <samp>-l</samp> option in order to
link an Objective-C or Objective-C++ program.
</p>
<a name="index-nostartfiles"></a>
</dd>
<dt><code>-nostartfiles</code></dt>
<dd><p>Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless <samp>-nostdlib</samp>,
<samp>-nolibc</samp>, or <samp>-nodefaultlibs</samp> is used.
</p>
<a name="index-nodefaultlibs"></a>
</dd>
<dt><code>-nodefaultlibs</code></dt>
<dd><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>
<a name="index-nolibc"></a>
</dd>
<dt><code>-nolibc</code></dt>
<dd><p>Do not use the C library or system libraries tightly coupled with it when
linking. Still link with the startup files, <samp>libgcc</samp> or toolchain
provided language support libraries such as <samp>libgnat</samp>, <samp>libgfortran</samp>
or <samp>libstdc++</samp> unless options preventing their inclusion are used as
well. This typically removes <samp>-lc</samp> from the link command line, as well
as system libraries that normally go with it and become meaningless when
absence of a C library is assumed, for example <samp>-lpthread</samp> or
<samp>-lm</samp> in some configurations. This is intended for bare-board
targets when there is indeed no C library available.
</p>
<a name="index-nostdlib"></a>
</dd>
<dt><code>-nostdlib</code></dt>
<dd><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>
<a name="index-nostdlib_002b_002b"></a>
</dd>
<dt><code>-nostdlib++</code></dt>
<dd><p>Do not implicitly link with standard C++ libraries.
</p>
<a name="index-e"></a>
<a name="index-entry"></a>
</dd>
<dt><code>-e <var>entry</var></code></dt>
<dt><code>--entry=<var>entry</var></code></dt>
<dd>
<p>Specify that the program entry point is <var>entry</var>. The argument is
interpreted by the linker; the GNU linker accepts either a symbol name
or an address.
</p>
<a name="index-pie"></a>
</dd>
<dt><code>-pie</code></dt>
<dd><p>Produce a dynamically linked 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>
<a name="index-no_002dpie"></a>
</dd>
<dt><code>-no-pie</code></dt>
<dd><p>Don’t produce a dynamically linked position independent executable.
</p>
<a name="index-static_002dpie"></a>
</dd>
<dt><code>-static-pie</code></dt>
<dd><p>Produce a static position independent executable on targets that support
it. A static position independent executable is similar to a static
executable, but can be loaded at any address without a dynamic linker.
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>
<a name="index-pthread-1"></a>
</dd>
<dt><code>-pthread</code></dt>
<dd><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>
<a name="index-r"></a>
</dd>
<dt><code>-r</code></dt>
<dd><p>Produce a relocatable object as output. This is also known as partial
linking.
</p>
<a name="index-rdynamic"></a>
</dd>
<dt><code>-rdynamic</code></dt>
<dd><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>
<a name="index-s"></a>
</dd>
<dt><code>-s</code></dt>
<dd><p>Remove all symbol table and relocation information from the executable.
</p>
<a name="index-static"></a>
</dd>
<dt><code>-static</code></dt>
<dd><p>On systems that support dynamic linking, this overrides <samp>-pie</samp>
and prevents linking with the shared libraries. On other systems, this
option has no effect.
</p>
<a name="index-shared"></a>
</dd>
<dt><code>-shared</code></dt>
<dd><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>
<a name="index-shared_002dlibgcc"></a>
<a name="index-static_002dlibgcc"></a>
</dd>
<dt><code>-shared-libgcc</code></dt>
<dt><code>-static-libgcc</code></dt>
<dd><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>
<a name="index-static_002dlibasan"></a>
</dd>
<dt><code>-static-libasan</code></dt>
<dd><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>
<a name="index-static_002dlibtsan"></a>
</dd>
<dt><code>-static-libtsan</code></dt>
<dd><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>
<a name="index-static_002dliblsan"></a>
</dd>
<dt><code>-static-liblsan</code></dt>
<dd><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>
<a name="index-static_002dlibubsan"></a>
</dd>
<dt><code>-static-libubsan</code></dt>
<dd><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>
<a name="index-static_002dlibstdc_002b_002b"></a>
</dd>
<dt><code>-static-libstdc++</code></dt>
<dd><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>
<a name="index-symbolic"></a>
</dd>
<dt><code>-symbolic</code></dt>
<dd><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>
<a name="index-T"></a>
<a name="index-linker-script"></a>
</dd>
<dt><code>-T <var>script</var></code></dt>
<dd><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>
<a name="index-Xlinker"></a>
</dd>
<dt><code>-Xlinker <var>option</var></code></dt>
<dd><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 "-assert definitions"</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>
<a name="index-Wl"></a>
</dd>
<dt><code>-Wl,<var>option</var></code></dt>
<dd><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>
<a name="index-u"></a>
</dd>
<dt><code>-u <var>symbol</var></code></dt>
<dd><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>
<a name="index-z"></a>
</dd>
<dt><code>-z <var>keyword</var></code></dt>
<dd><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, ‘<samp>gcc -shared</samp>’
needs to build supplementary stub code for constructors to work. On
multi-libbed systems, ‘<samp>gcc -shared</samp>’ 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. <samp>-shared</samp> suppresses the addition of startup code
to alter the floating-point environment as done with <samp>-ffast-math</samp>,
<samp>-Ofast</samp> or <samp>-funsafe-math-optimizations</samp> on some targets.</p>
</div>
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