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<title>Using the GNU Compiler Collection (GCC)</title>
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<h3 class="section">Options for Linking</h3>
<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.
<dl>
<dt><code></code><var>object-file-name</var><code></code>
<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>
<dd><dt><code>-S</code>
<dd><dt><code>-E</code>
<dd>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%20Options">Overall Options</a>.
<br><dt><code>-l</code><var>library</var><code></code>
<dd><dt><code>-l </code><var>library</var><code></code>
<dd>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, <code>foo.o -lz bar.o</code> searches library <code>z</code>
after file <code>foo.o</code> but before <code>bar.o</code>. If <code>bar.o</code> refers
to functions in <code>z</code>, those functions may not be loaded.
<p>The linker searches a standard list of directories for the library,
which is actually a file named <code>lib</code><var>library</var><code>.a</code>. 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 <code>-L</code>.
<p>Normally the files found this way are library files--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 <code>-l</code> option and specifying a file name
is that <code>-l</code> surrounds <var>library</var> with <code>lib</code> and <code>.a</code>
and searches several directories.
<br><dt><code>-lobjc</code>
<dd>You need this special case of the <code>-l</code> option in order to
link an Objective-C program.
<br><dt><code>-nostartfiles</code>
<dd>Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless <code>-nostdlib</code>
or <code>-nodefaultlibs</code> is used.
<br><dt><code>-nodefaultlibs</code>
<dd>Do not use the standard system libraries when linking.
Only the libraries you specify will be passed to the linker.
The standard startup files are used normally, unless <code>-nostartfiles</code>
is used. The compiler may generate calls to memcmp, memset, and memcpy
for System V (and ISO C) environments or to bcopy and bzero for
BSD environments. 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>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. The compiler may generate calls to memcmp, memset, and memcpy
for System V (and ISO C) environments or to bcopy and bzero for
BSD environments. 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>One of the standard libraries bypassed by <code>-nostdlib</code> and
<code>-nodefaultlibs</code> is <code>libgcc.a</code>, 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 <code>libgcc.a</code>.)
In most cases, you need <code>libgcc.a</code> even when you want to avoid
other standard libraries. In other words, when you specify <code>-nostdlib</code>
or <code>-nodefaultlibs</code> you should usually specify <code>-lgcc</code> as well.
This ensures that you have no unresolved references to internal GCC
library subroutines. (For example, <code>__main</code>, used to ensure C++
constructors will be called; see <a href="../gccint/Collect2.html#Collect2"><code>collect2</code></a>.)
<br><dt><code>-s</code>
<dd>Remove all symbol table and relocation information from the executable.
<br><dt><code>-static</code>
<dd>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>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 (<code>-fpic</code>, <code>-fPIC</code>, or model suboptions)
when you specify this option.<a rel="footnote" href="#fn-1"><sup>1</sup></a>
<br><dt><code>-shared-libgcc</code>
<dd><dt><code>-static-libgcc</code>
<dd>On systems that provide <code>libgcc</code> as a shared library, these options
force the use of either the shared or static version respectively.
If no shared version of <code>libgcc</code> 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 <code>libgcc</code> 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 <code>libgcc</code>.
<p>Therefore, the G++ and GCJ drivers automatically add
<code>-shared-libgcc</code> 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 <code>libgcc</code>.
If GCC finds, at its configuration time, that you have a GNU linker that
does not support option <code>--eh-frame-hdr</code>, it will link the shared
version of <code>libgcc</code> into shared libraries by default. Otherwise,
it will take advantage of the linker and optimize away the linking with
the shared version of <code>libgcc</code>, 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
<code>-shared-libgcc</code>, such that it is linked with the shared
<code>libgcc</code>.
<br><dt><code>-symbolic</code>
<dd>Bind references to global symbols when building a shared object. Warn
about any unresolved references (unless overridden by the link editor
option <code>-Xlinker -z -Xlinker defs</code>). Only a few systems support
this option.
<br><dt><code>-Xlinker </code><var>option</var><code></code>
<dd>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 an argument, you must use
<code>-Xlinker</code> twice, once for the option and once for the argument.
For example, to pass <code>-assert definitions</code>, you must write
<code>-Xlinker -assert -Xlinker definitions</code>. It does not work to write
<code>-Xlinker "-assert definitions"</code>, because this passes the entire
string as a single argument, which is not what the linker expects.
<br><dt><code>-Wl,</code><var>option</var><code></code>
<dd>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.
<br><dt><code>-u </code><var>symbol</var><code></code>
<dd>Pretend the symbol <var>symbol</var> is undefined, to force linking of
library modules to define it. You can use <code>-u</code> multiple times with
different symbols to force loading of additional library modules.
</dl>
<div class="footnote">
<hr>
<h4>Footnotes</h4>
<ol type="1">
<li><a name="fn-1"></a>
<p>On some systems, <code>gcc -shared</code>
needs to build supplementary stub code for constructors to work. On
multi-libbed systems, <code>gcc -shared</code> 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>
</ol><hr></div>
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