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<h3 class="section">17.7 Compiling and injecting code in <span class="sc">gdb</span></h3>
<p><a name="index-injecting-code-1192"></a><a name="index-writing-into-executables-1193"></a><a name="index-compiling-code-1194"></a>
<span class="sc">gdb</span> supports on-demand compilation and code injection into
programs running under <span class="sc">gdb</span>. GCC 5.0 or higher built with
<samp><span class="file">libcc1.so</span></samp> must be installed for this functionality to be enabled.
This functionality is implemented with the following commands.
<a name="index-compile-code-1195"></a>
<dl><dt><code>compile code </code><var>source-code</var><dt><code>compile code -raw </code><var>&ndash;</var> <var>source-code</var><dd>Compile <var>source-code</var> with the compiler language found as the current
language in <span class="sc">gdb</span> (see <a href="Languages.html#Languages">Languages</a>). If compilation and
injection is not supported with the current language specified in
<span class="sc">gdb</span>, or the compiler does not support this feature, an error
message will be printed. If <var>source-code</var> compiles and links
successfully, <span class="sc">gdb</span> will load the object-code emitted,
and execute it within the context of the currently selected inferior.
It is important to note that the compiled code is executed immediately.
After execution, the compiled code is removed from <span class="sc">gdb</span> and any
new types or variables you have defined will be deleted.
<p>The command allows you to specify <var>source-code</var> in two ways.
The simplest method is to provide a single line of code to the command.
E.g.:
<pre class="smallexample"> compile code printf ("hello world\n");
</pre>
<p>If you specify options on the command line as well as source code, they
may conflict. The &lsquo;<samp><span class="samp">--</span></samp>&rsquo; delimiter can be used to separate options
from actual source code. E.g.:
<pre class="smallexample"> compile code -r -- printf ("hello world\n");
</pre>
<p>Alternatively you can enter source code as multiple lines of text. To
enter this mode, invoke the &lsquo;<samp><span class="samp">compile code</span></samp>&rsquo; command without any text
following the command. This will start the multiple-line editor and
allow you to type as many lines of source code as required. When you
have completed typing, enter &lsquo;<samp><span class="samp">end</span></samp>&rsquo; on its own line to exit the
editor.
<pre class="smallexample"> compile code
&gt;printf ("hello\n");
&gt;printf ("world\n");
&gt;end
</pre>
<p>Specifying &lsquo;<samp><span class="samp">-raw</span></samp>&rsquo;, prohibits <span class="sc">gdb</span> from wrapping the
provided <var>source-code</var> in a callable scope. In this case, you must
specify the entry point of the code by defining a function named
<code>_gdb_expr_</code>. The &lsquo;<samp><span class="samp">-raw</span></samp>&rsquo; code cannot access variables of the
inferior. Using &lsquo;<samp><span class="samp">-raw</span></samp>&rsquo; option may be needed for example when
<var>source-code</var> requires &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; lines which may conflict with
inferior symbols otherwise.
<p><a name="index-compile-file-1196"></a><br><dt><code>compile file </code><var>filename</var><dt><code>compile file -raw </code><var>filename</var><dd>Like <code>compile code</code>, but take the source code from <var>filename</var>.
<pre class="smallexample"> compile file /home/user/example.c
</pre>
</dl>
<dl>
<dt><code>compile print </code><var>expr</var><dt><code>compile print /</code><var>f</var> <var>expr</var><dd>Compile and execute <var>expr</var> with the compiler language found as the
current language in <span class="sc">gdb</span> (see <a href="Languages.html#Languages">Languages</a>). By default the
value of <var>expr</var> is printed in a format appropriate to its data type;
you can choose a different format by specifying &lsquo;<samp><span class="samp">/</span><var>f</var></samp>&rsquo;, where
<var>f</var> is a letter specifying the format; see <a href="Output-Formats.html#Output-Formats">Output Formats</a>.
<br><dt><code>compile print</code><dt><code>compile print /</code><var>f</var><dd><a name="index-reprint-the-last-value-1197"></a>Alternatively you can enter the expression (source code producing it) as
multiple lines of text. To enter this mode, invoke the &lsquo;<samp><span class="samp">compile print</span></samp>&rsquo;
command without any text following the command. This will start the
multiple-line editor.
</dl>
<p class="noindent">The process of compiling and injecting the code can be inspected using:
<dl>
<a name="set-debug-compile"></a><dt><code>set debug compile</code><dd><a name="index-compile-command-debugging-info-1198"></a>Turns on or off display of <span class="sc">gdb</span> process of compiling and
injecting the code. The default is off.
<br><dt><code>show debug compile</code><dd>Displays the current state of displaying <span class="sc">gdb</span> process of
compiling and injecting the code.
<p><a name="set-debug-compile_002dcplus_002dtypes"></a><br><dt><code>set debug compile-cplus-types</code><dd><a name="index-compile-C_0040t_007b_002b_002b_007d-type-conversion-1199"></a>Turns on or off the display of C<tt>++</tt> type conversion debugging information.
The default is off.
<br><dt><code>show debug compile-cplus-types</code><dd>Displays the current state of displaying debugging information for
C<tt>++</tt> type conversion.
</dl>
<h4 class="subsection">17.7.1 Compilation options for the <code>compile</code> command</h4>
<p><span class="sc">gdb</span> needs to specify the right compilation options for the code
to be injected, in part to make its ABI compatible with the inferior
and in part to make the injected code compatible with <span class="sc">gdb</span>'s
injecting process.
<p class="noindent">The options used, in increasing precedence:
<dl>
<dt>target architecture and OS options (<code>gdbarch</code>)<dd>These options depend on target processor type and target operating
system, usually they specify at least 32-bit (<code>-m32</code>) or 64-bit
(<code>-m64</code>) compilation option.
<br><dt>compilation options recorded in the target<dd><span class="sc">gcc</span> (since version 4.7) stores the options used for compilation
into <code>DW_AT_producer</code> part of DWARF debugging information according
to the <span class="sc">gcc</span> option <code>-grecord-gcc-switches</code>. One has to
explicitly specify <code>-g</code> during inferior compilation otherwise
<span class="sc">gcc</span> produces no DWARF. This feature is only relevant for
platforms where <code>-g</code> produces DWARF by default, otherwise one may
try to enforce DWARF by using <code>-gdwarf-4</code>.
<br><dt>compilation options set by <code>set compile-args</code><dd></dl>
<p class="noindent">You can override compilation options using the following command:
<dl>
<dt><code>set compile-args</code><dd><a name="index-compile-command-options-override-1200"></a>Set compilation options used for compiling and injecting code with the
<code>compile</code> commands. These options override any conflicting ones
from the target architecture and/or options stored during inferior
compilation.
<br><dt><code>show compile-args</code><dd>Displays the current state of compilation options override.
This does not show all the options actually used during compilation,
use <a href="set-debug-compile.html#set-debug-compile">set debug compile</a> for that.
</dl>
<h4 class="subsection">17.7.2 Caveats when using the <code>compile</code> command</h4>
<p>There are a few caveats to keep in mind when using the <code>compile</code>
command. As the caveats are different per language, the table below
highlights specific issues on a per language basis.
<dl>
<dt>C code examples and caveats<dd>When the language in <span class="sc">gdb</span> is set to &lsquo;<samp><span class="samp">C</span></samp>&rsquo;, the compiler will
attempt to compile the source code with a &lsquo;<samp><span class="samp">C</span></samp>&rsquo; compiler. The source
code provided to the <code>compile</code> command will have much the same
access to variables and types as it normally would if it were part of
the program currently being debugged in <span class="sc">gdb</span>.
<p>Below is a sample program that forms the basis of the examples that
follow. This program has been compiled and loaded into <span class="sc">gdb</span>,
much like any other normal debugging session.
<pre class="smallexample"> void function1 (void)
{
int i = 42;
printf ("function 1\n");
}
void function2 (void)
{
int j = 12;
function1 ();
}
int main(void)
{
int k = 6;
int *p;
function2 ();
return 0;
}
</pre>
<p>For the purposes of the examples in this section, the program above has
been compiled, loaded into <span class="sc">gdb</span>, stopped at the function
<code>main</code>, and <span class="sc">gdb</span> is awaiting input from the user.
<p>To access variables and types for any program in <span class="sc">gdb</span>, the
program must be compiled and packaged with debug information. The
<code>compile</code> command is not an exception to this rule. Without debug
information, you can still use the <code>compile</code> command, but you will
be very limited in what variables and types you can access.
<p>So with that in mind, the example above has been compiled with debug
information enabled. The <code>compile</code> command will have access to
all variables and types (except those that may have been optimized
out). Currently, as <span class="sc">gdb</span> has stopped the program in the
<code>main</code> function, the <code>compile</code> command would have access to
the variable <code>k</code>. You could invoke the <code>compile</code> command
and type some source code to set the value of <code>k</code>. You can also
read it, or do anything with that variable you would normally do in
<code>C</code>. Be aware that changes to inferior variables in the
<code>compile</code> command are persistent. In the following example:
<pre class="smallexample"> compile code k = 3;
</pre>
<p class="noindent">the variable <code>k</code> is now 3. It will retain that value until
something else in the example program changes it, or another
<code>compile</code> command changes it.
<p>Normal scope and access rules apply to source code compiled and
injected by the <code>compile</code> command. In the example, the variables
<code>j</code> and <code>k</code> are not accessible yet, because the program is
currently stopped in the <code>main</code> function, where these variables
are not in scope. Therefore, the following command
<pre class="smallexample"> compile code j = 3;
</pre>
<p class="noindent">will result in a compilation error message.
<p>Once the program is continued, execution will bring these variables in
scope, and they will become accessible; then the code you specify via
the <code>compile</code> command will be able to access them.
<p>You can create variables and types with the <code>compile</code> command as
part of your source code. Variables and types that are created as part
of the <code>compile</code> command are not visible to the rest of the program for
the duration of its run. This example is valid:
<pre class="smallexample"> compile code int ff = 5; printf ("ff is %d\n", ff);
</pre>
<p>However, if you were to type the following into <span class="sc">gdb</span> after that
command has completed:
<pre class="smallexample"> compile code printf ("ff is %d\n'', ff);
</pre>
<p class="noindent">a compiler error would be raised as the variable <code>ff</code> no longer
exists. Object code generated and injected by the <code>compile</code>
command is removed when its execution ends. Caution is advised
when assigning to program variables values of variables created by the
code submitted to the <code>compile</code> command. This example is valid:
<pre class="smallexample"> compile code int ff = 5; k = ff;
</pre>
<p>The value of the variable <code>ff</code> is assigned to <code>k</code>. The variable
<code>k</code> does not require the existence of <code>ff</code> to maintain the value
it has been assigned. However, pointers require particular care in
assignment. If the source code compiled with the <code>compile</code> command
changed the address of a pointer in the example program, perhaps to a
variable created in the <code>compile</code> command, that pointer would point
to an invalid location when the command exits. The following example
would likely cause issues with your debugged program:
<pre class="smallexample"> compile code int ff = 5; p = &amp;ff;
</pre>
<p>In this example, <code>p</code> would point to <code>ff</code> when the
<code>compile</code> command is executing the source code provided to it.
However, as variables in the (example) program persist with their
assigned values, the variable <code>p</code> would point to an invalid
location when the command exists. A general rule should be followed
in that you should either assign <code>NULL</code> to any assigned pointers,
or restore a valid location to the pointer before the command exits.
<p>Similar caution must be exercised with any structs, unions, and typedefs
defined in <code>compile</code> command. Types defined in the <code>compile</code>
command will no longer be available in the next <code>compile</code> command.
Therefore, if you cast a variable to a type defined in the
<code>compile</code> command, care must be taken to ensure that any future
need to resolve the type can be achieved.
<pre class="smallexample"> (gdb) compile code static struct a { int a; } v = { 42 }; argv = &amp;v;
(gdb) compile code printf ("%d\n", ((struct a *) argv)-&gt;a);
gdb command line:1:36: error: dereferencing pointer to incomplete type ‘struct a’
Compilation failed.
(gdb) compile code struct a { int a; }; printf ("%d\n", ((struct a *) argv)-&gt;a);
42
</pre>
<p>Variables that have been optimized away by the compiler are not
accessible to the code submitted to the <code>compile</code> command.
Access to those variables will generate a compiler error which <span class="sc">gdb</span>
will print to the console.
</dl>
<h4 class="subsection">17.7.3 Compiler search for the <code>compile</code> command</h4>
<p><span class="sc">gdb</span> needs to find <span class="sc">gcc</span> for the inferior being debugged
which may not be obvious for remote targets of different architecture
than where <span class="sc">gdb</span> is running. Environment variable <code>PATH</code> on
<span class="sc">gdb</span> host is searched for <span class="sc">gcc</span> binary matching the
target architecture and operating system. This search can be overriden
by <code>set compile-gcc</code> <span class="sc">gdb</span> command below. <code>PATH</code> is
taken from shell that executed <span class="sc">gdb</span>, it is not the value set by
<span class="sc">gdb</span> command <code>set environment</code>). See <a href="Environment.html#Environment">Environment</a>.
<p>Specifically <code>PATH</code> is searched for binaries matching regular expression
<var>arch</var><code>(-[^-]*)?-</code><var>os</var><code>-gcc</code> according to the inferior target being
debugged. <var>arch</var> is processor name &mdash; multiarch is supported, so for
example both <code>i386</code> and <code>x86_64</code> targets look for pattern
<code>(x86_64|i.86)</code> and both <code>s390</code> and <code>s390x</code> targets look
for pattern <code>s390x?</code>. <var>os</var> is currently supported only for
pattern <code>linux(-gnu)?</code>.
<p>On Posix hosts the compiler driver <span class="sc">gdb</span> needs to find also
shared library <samp><span class="file">libcc1.so</span></samp> from the compiler. It is searched in
default shared library search path (overridable with usual environment
variable <code>LD_LIBRARY_PATH</code>), unrelated to <code>PATH</code> or <code>set
compile-gcc</code> settings. Contrary to it <samp><span class="file">libcc1plugin.so</span></samp> is found
according to the installation of the found compiler &mdash; as possibly
specified by the <code>set compile-gcc</code> command.
<dl>
<dt><code>set compile-gcc</code><dd><a name="index-compile-command-driver-filename-override-1201"></a>Set compilation command used for compiling and injecting code with the
<code>compile</code> commands. If this option is not set (it is set to
an empty string), the search described above will occur &mdash; that is the
default.
<br><dt><code>show compile-gcc</code><dd>Displays the current compile command <span class="sc">gcc</span> driver filename.
If set, it is the main command <samp><span class="command">gcc</span></samp>, found usually for example
under name <samp><span class="file">x86_64-linux-gnu-gcc</span></samp>.
</dl>
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