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<a name="Inferiors-and-Programs"></a>
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<hr>
<a name="Debugging-Multiple-Inferiors-and-Programs"></a>
<h3 class="section">4.9 Debugging Multiple Inferiors and Programs</h3>

<p><small>GDB</small> lets you run and debug multiple programs in a single
session.  In addition, <small>GDB</small> on some systems may let you run
several programs simultaneously (otherwise you have to exit from one
before starting another).  In the most general case, you can have
multiple threads of execution in each of multiple processes, launched
from multiple executables.
</p>
<a name="index-inferior"></a>
<p><small>GDB</small> represents the state of each program execution with an
object called an <em>inferior</em>.  An inferior typically corresponds to
a process, but is more general and applies also to targets that do not
have processes.  Inferiors may be created before a process runs, and
may be retained after a process exits.  Inferiors have unique
identifiers that are different from process ids.  Usually each
inferior will also have its own distinct address space, although some
embedded targets may have several inferiors running in different parts
of a single address space.  Each inferior may in turn have multiple
threads running in it.
</p>
<p>To find out what inferiors exist at any moment, use <code>info&nbsp;inferiors</code><!-- /@w -->:
</p>
<dl compact="compact">
<dd><a name="index-info-inferiors-_005b-id_2026-_005d"></a>
</dd>
<dt><code>info inferiors</code></dt>
<dd><p>Print a list of all inferiors currently being managed by <small>GDB</small>.
By default all inferiors are printed, but the argument <var>id</var>&hellip;
&ndash; a space separated list of inferior numbers &ndash; can be used to limit
the display to just the requested inferiors.
</p>
<p><small>GDB</small> displays for each inferior (in this order):
</p>
<ol>
<li> the inferior number assigned by <small>GDB</small>

</li><li> the target system&rsquo;s inferior identifier

</li><li> the name of the executable the inferior is running.

</li></ol>

<p>An asterisk &lsquo;<samp>*</samp>&rsquo; preceding the <small>GDB</small> inferior number
indicates the current inferior.
</p>
<p>For example,
</p></dd>
</dl>

<div class="smallexample">
<pre class="smallexample">(gdb) info inferiors
  Num  Description       Executable
  2    process 2307      hello
* 1    process 3401      goodbye
</pre></div>

<p>To switch focus between inferiors, use the <code>inferior</code> command:
</p>
<dl compact="compact">
<dd><a name="index-inferior-infno"></a>
</dd>
<dt><code>inferior <var>infno</var></code></dt>
<dd><p>Make inferior number <var>infno</var> the current inferior.  The argument
<var>infno</var> is the inferior number assigned by <small>GDB</small>, as shown
in the first field of the &lsquo;<samp>info inferiors</samp>&rsquo; display.
</p></dd>
</dl>

<a name="index-_0024_005finferior_002c-convenience-variable"></a>
<p>The debugger convenience variable &lsquo;<samp>$_inferior</samp>&rsquo; contains the
number of the current inferior.  You may find this useful in writing
breakpoint conditional expressions, command scripts, and so forth.
See <a href="Convenience-Vars.html#Convenience-Vars">Convenience Variables</a>, for general
information on convenience variables.
</p>
<p>You can get multiple executables into a debugging session via the
<code>add-inferior</code> and <code><span class="nolinebreak">clone-inferior</span></code><!-- /@w --> commands.  On some
systems <small>GDB</small> can add inferiors to the debug session
automatically by following calls to <code>fork</code> and <code>exec</code>.  To
remove inferiors from the debugging session use the
<code><span class="nolinebreak">remove-inferiors</span></code><!-- /@w --> command.
</p>
<dl compact="compact">
<dd><a name="index-add_002dinferior"></a>
</dd>
<dt><code>add-inferior [ -copies <var>n</var> ] [ -exec <var>executable</var> ]</code></dt>
<dd><p>Adds <var>n</var> inferiors to be run using <var>executable</var> as the
executable; <var>n</var> defaults to 1.  If no executable is specified,
the inferiors begins empty, with no program.  You can still assign or
change the program assigned to the inferior at any time by using the
<code>file</code> command with the executable name as its argument.
</p>
<a name="index-clone_002dinferior"></a>
</dd>
<dt><code>clone-inferior [ -copies <var>n</var> ] [ <var>infno</var> ]</code></dt>
<dd><p>Adds <var>n</var> inferiors ready to execute the same program as inferior
<var>infno</var>; <var>n</var> defaults to 1, and <var>infno</var> defaults to the
number of the current inferior.  This is a convenient command when you
want to run another instance of the inferior you are debugging.
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) info inferiors
  Num  Description       Executable
* 1    process 29964     helloworld
(gdb) clone-inferior
Added inferior 2.
1 inferiors added.
(gdb) info inferiors
  Num  Description       Executable
  2    &lt;null&gt;            helloworld
* 1    process 29964     helloworld
</pre></div>

<p>You can now simply switch focus to inferior 2 and run it.
</p>
<a name="index-remove_002dinferiors"></a>
</dd>
<dt><code>remove-inferiors <var>infno</var>&hellip;</code></dt>
<dd><p>Removes the inferior or inferiors <var>infno</var>&hellip;.  It is not
possible to remove an inferior that is running with this command.  For
those, use the <code>kill</code> or <code>detach</code> command first.
</p>
</dd>
</dl>

<p>To quit debugging one of the running inferiors that is not the current
inferior, you can either detach from it by using the <code>detach&nbsp;inferior</code><!-- /@w --> command (allowing it to run independently), or kill it
using the <code>kill&nbsp;inferiors</code><!-- /@w --> command:
</p>
<dl compact="compact">
<dd><a name="index-detach-inferiors-infno_2026"></a>
</dd>
<dt><code>detach inferior <var>infno</var>&hellip;</code></dt>
<dd><p>Detach from the inferior or inferiors identified by <small>GDB</small>
inferior number(s) <var>infno</var>&hellip;.  Note that the inferior&rsquo;s entry
still stays on the list of inferiors shown by <code>info inferiors</code>,
but its Description will show &lsquo;<samp>&lt;null&gt;</samp>&rsquo;.
</p>
<a name="index-kill-inferiors-infno_2026"></a>
</dd>
<dt><code>kill inferiors <var>infno</var>&hellip;</code></dt>
<dd><p>Kill the inferior or inferiors identified by <small>GDB</small> inferior
number(s) <var>infno</var>&hellip;.  Note that the inferior&rsquo;s entry still
stays on the list of inferiors shown by <code>info inferiors</code>, but its
Description will show &lsquo;<samp>&lt;null&gt;</samp>&rsquo;.
</p></dd>
</dl>

<p>After the successful completion of a command such as <code>detach</code>,
<code>detach inferiors</code>, <code>kill</code> or <code>kill inferiors</code>, or after
a normal process exit, the inferior is still valid and listed with
<code>info inferiors</code>, ready to be restarted.
</p>

<p>To be notified when inferiors are started or exit under <small>GDB</small>&rsquo;s
control use <code>set&nbsp;print&nbsp;<span class="nolinebreak">inferior-events</span></code><!-- /@w -->:
</p>
<dl compact="compact">
<dd><a name="index-set-print-inferior_002devents"></a>
<a name="index-print-messages-on-inferior-start-and-exit"></a>
</dd>
<dt><code>set print inferior-events</code></dt>
<dt><code>set print inferior-events on</code></dt>
<dt><code>set print inferior-events off</code></dt>
<dd><p>The <code>set print inferior-events</code> command allows you to enable or
disable printing of messages when <small>GDB</small> notices that new
inferiors have started or that inferiors have exited or have been
detached.  By default, these messages will not be printed.
</p>
<a name="index-show-print-inferior_002devents"></a>
</dd>
<dt><code>show print inferior-events</code></dt>
<dd><p>Show whether messages will be printed when <small>GDB</small> detects that
inferiors have started, exited or have been detached.
</p></dd>
</dl>

<p>Many commands will work the same with multiple programs as with a
single program: e.g., <code>print myglobal</code> will simply display the
value of <code>myglobal</code> in the current inferior.
</p>

<p>Occasionaly, when debugging <small>GDB</small> itself, it may be useful to
get more info about the relationship of inferiors, programs, address
spaces in a debug session.  You can do that with the <code>maint&nbsp;info&nbsp;<span class="nolinebreak">program-spaces</span></code><!-- /@w --> command.
</p>
<dl compact="compact">
<dd><a name="index-maint-info-program_002dspaces"></a>
</dd>
<dt><code>maint info program-spaces</code></dt>
<dd><p>Print a list of all program spaces currently being managed by
<small>GDB</small>.
</p>
<p><small>GDB</small> displays for each program space (in this order):
</p>
<ol>
<li> the program space number assigned by <small>GDB</small>

</li><li> the name of the executable loaded into the program space, with e.g.,
the <code>file</code> command.

</li></ol>

<p>An asterisk &lsquo;<samp>*</samp>&rsquo; preceding the <small>GDB</small> program space number
indicates the current program space.
</p>
<p>In addition, below each program space line, <small>GDB</small> prints extra
information that isn&rsquo;t suitable to display in tabular form.  For
example, the list of inferiors bound to the program space.
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) maint info program-spaces
  Id   Executable
* 1    hello
  2    goodbye
        Bound inferiors: ID 1 (process 21561)
</pre></div>

<p>Here we can see that no inferior is running the program <code>hello</code>,
while <code>process 21561</code> is running the program <code>goodbye</code>.  On
some targets, it is possible that multiple inferiors are bound to the
same program space.  The most common example is that of debugging both
the parent and child processes of a <code>vfork</code> call.  For example,
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) maint info program-spaces
  Id   Executable
* 1    vfork-test
        Bound inferiors: ID 2 (process 18050), ID 1 (process 18045)
</pre></div>

<p>Here, both inferior 2 and inferior 1 are running in the same program
space as a result of inferior 1 having executed a <code>vfork</code> call.
</p></dd>
</dl>

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