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<title>Command aliases default args (Debugging with GDB)</title>

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<a name="Command-aliases-default-args"></a>
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<hr>
<a name="Automatically-prepend-default-arguments-to-user_002ddefined-aliases"></a>
<h3 class="section">3.5 Automatically prepend default arguments to user-defined aliases</h3>

<p>You can tell <small>GDB</small> to always prepend some default arguments to
the list of arguments provided explicitly by the user when using a
user-defined alias.
</p>
<p>If you repeatedly use the same arguments or options for a command, you
can define an alias for this command and tell <small>GDB</small> to
automatically prepend these arguments or options to the list of
arguments you type explicitly when using the alias<a name="DOCF3" href="#FOOT3"><sup>3</sup></a>.
</p>
<p>For example, if you often use the command <code>thread apply all</code>
specifying to work on the threads in ascending order and to continue in case it
encounters an error, you can tell <small>GDB</small> to automatically preprend
the <code>-ascending</code> and <code>-c</code> options by using:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) alias thread apply asc-all = thread apply all -ascending -c
</pre></div>

<p>Once you have defined this alias with its default args, any time you type
the <code>thread apply asc-all</code> followed by <code>some arguments</code>,
<small>GDB</small> will execute  <code>thread apply all -ascending -c some arguments</code>.
</p>
<p>To have even less to type, you can also define a one word alias:
</p><div class="smallexample">
<pre class="smallexample">(gdb) alias t_a_c = thread apply all -ascending -c
</pre></div>

<p>As usual, unambiguous abbreviations can be used for <var>alias</var>
and <var>default-args</var>.
</p>
<p>The different aliases of a command do not share their default args.
For example, you define a new alias <code>bt_ALL</code> showing all possible
information and another alias <code>bt_SMALL</code> showing very limited information
using:
</p><div class="smallexample">
<pre class="smallexample">(gdb) alias bt_ALL = backtrace -entry-values both -frame-arg all \
   -past-main -past-entry -full
(gdb) alias bt_SMALL = backtrace -entry-values no -frame-arg none \
   -past-main off -past-entry off
</pre></div>

<p>(For more on using the <code>alias</code> command, see <a href="Aliases.html#Aliases">Aliases</a>.)
</p>
<p>Default args are not limited to the arguments and options of <var>command</var>,
but can specify nested commands if <var>command</var> accepts such a nested command
as argument.
For example, the below defines <code>faalocalsoftype</code> that lists the
frames having locals of a certain type, together with the matching
local vars:
</p><div class="smallexample">
<pre class="smallexample">(gdb) alias faalocalsoftype = frame apply all info locals -q -t
(gdb) faalocalsoftype int
#1  0x55554f5e in sleeper_or_burner (v=0xdf50) at sleepers.c:86
i = 0
ret = 21845
</pre></div>

<p>This is also very useful to define an alias for a set of nested <code>with</code>
commands to have a particular combination of temporary settings.  For example,
the below defines the alias <code>pp10</code> that pretty prints an expression
argument, with a maximum of 10 elements if the expression is a string or
an array:
</p><div class="smallexample">
<pre class="smallexample">(gdb) alias pp10 = with print pretty -- with print elements 10 -- print
</pre></div>
<p>This defines the alias  <code>pp10</code> as being a sequence of 3 commands.
The first part <code>with print pretty --</code> temporarily activates the setting
<code>set print pretty</code>, then launches the command that follows the separator
<code>--</code>.
The command following the first part is also a <code>with</code> command that
temporarily changes the setting <code>set print elements</code> to 10, then
launches the command that follows the second separator <code>--</code>.
The third part <code>print</code> is the command the <code>pp10</code> alias will launch,
using the temporary values of the settings and the arguments explicitly given
by the user.
For more information about the <code>with</code> command usage,
see <a href="Command-Settings.html#Command-Settings">Command Settings</a>.
</p>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
<p><small>GDB</small>
could easily accept default arguments for pre-defined commands and aliases,
but it was deemed this would be confusing, and so is not allowed.</p>
</div>
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