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<title>Debugging with GDB: Command aliases default args</title>
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<a name="Command-aliases-default-args"></a>
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<div class="header">
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<p>
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Next: <a href="Help.html#Help" accesskey="n" rel="next">Help</a>, Previous: <a href="Command-Options.html#Command-Options" accesskey="p" rel="previous">Command Options</a>, Up: <a href="Commands.html#Commands" accesskey="u" rel="up">Commands</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Automatically-Prepend-Default-Arguments-to-User_002dDefined-Aliases"></a>
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<h3 class="section">3.5 Automatically Prepend Default Arguments to User-Defined Aliases</h3>
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<p>You can tell <small>GDB</small> to always prepend some default arguments to
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the list of arguments provided explicitly by the user when using a
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user-defined alias.
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</p>
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<p>If you repeatedly use the same arguments or options for a command, you
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can define an alias for this command and tell <small>GDB</small> to
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automatically prepend these arguments or options to the list of
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arguments you type explicitly when using the alias<a name="DOCF3" href="#FOOT3"><sup>3</sup></a>.
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</p>
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<p>For example, if you often use the command <code>thread apply all</code>
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specifying to work on the threads in ascending order and to continue in case it
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encounters an error, you can tell <small>GDB</small> to automatically preprend
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the <code>-ascending</code> and <code>-c</code> options by using:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) alias thread apply asc-all = thread apply all -ascending -c
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</pre></div>
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<p>Once you have defined this alias with its default args, any time you type
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the <code>thread apply asc-all</code> followed by <code>some arguments</code>,
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<small>GDB</small> will execute <code>thread apply all -ascending -c some arguments</code>.
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</p>
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<p>To have even less to type, you can also define a one word alias:
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</p><div class="smallexample">
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<pre class="smallexample">(gdb) alias t_a_c = thread apply all -ascending -c
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</pre></div>
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<p>As usual, unambiguous abbreviations can be used for <var>alias</var>
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and <var>default-args</var>.
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</p>
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<p>The different aliases of a command do not share their default args.
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For example, you define a new alias <code>bt_ALL</code> showing all possible
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information and another alias <code>bt_SMALL</code> showing very limited information
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using:
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</p><div class="smallexample">
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<pre class="smallexample">(gdb) alias bt_ALL = backtrace -entry-values both -frame-arg all \
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-past-main -past-entry -full
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(gdb) alias bt_SMALL = backtrace -entry-values no -frame-arg none \
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-past-main off -past-entry off
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</pre></div>
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<p>(For more on using the <code>alias</code> command, see <a href="Aliases.html#Aliases">Aliases</a>.)
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</p>
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<p>Default args are not limited to the arguments and options of <var>command</var>,
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but can specify nested commands if <var>command</var> accepts such a nested command
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as argument.
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For example, the below defines <code>faalocalsoftype</code> that lists the
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frames having locals of a certain type, together with the matching
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local vars:
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</p><div class="smallexample">
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<pre class="smallexample">(gdb) alias faalocalsoftype = frame apply all info locals -q -t
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(gdb) faalocalsoftype int
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#1 0x55554f5e in sleeper_or_burner (v=0xdf50) at sleepers.c:86
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i = 0
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ret = 21845
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</pre></div>
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<p>This is also very useful to define an alias for a set of nested <code>with</code>
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commands to have a particular combination of temporary settings. For example,
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the below defines the alias <code>pp10</code> that pretty prints an expression
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argument, with a maximum of 10 elements if the expression is a string or
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an array:
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</p><div class="smallexample">
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<pre class="smallexample">(gdb) alias pp10 = with print pretty -- with print elements 10 -- print
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</pre></div>
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<p>This defines the alias <code>pp10</code> as being a sequence of 3 commands.
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The first part <code>with print pretty --</code> temporarily activates the setting
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<code>set print pretty</code>, then launches the command that follows the separator
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<code>--</code>.
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The command following the first part is also a <code>with</code> command that
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temporarily changes the setting <code>set print elements</code> to 10, then
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launches the command that follows the second separator <code>--</code>.
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The third part <code>print</code> is the command the <code>pp10</code> alias will launch,
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using the temporary values of the settings and the arguments explicitly given
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by the user.
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For more information about the <code>with</code> command usage,
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see <a href="Command-Settings.html#Command-Settings">Command Settings</a>.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
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<p><small>GDB</small>
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could easily accept default arguments for pre-defined commands and aliases,
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but it was deemed this would be confusing, and so is not allowed.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Help.html#Help" accesskey="n" rel="next">Help</a>, Previous: <a href="Command-Options.html#Command-Options" accesskey="p" rel="previous">Command Options</a>, Up: <a href="Commands.html#Commands" accesskey="u" rel="up">Commands</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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