mingw/info/gdb/Convenience-Vars.html

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  19 <h3 class="section">Convenience variables</h3>
  20
  21    GDB provides <dfn>convenience variables</dfn> that you can use within
  22 GDB to hold on to a value and refer to it later.  These variables
  23 exist entirely within GDB; they are not part of your program, and
  24 setting a convenience variable has no direct effect on further execution
  25 of your program.  That is why you can use them freely.
  26
  27    <p>Convenience variables are prefixed with <code>$</code>.  Any name preceded by
  28 <code>$</code> can be used for a convenience variable, unless it is one of
  29 the predefined machine-specific register names (see <a href="Registers.html#Registers">Registers</a>).
  30 (Value history references, in contrast, are <em>numbers</em> preceded
  31 by <code>$</code>.  See <a href="Value-History.html#Value%20History">Value history</a>.)
  32
  33    <p>You can save a value in a convenience variable with an assignment
  34 expression, just as you would set a variable in your program.
  35 For example:
  36
  37 <pre class="example">     set $foo = *object_ptr
  38      </pre>
  39
  40 <p>would save in <code>$foo</code> the value contained in the object pointed to by
  41 <code>object_ptr</code>.
  42
  43    <p>Using a convenience variable for the first time creates it, but its
  44 value is <code>void</code> until you assign a new value.  You can alter the
  45 value with another assignment at any time.
  46
  47    <p>Convenience variables have no fixed types.  You can assign a convenience
  48 variable any type of value, including structures and arrays, even if
  49 that variable already has a value of a different type.  The convenience
  50 variable, when used as an expression, has the type of its current value.
  51
  52      <dl>
  53 <dt><code>show convenience</code>
  54      <dd>Print a list of convenience variables used so far, and their values.
  55 Abbreviated <code>show conv</code>.
  56 </dl>
  57
  58    <p>One of the ways to use a convenience variable is as a counter to be
  59 incremented or a pointer to be advanced.  For example, to print
  60 a field from successive elements of an array of structures:
  61
  62 <pre class="example">     set $i = 0
  63      print bar[$i++]-&gt;contents
  64      </pre>
  65
  66 <p>Repeat that command by typing &lt;RET&gt;.
  67
  68    <p>Some convenience variables are created automatically by GDB and given
  69 values likely to be useful.
  70
  71      <dl>
  72 <dt><code>$_</code>
  73      <dd>The variable <code>$_</code> is automatically set by the <code>x</code> command to
  74 the last address examined (see <a href="Memory.html#Memory">Examining memory</a>).  Other
  75 commands which provide a default address for <code>x</code> to examine also
  76 set <code>$_</code> to that address; these commands include <code>info line</code>
  77 and <code>info breakpoint</code>.  The type of <code>$_</code> is <code>void *</code>
  78 except when set by the <code>x</code> command, in which case it is a pointer
  79 to the type of <code>$__</code>.
  80
  81      <br><dt><code>$__</code>
  82      <dd>The variable <code>$__</code> is automatically set by the <code>x</code> command
  83 to the value found in the last address examined.  Its type is chosen
  84 to match the format in which the data was printed.
  85
  86      <br><dt><code>$_exitcode</code>
  87      <dd>The variable <code>$_exitcode</code> is automatically set to the exit code when
  88 the program being debugged terminates.
  89 </dl>
  90
  91    <p>On HP-UX systems, if you refer to a function or variable name that
  92 begins with a dollar sign, GDB searches for a user or system
  93 name first, before it searches for a convenience variable.
  94
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