mingw/info/gdb/Assignment.html

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  18 <h3 class="section">Assignment to variables</h3>
  19
  20    <p>To alter the value of a variable, evaluate an assignment expression.
  21 See <a href="Expressions.html#Expressions">Expressions</a>.  For example,
  22
  23 <pre class="example">     print x=4
  24      </pre>
  25
  26 <p>stores the value 4 into the variable <code>x</code>, and then prints the
  27 value of the assignment expression (which is 4).
  28 See <a href="Languages.html#Languages">Using GDB with Different Languages</a>, for more
  29 information on operators in supported languages.
  30
  31    <p>If you are not interested in seeing the value of the assignment, use the
  32 <code>set</code> command instead of the <code>print</code> command.  <code>set</code> is
  33 really the same as <code>print</code> except that the expression's value is
  34 not printed and is not put in the value history (see <a href="Value-History.html#Value%20History">Value history</a>).  The expression is evaluated only for its effects.
  35
  36    <p>If the beginning of the argument string of the <code>set</code> command
  37 appears identical to a <code>set</code> subcommand, use the <code>set
  38 variable</code> command instead of just <code>set</code>.  This command is identical
  39 to <code>set</code> except for its lack of subcommands.  For example, if your
  40 program has a variable <code>width</code>, you get an error if you try to set
  41 a new value with just <code>set width=13</code>, because GDB has the
  42 command <code>set width</code>:
  43
  44 <pre class="example">     (gdb) whatis width
  45      type = double
  46      (gdb) p width
  47      $4 = 13
  48      (gdb) set width=47
  49      Invalid syntax in expression.
  50      </pre>
  51
  52 <p>The invalid expression, of course, is <code>=47</code>.  In
  53 order to actually set the program's variable <code>width</code>, use
  54
  55 <pre class="example">     (gdb) set var width=47
  56      </pre>
  57
  58    <p>Because the <code>set</code> command has many subcommands that can conflict
  59 with the names of program variables, it is a good idea to use the
  60 <code>set variable</code> command instead of just <code>set</code>.  For example, if
  61 your program has a variable <code>g</code>, you run into problems if you try
  62 to set a new value with just <code>set g=4</code>, because GDB has
  63 the command <code>set gnutarget</code>, abbreviated <code>set g</code>:
  64
  65 <pre class="example">     (gdb) whatis g
  66      type = double
  67      (gdb) p g
  68      $1 = 1
  69      (gdb) set g=4
  70      (gdb) p g
  71      $2 = 1
  72      (gdb) r
  73      The program being debugged has been started already.
  74      Start it from the beginning? (y or n) y
  75      Starting program: /home/smith/cc_progs/a.out
  76      "/home/smith/cc_progs/a.out": can't open to read symbols:
  77                                       Invalid bfd target.
  78      (gdb) show g
  79      The current BFD target is "=4".
  80      </pre>
  81
  82 <p>The program variable <code>g</code> did not change, and you silently set the
  83 <code>gnutarget</code> to an invalid value.  In order to set the variable
  84 <code>g</code>, use
  85
  86 <pre class="example">     (gdb) set var g=4
  87      </pre>
  88
  89    GDB allows more implicit conversions in assignments than C; you can
  90 freely store an integer value into a pointer variable or vice versa,
  91 and you can convert any structure to any other structure that is the
  92 same length or shorter.
  93
  94    <p>To store values into arbitrary places in memory, use the <code>{...}</code>
  95 construct to generate a value of specified type at a specified address
  96 (see <a href="Expressions.html#Expressions">Expressions</a>).  For example, <code>{int}0x83040</code> refers
  97 to memory location <code>0x83040</code> as an integer (which implies a certain size
  98 and representation in memory), and
  99
 100 <pre class="example">     set {int}0x83040 = 4
 101      </pre>
 102
 103 <p>stores the value 4 into that memory location.
 104
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