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| <h3 class="section">17.1 Assignment to Variables</h3> |
| |
| <p><a name="index-assignment-924"></a><a name="index-setting-variables-925"></a>To alter the value of a variable, evaluate an assignment expression. |
| See <a href="Expressions.html#Expressions">Expressions</a>. For example, |
| |
| <pre class="smallexample"> print x=4 |
| </pre> |
| <p class="noindent">stores the value 4 into the variable <code>x</code>, and then prints the |
| value of the assignment expression (which is 4). |
| See <a href="Languages.html#Languages">Using <span class="sc">gdb</span> with Different Languages</a>, for more |
| information on operators in supported languages. |
| |
| <p><a name="index-set-variable-926"></a><a name="index-variables_002c-setting-927"></a>If you are not interested in seeing the value of the assignment, use the |
| <code>set</code> command instead of the <code>print</code> command. <code>set</code> is |
| really the same as <code>print</code> except that the expression's value is |
| not printed and is not put in the value history (see <a href="Value-History.html#Value-History">Value History</a>). The expression is evaluated only for its effects. |
| |
| <p>If the beginning of the argument string of the <code>set</code> command |
| appears identical to a <code>set</code> subcommand, use the <code>set |
| variable</code> command instead of just <code>set</code>. This command is identical |
| to <code>set</code> except for its lack of subcommands. For example, if your |
| program has a variable <code>width</code>, you get an error if you try to set |
| a new value with just ‘<samp><span class="samp">set width=13</span></samp>’, because <span class="sc">gdb</span> has the |
| command <code>set width</code>: |
| |
| <pre class="smallexample"> (gdb) whatis width |
| type = double |
| (gdb) p width |
| $4 = 13 |
| (gdb) set width=47 |
| Invalid syntax in expression. |
| </pre> |
| <p class="noindent">The invalid expression, of course, is ‘<samp><span class="samp">=47</span></samp>’. In |
| order to actually set the program's variable <code>width</code>, use |
| |
| <pre class="smallexample"> (gdb) set var width=47 |
| </pre> |
| <p>Because the <code>set</code> command has many subcommands that can conflict |
| with the names of program variables, it is a good idea to use the |
| <code>set variable</code> command instead of just <code>set</code>. For example, if |
| your program has a variable <code>g</code>, you run into problems if you try |
| to set a new value with just ‘<samp><span class="samp">set g=4</span></samp>’, because <span class="sc">gdb</span> has |
| the command <code>set gnutarget</code>, abbreviated <code>set g</code>: |
| |
| <pre class="smallexample"> (gdb) whatis g |
| type = double |
| (gdb) p g |
| $1 = 1 |
| (gdb) set g=4 |
| (gdb) p g |
| $2 = 1 |
| (gdb) r |
| The program being debugged has been started already. |
| Start it from the beginning? (y or n) y |
| Starting program: /home/smith/cc_progs/a.out |
| "/home/smith/cc_progs/a.out": can't open to read symbols: |
| Invalid bfd target. |
| (gdb) show g |
| The current BFD target is "=4". |
| </pre> |
| <p class="noindent">The program variable <code>g</code> did not change, and you silently set the |
| <code>gnutarget</code> to an invalid value. In order to set the variable |
| <code>g</code>, use |
| |
| <pre class="smallexample"> (gdb) set var g=4 |
| </pre> |
| <p><span class="sc">gdb</span> allows more implicit conversions in assignments than C; you can |
| freely store an integer value into a pointer variable or vice versa, |
| and you can convert any structure to any other structure that is the |
| same length or shorter. |
| <!-- FIXME: how do structs align/pad in these conversions? --> |
| <!-- /doc@cygnus.com 18dec1990 --> |
| |
| <p>To store values into arbitrary places in memory, use the ‘<samp><span class="samp">{...}</span></samp>’ |
| construct to generate a value of specified type at a specified address |
| (see <a href="Expressions.html#Expressions">Expressions</a>). For example, <code>{int}0x83040</code> refers |
| to memory location <code>0x83040</code> as an integer (which implies a certain size |
| and representation in memory), and |
| |
| <pre class="smallexample"> set {int}0x83040 = 4 |
| </pre> |
| <p class="noindent">stores the value 4 into that memory location. |
| |
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