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177 lines
7.4 KiB
HTML
177 lines
7.4 KiB
HTML
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<head>
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<title>Assignment (Debugging with GDB)</title>
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Altering.html#Altering" rel="up" title="Altering">
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</head>
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<body lang="en">
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<a name="Assignment"></a>
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<div class="header">
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<p>
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Next: <a href="Jumping.html#Jumping" accesskey="n" rel="next">Jumping</a>, Up: <a href="Altering.html#Altering" accesskey="u" rel="up">Altering</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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</div>
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<hr>
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<a name="Assignment-to-Variables"></a>
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<h3 class="section">17.1 Assignment to Variables</h3>
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<a name="index-assignment"></a>
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<a name="index-setting-variables"></a>
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<p>To alter the value of a variable, evaluate an assignment expression.
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See <a href="Expressions.html#Expressions">Expressions</a>. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">print x=4
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</pre></div>
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<p>stores the value 4 into the variable <code>x</code>, and then prints the
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value of the assignment expression (which is 4).
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See <a href="Languages.html#Languages">Using <small>GDB</small> with Different Languages</a>, for more
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information on operators in supported languages.
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</p>
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<a name="index-set-variable"></a>
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<a name="index-variables_002c-setting"></a>
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<p>If you are not interested in seeing the value of the assignment, use the
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<code>set</code> command instead of the <code>print</code> command. <code>set</code> is
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really the same as <code>print</code> except that the expression’s value is
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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.
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</p>
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<p>If the beginning of the argument string of the <code>set</code> command
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appears identical to a <code>set</code> subcommand, use the <code>set
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variable</code> command instead of just <code>set</code>. This command is identical
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to <code>set</code> except for its lack of subcommands. For example, if your
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program has a variable <code>width</code>, you get an error if you try to set
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a new value with just ‘<samp>set width=13</samp>’, because <small>GDB</small> has the
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command <code>set width</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) whatis width
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type = double
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(gdb) p width
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$4 = 13
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(gdb) set width=47
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Invalid syntax in expression.
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</pre></div>
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<p>The invalid expression, of course, is ‘<samp>=47</samp>’. In
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order to actually set the program’s variable <code>width</code>, use
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) set var width=47
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</pre></div>
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<p>Because the <code>set</code> command has many subcommands that can conflict
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with the names of program variables, it is a good idea to use the
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<code>set variable</code> command instead of just <code>set</code>. For example, if
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your program has a variable <code>g</code>, you run into problems if you try
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to set a new value with just ‘<samp>set g=4</samp>’, because <small>GDB</small> has
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the command <code>set gnutarget</code>, abbreviated <code>set g</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) whatis g
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type = double
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(gdb) p g
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$1 = 1
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(gdb) set g=4
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(gdb) p g
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$2 = 1
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(gdb) r
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The program being debugged has been started already.
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Start it from the beginning? (y or n) y
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Starting program: /home/smith/cc_progs/a.out
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"/home/smith/cc_progs/a.out": can't open to read symbols:
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Invalid bfd target.
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(gdb) show g
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The current BFD target is "=4".
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</pre></div>
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<p>The program variable <code>g</code> did not change, and you silently set the
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<code>gnutarget</code> to an invalid value. In order to set the variable
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<code>g</code>, use
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) set var g=4
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</pre></div>
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<p><small>GDB</small> allows more implicit conversions in assignments than C; you can
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freely store an integer value into a pointer variable or vice versa,
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and you can convert any structure to any other structure that is the
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same length or shorter.
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</p>
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<p>To store values into arbitrary places in memory, use the ‘<samp>{…}</samp>’
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construct to generate a value of specified type at a specified address
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(see <a href="Expressions.html#Expressions">Expressions</a>). For example, <code>{int}0x83040</code> refers
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to memory location <code>0x83040</code> as an integer (which implies a certain size
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and representation in memory), and
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</p>
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<div class="smallexample">
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<pre class="smallexample">set {int}0x83040 = 4
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</pre></div>
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<p>stores the value 4 into that memory location.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Jumping.html#Jumping" accesskey="n" rel="next">Jumping</a>, Up: <a href="Altering.html#Altering" accesskey="u" rel="up">Altering</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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</div>
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</body>
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</html>
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