132 lines
6.1 KiB
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132 lines
6.1 KiB
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<head>
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<title>Assignment - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Altering.html#Altering" title="Altering">
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Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Free Software'' and ``Free Software Needs
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: ``You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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<body>
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<div class="node">
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<a name="Assignment"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Jumping.html#Jumping">Jumping</a>,
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Up: <a rel="up" accesskey="u" href="Altering.html#Altering">Altering</a>
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<hr>
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</div>
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<h3 class="section">17.1 Assignment to Variables</h3>
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<p><a name="index-assignment-1164"></a><a name="index-setting-variables-1165"></a>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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<pre class="smallexample"> print x=4
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</pre>
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<p class="noindent">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 <span class="sc">gdb</span> with Different Languages</a>, for more
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information on operators in supported languages.
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<p><a name="index-set-variable-1166"></a><a name="index-variables_002c-setting-1167"></a>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>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><span class="samp">set width=13</span></samp>’, because <span class="sc">gdb</span> has the
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command <code>set width</code>:
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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>
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<p class="noindent">The invalid expression, of course, is ‘<samp><span class="samp">=47</span></samp>’. In
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order to actually set the program's variable <code>width</code>, use
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<pre class="smallexample"> (gdb) set var width=47
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</pre>
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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><span class="samp">set g=4</span></samp>’, because <span class="sc">gdb</span> has
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the command <code>set gnutarget</code>, abbreviated <code>set g</code>:
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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>
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<p class="noindent">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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<pre class="smallexample"> (gdb) set var g=4
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</pre>
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<p><span class="sc">gdb</span> 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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<!-- FIXME: how do structs align/pad in these conversions? -->
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<!-- /doc@cygnus.com 18dec1990 -->
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<p>To store values into arbitrary places in memory, use the ‘<samp><span class="samp">{...}</span></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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<pre class="smallexample"> set {int}0x83040 = 4
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</pre>
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<p class="noindent">stores the value 4 into that memory location.
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</body></html>
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