aarch64-linux-gnu-9.2/share/doc/gdb/Assignment.html - toolchains/linux-x86/gcc - Git at Google

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 <h3 class="section">17.1 Assignment to Variables</h3>

 <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.
 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-1166"></a><a name="index-variables_002c-setting-1167"></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 &lsquo;<samp><span class="samp">set width=13</span></samp>&rsquo;, 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 &lsquo;<samp><span class="samp">=47</span></samp>&rsquo;.  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 &lsquo;<samp><span class="samp">set g=4</span></samp>&rsquo;, 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 &lsquo;<samp><span class="samp">{...}</span></samp>&rsquo;
 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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	<h3 class="section">17.1 Assignment to Variables</h3>

	<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.
	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-1166"></a><a name="index-variables_002c-setting-1167"></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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