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+<title>Debugging with GDB: Variables</title>
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+<a name="Variables"></a>
+<div class="header">
+<p>
+Next: <a href="Arrays.html#Arrays" accesskey="n" rel="next">Arrays</a>, Previous: <a href="Ambiguous-Expressions.html#Ambiguous-Expressions" accesskey="p" rel="previous">Ambiguous Expressions</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> &nbsp; [<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>
+</div>
+<hr>
+<a name="Program-Variables"></a>
+<h3 class="section">10.3 Program Variables</h3>
+
+<p>The most common kind of expression to use is the name of a variable
+in your program.
+</p>
+<p>Variables in expressions are understood in the selected stack frame
+(see <a href="Selection.html#Selection">Selecting a Frame</a>); they must be either:
+</p>
+<ul>
+<li> global (or file-static)
+</li></ul>
+
+<p>or
+</p>
+<ul>
+<li> visible according to the scope rules of the
+programming language from the point of execution in that frame
+</li></ul>
+
+<p>This means that in the function
+</p>
+<div class="smallexample">
+<pre class="smallexample">foo (a)
+ int a;
+{
+ bar (a);
+ {
+ int b = test ();
+ bar (b);
+ }
+}
+</pre></div>
+
+<p>you can examine and use the variable <code>a</code> whenever your program is
+executing within the function <code>foo</code>, but you can only use or
+examine the variable <code>b</code> while your program is executing inside
+the block where <code>b</code> is declared.
+</p>
+<a name="index-variable-name-conflict"></a>
+<p>There is an exception: you can refer to a variable or function whose
+scope is a single source file even if the current execution point is not
+in this file. But it is possible to have more than one such variable or
+function with the same name (in different source files). If that
+happens, referring to that name has unpredictable effects. If you wish,
+you can specify a static variable in a particular function or file by
+using the colon-colon (<code>::</code>) notation:
+</p>
+<a name="index-colon_002dcolon_002c-context-for-variables_002ffunctions"></a>
+<a name="index-_003a_003a_002c-context-for-variables_002ffunctions"></a>
+<div class="smallexample">
+<pre class="smallexample"><var>file</var>::<var>variable</var>
+<var>function</var>::<var>variable</var>
+</pre></div>
+
+<p>Here <var>file</var> or <var>function</var> is the name of the context for the
+static <var>variable</var>. In the case of file names, you can use quotes to
+make sure <small>GDB</small> parses the file name as a single word&mdash;for example,
+to print a global value of <code>x</code> defined in <samp>f2.c</samp>:
+</p>
+<div class="smallexample">
+<pre class="smallexample">(gdb) p 'f2.c'::x
+</pre></div>
+
+<p>The <code>::</code> notation is normally used for referring to
+static variables, since you typically disambiguate uses of local variables
+in functions by selecting the appropriate frame and using the
+simple name of the variable. However, you may also use this notation
+to refer to local variables in frames enclosing the selected frame:
+</p>
+<div class="smallexample">
+<pre class="smallexample">void
+foo (int a)
+{
+ if (a &lt; 10)
+ bar (a);
+ else
+ process (a); /* Stop here */
+}
+
+int
+bar (int a)
+{
+ foo (a + 5);
+}
+</pre></div>
+
+<p>For example, if there is a breakpoint at the commented line,
+here is what you might see
+when the program stops after executing the call <code>bar(0)</code>:
+</p>
+<div class="smallexample">
+<pre class="smallexample">(gdb) p a
+$1 = 10
+(gdb) p bar::a
+$2 = 5
+(gdb) up 2
+#2 0x080483d0 in foo (a=5) at foobar.c:12
+(gdb) p a
+$3 = 5
+(gdb) p bar::a
+$4 = 0
+</pre></div>
+
+<a name="index-C_002b_002b-scope-resolution"></a>
+<p>These uses of &lsquo;<samp>::</samp>&rsquo; are very rarely in conflict with the very
+similar use of the same notation in C<tt>++</tt>. When they are in
+conflict, the C<tt>++</tt> meaning takes precedence; however, this can be
+overridden by quoting the file or function name with single quotes.
+</p>
+<p>For example, suppose the program is stopped in a method of a class
+that has a field named <code>includefile</code>, and there is also an
+include file named <samp>includefile</samp> that defines a variable,
+<code>some_global</code>.
+</p>
+<div class="smallexample">
+<pre class="smallexample">(gdb) p includefile
+$1 = 23
+(gdb) p includefile::some_global
+A syntax error in expression, near `'.
+(gdb) p 'includefile'::some_global
+$2 = 27
+</pre></div>
+
+<a name="index-wrong-values"></a>
+<a name="index-variable-values_002c-wrong"></a>
+<a name="index-function-entry_002fexit_002c-wrong-values-of-variables"></a>
+<a name="index-optimized-code_002c-wrong-values-of-variables"></a>
+<blockquote>
+<p><em>Warning:</em> Occasionally, a local variable may appear to have the
+wrong value at certain points in a function&mdash;just after entry to a new
+scope, and just before exit.
+</p></blockquote>
+<p>You may see this problem when you are stepping by machine instructions.
+This is because, on most machines, it takes more than one instruction to
+set up a stack frame (including local variable definitions); if you are
+stepping by machine instructions, variables may appear to have the wrong
+values until the stack frame is completely built. On exit, it usually
+also takes more than one machine instruction to destroy a stack frame;
+after you begin stepping through that group of instructions, local
+variable definitions may be gone.
+</p>
+<p>This may also happen when the compiler does significant optimizations.
+To be sure of always seeing accurate values, turn off all optimization
+when compiling.
+</p>
+<a name="index-_0060_0060No-symbol-_0022foo_0022-in-current-context_0027_0027"></a>
+<p>Another possible effect of compiler optimizations is to optimize
+unused variables out of existence, or assign variables to registers (as
+opposed to memory addresses). Depending on the support for such cases
+offered by the debug info format used by the compiler, <small>GDB</small>
+might not be able to display values for such local variables. If that
+happens, <small>GDB</small> will print a message like this:
+</p>
+<div class="smallexample">
+<pre class="smallexample">No symbol &quot;foo&quot; in current context.
+</pre></div>
+
+<p>To solve such problems, either recompile without optimizations, or use a
+different debug info format, if the compiler supports several such
+formats. See <a href="Compilation.html#Compilation">Compilation</a>, for more information on choosing compiler
+options. See <a href="C.html#C">C and C<tt>++</tt></a>, for more information about debug
+info formats that are best suited to C<tt>++</tt> programs.
+</p>
+<p>If you ask to print an object whose contents are unknown to
+<small>GDB</small>, e.g., because its data type is not completely specified
+by the debug information, <small>GDB</small> will say &lsquo;<samp>&lt;incomplete
+type&gt;</samp>&rsquo;. See <a href="Symbols.html#Symbols">incomplete type</a>, for more about this.
+</p>
+<a name="index-no-debug-info-variables"></a>
+<p>If you try to examine or use the value of a (global) variable for
+which <small>GDB</small> has no type information, e.g., because the program
+includes no debug information, <small>GDB</small> displays an error message.
+See <a href="Symbols.html#Symbols">unknown type</a>, for more about unknown types. If you
+cast the variable to its declared type, <small>GDB</small> gets the
+variable&rsquo;s value using the cast-to type as the variable&rsquo;s type. For
+example, in a C program:
+</p>
+<div class="smallexample">
+<pre class="smallexample"> (gdb) p var
+ 'var' has unknown type; cast it to its declared type
+ (gdb) p (float) var
+ $1 = 3.14
+</pre></div>
+
+<p>If you append <kbd>@entry</kbd> string to a function parameter name you get its
+value at the time the function got called. If the value is not available an
+error message is printed. Entry values are available only with some compilers.
+Entry values are normally also printed at the function parameter list according
+to <a href="Print-Settings.html#set-print-entry_002dvalues">set print entry-values</a>.
+</p>
+<div class="smallexample">
+<pre class="smallexample">Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
+29 i++;
+(gdb) next
+30 e (i);
+(gdb) print i
+$1 = 31
+(gdb) print i@entry
+$2 = 30
+</pre></div>
+
+<p>Strings are identified as arrays of <code>char</code> values without specified
+signedness. Arrays of either <code>signed char</code> or <code>unsigned char</code> get
+printed as arrays of 1 byte sized integers. <code>-fsigned-char</code> or
+<code>-funsigned-char</code> <small>GCC</small> options have no effect as <small>GDB</small>
+defines literal string type <code>&quot;char&quot;</code> as <code>char</code> without a sign.
+For program code
+</p>
+<div class="smallexample">
+<pre class="smallexample">char var0[] = &quot;A&quot;;
+signed char var1[] = &quot;A&quot;;
+</pre></div>
+
+<p>You get during debugging
+</p><div class="smallexample">
+<pre class="smallexample">(gdb) print var0
+$1 = &quot;A&quot;
+(gdb) print var1
+$2 = {65 'A', 0 '\0'}
+</pre></div>
+
+<hr>
+<div class="header">
+<p>
+Next: <a href="Arrays.html#Arrays" accesskey="n" rel="next">Arrays</a>, Previous: <a href="Ambiguous-Expressions.html#Ambiguous-Expressions" accesskey="p" rel="previous">Ambiguous Expressions</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> &nbsp; [<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>
+</div>
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