242 lines
11 KiB
HTML
242 lines
11 KiB
HTML
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<html lang="en">
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<head>
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<title>Variables - Debugging with GDB</title>
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<link rel="up" href="Data.html#Data" title="Data">
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<link rel="prev" href="Ambiguous-Expressions.html#Ambiguous-Expressions" title="Ambiguous Expressions">
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under the terms of the GNU Free Documentation License, Version 1.3 or
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</head>
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<body>
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<div class="node">
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<a name="Variables"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Arrays.html#Arrays">Arrays</a>,
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Previous: <a rel="previous" accesskey="p" href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous Expressions</a>,
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Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a>
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<hr>
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</div>
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<h3 class="section">10.3 Program Variables</h3>
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<p>The most common kind of expression to use is the name of a variable
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in your program.
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<p>Variables in expressions are understood in the selected stack frame
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(see <a href="Selection.html#Selection">Selecting a Frame</a>); they must be either:
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<ul>
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<li>global (or file-static)
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</ul>
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<p class="noindent">or
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<ul>
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<li>visible according to the scope rules of the
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programming language from the point of execution in that frame
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</ul>
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<p class="noindent">This means that in the function
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<pre class="smallexample"> foo (a)
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int a;
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{
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bar (a);
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{
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int b = test ();
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bar (b);
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}
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}
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</pre>
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<p class="noindent">you can examine and use the variable <code>a</code> whenever your program is
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executing within the function <code>foo</code>, but you can only use or
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examine the variable <code>b</code> while your program is executing inside
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the block where <code>b</code> is declared.
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<p><a name="index-variable-name-conflict-636"></a>There is an exception: you can refer to a variable or function whose
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scope is a single source file even if the current execution point is not
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in this file. But it is possible to have more than one such variable or
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function with the same name (in different source files). If that
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happens, referring to that name has unpredictable effects. If you wish,
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you can specify a static variable in a particular function or file by
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using the colon-colon (<code>::</code>) notation:
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<p><a name="index-colon_002dcolon_002c-context-for-variables_002ffunctions-637"></a><!-- info cannot cope with a :: index entry, but why deprive hard copy readers? -->
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<a name="index-g_t_0040code_007b_003a_003a_007d_002c-context-for-variables_002ffunctions-638"></a>
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<pre class="smallexample"> <var>file</var>::<var>variable</var>
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<var>function</var>::<var>variable</var>
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</pre>
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<p class="noindent">Here <var>file</var> or <var>function</var> is the name of the context for the
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static <var>variable</var>. In the case of file names, you can use quotes to
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make sure <span class="sc">gdb</span> parses the file name as a single word—for example,
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to print a global value of <code>x</code> defined in <samp><span class="file">f2.c</span></samp>:
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<pre class="smallexample"> (gdb) p 'f2.c'::x
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</pre>
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<p>The <code>::</code> notation is normally used for referring to
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static variables, since you typically disambiguate uses of local variables
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in functions by selecting the appropriate frame and using the
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simple name of the variable. However, you may also use this notation
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to refer to local variables in frames enclosing the selected frame:
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<pre class="smallexample"> void
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foo (int a)
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{
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if (a < 10)
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bar (a);
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else
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process (a); /* Stop here */
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}
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int
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bar (int a)
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{
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foo (a + 5);
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}
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</pre>
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<p class="noindent">For example, if there is a breakpoint at the commented line,
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here is what you might see
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when the program stops after executing the call <code>bar(0)</code>:
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<pre class="smallexample"> (gdb) p a
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$1 = 10
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(gdb) p bar::a
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$2 = 5
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(gdb) up 2
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#2 0x080483d0 in foo (a=5) at foobar.c:12
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(gdb) p a
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$3 = 5
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(gdb) p bar::a
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$4 = 0
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</pre>
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<p><a name="index-C_0040t_007b_002b_002b_007d-scope-resolution-639"></a>These uses of ‘<samp><span class="samp">::</span></samp>’ are very rarely in conflict with the very
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similar use of the same notation in C<tt>++</tt>. When they are in
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conflict, the C<tt>++</tt> meaning takes precedence; however, this can be
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overridden by quoting the file or function name with single quotes.
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<p>For example, suppose the program is stopped in a method of a class
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that has a field named <code>includefile</code>, and there is also an
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include file named <samp><span class="file">includefile</span></samp> that defines a variable,
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<code>some_global</code>.
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<pre class="smallexample"> (gdb) p includefile
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$1 = 23
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(gdb) p includefile::some_global
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A syntax error in expression, near `'.
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(gdb) p 'includefile'::some_global
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$2 = 27
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</pre>
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<p><a name="index-wrong-values-640"></a><a name="index-variable-values_002c-wrong-641"></a><a name="index-function-entry_002fexit_002c-wrong-values-of-variables-642"></a><a name="index-optimized-code_002c-wrong-values-of-variables-643"></a><blockquote>
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<em>Warning:</em> Occasionally, a local variable may appear to have the
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wrong value at certain points in a function—just after entry to a new
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scope, and just before exit.
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</blockquote>
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You may see this problem when you are stepping by machine instructions.
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This is because, on most machines, it takes more than one instruction to
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set up a stack frame (including local variable definitions); if you are
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stepping by machine instructions, variables may appear to have the wrong
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values until the stack frame is completely built. On exit, it usually
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also takes more than one machine instruction to destroy a stack frame;
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after you begin stepping through that group of instructions, local
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variable definitions may be gone.
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<p>This may also happen when the compiler does significant optimizations.
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To be sure of always seeing accurate values, turn off all optimization
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when compiling.
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<p><a name="index-g_t_0060_0060No-symbol-_0022foo_0022-in-current-context_0027_0027-644"></a>Another possible effect of compiler optimizations is to optimize
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unused variables out of existence, or assign variables to registers (as
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opposed to memory addresses). Depending on the support for such cases
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offered by the debug info format used by the compiler, <span class="sc">gdb</span>
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might not be able to display values for such local variables. If that
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happens, <span class="sc">gdb</span> will print a message like this:
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<pre class="smallexample"> No symbol "foo" in current context.
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</pre>
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<p>To solve such problems, either recompile without optimizations, or use a
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different debug info format, if the compiler supports several such
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formats. See <a href="Compilation.html#Compilation">Compilation</a>, for more information on choosing compiler
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options. See <a href="C.html#C">C and C<tt>++</tt></a>, for more information about debug
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info formats that are best suited to C<tt>++</tt> programs.
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<p>If you ask to print an object whose contents are unknown to
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<span class="sc">gdb</span>, e.g., because its data type is not completely specified
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by the debug information, <span class="sc">gdb</span> will say ‘<samp><span class="samp"><incomplete
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type></span></samp>’. See <a href="Symbols.html#Symbols">incomplete type</a>, for more about this.
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<p><a name="index-no-debug-info-variables-645"></a>If you try to examine or use the value of a (global) variable for
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which <span class="sc">gdb</span> has no type information, e.g., because the program
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includes no debug information, <span class="sc">gdb</span> displays an error message.
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See <a href="Symbols.html#Symbols">unknown type</a>, for more about unknown types. If you
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cast the variable to its declared type, <span class="sc">gdb</span> gets the
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variable's value using the cast-to type as the variable's type. For
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example, in a C program:
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<pre class="smallexample"> (gdb) p var
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'var' has unknown type; cast it to its declared type
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(gdb) p (float) var
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$1 = 3.14
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</pre>
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<p>If you append <kbd>@entry</kbd> string to a function parameter name you get its
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value at the time the function got called. If the value is not available an
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error message is printed. Entry values are available only with some compilers.
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Entry values are normally also printed at the function parameter list according
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to <a href="set-print-entry_002dvalues.html#set-print-entry_002dvalues">set print entry-values</a>.
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<pre class="smallexample"> Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
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29 i++;
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(gdb) next
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30 e (i);
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(gdb) print i
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$1 = 31
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(gdb) print i@entry
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$2 = 30
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</pre>
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<p>Strings are identified as arrays of <code>char</code> values without specified
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signedness. Arrays of either <code>signed char</code> or <code>unsigned char</code> get
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printed as arrays of 1 byte sized integers. <code>-fsigned-char</code> or
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<code>-funsigned-char</code> <span class="sc">gcc</span> options have no effect as <span class="sc">gdb</span>
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defines literal string type <code>"char"</code> as <code>char</code> without a sign.
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For program code
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<pre class="smallexample"> char var0[] = "A";
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signed char var1[] = "A";
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</pre>
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<p>You get during debugging
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<pre class="smallexample"> (gdb) print var0
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$1 = "A"
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(gdb) print var1
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$2 = {65 'A', 0 '\0'}
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</pre>
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</body></html>
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