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| <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>Variables in expressions are understood in the selected stack frame |
| (see <a href="Selection.html#Selection">Selecting a Frame</a>); they must be either: |
| |
| <ul> |
| <li>global (or file-static) |
| </ul> |
| |
| <p class="noindent">or |
| |
| <ul> |
| <li>visible according to the scope rules of the |
| programming language from the point of execution in that frame |
| </ul> |
| |
| <p class="noindent">This means that in the function |
| |
| <pre class="smallexample"> foo (a) |
| int a; |
| { |
| bar (a); |
| { |
| int b = test (); |
| bar (b); |
| } |
| } |
| </pre> |
| <p class="noindent">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-505"></a>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, |
| using the colon-colon (<code>::</code>) notation: |
| |
| <p><a name="index-colon_002dcolon_002c-context-for-variables_002ffunctions-506"></a><!-- info cannot cope with a :: index entry, but why deprive hard copy readers? --> |
| <a name="index-g_t_0040code_007b_003a_003a_007d_002c-context-for-variables_002ffunctions-507"></a> |
| <pre class="smallexample"> <var>file</var>::<var>variable</var> |
| <var>function</var>::<var>variable</var> |
| </pre> |
| <p class="noindent">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 <span class="sc">gdb</span> parses the file name as a single word—for example, |
| to print a global value of <code>x</code> defined in <samp><span class="file">f2.c</span></samp>: |
| |
| <pre class="smallexample"> (gdb) p 'f2.c'::x |
| </pre> |
| <p><a name="index-C_0040t_007b_002b_002b_007d-scope-resolution-508"></a>This use of ‘<samp><span class="samp">::</span></samp>’ is very rarely in conflict with the very similar |
| use of the same notation in C<tt>++</tt>. <span class="sc">gdb</span> also supports use of the C<tt>++</tt> |
| scope resolution operator in <span class="sc">gdb</span> expressions. |
| <!-- FIXME: Um, so what happens in one of those rare cases where it's in --> |
| <!-- conflict?? -mew --> |
| |
| <p><a name="index-wrong-values-509"></a><a name="index-variable-values_002c-wrong-510"></a><a name="index-function-entry_002fexit_002c-wrong-values-of-variables-511"></a><a name="index-optimized-code_002c-wrong-values-of-variables-512"></a><blockquote> |
| <em>Warning:</em> Occasionally, a local variable may appear to have the |
| wrong value at certain points in a function—just after entry to a new |
| scope, and just before exit. |
| </blockquote> |
| 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>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-g_t_0060_0060No-symbol-_0022foo_0022-in-current-context_0027_0027-513"></a>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, <span class="sc">gdb</span> |
| might not be able to display values for such local variables. If that |
| happens, <span class="sc">gdb</span> will print a message like this: |
| |
| <pre class="smallexample"> No symbol "foo" in current context. |
| </pre> |
| <p>To solve such problems, either recompile without optimizations, or use a |
| different debug info format, if the compiler supports several such |
| formats. For example, <span class="sc">gcc</span>, the <span class="sc">gnu</span> C/C<tt>++</tt> compiler, |
| usually supports the <samp><span class="option">-gstabs+</span></samp> option. <samp><span class="option">-gstabs+</span></samp> |
| produces debug info in a format that is superior to formats such as |
| COFF. You may be able to use DWARF 2 (<samp><span class="option">-gdwarf-2</span></samp>), which is also |
| an effective form for debug info. See <a href="../gcc/Debugging-Options.html#Debugging-Options">Options for Debugging Your Program or GCC</a>. |
| 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>If you ask to print an object whose contents are unknown to |
| <span class="sc">gdb</span>, e.g., because its data type is not completely specified |
| by the debug information, <span class="sc">gdb</span> will say ‘<samp><span class="samp"><incomplete |
| type></span></samp>’. See <a href="Symbols.html#Symbols">incomplete type</a>, for more about this. |
| |
| <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> <span class="sc">gcc</span> options have no effect as <span class="sc">gdb</span> |
| defines literal string type <code>"char"</code> as <code>char</code> without a sign. |
| For program code |
| |
| <pre class="smallexample"> char var0[] = "A"; |
| signed char var1[] = "A"; |
| </pre> |
| <p>You get during debugging |
| <pre class="smallexample"> (gdb) print var0 |
| $1 = "A" |
| (gdb) print var1 |
| $2 = {65 'A', 0 '\0'} |
| </pre> |
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