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| <h2 class="chapter">10 Examining Data</h2> |
| |
| <p><a name="index-printing-data-616"></a><a name="index-examining-data-617"></a><a name="index-print-618"></a><a name="index-inspect-619"></a>The usual way to examine data in your program is with the <code>print</code> |
| command (abbreviated <code>p</code>), or its synonym <code>inspect</code>. It |
| evaluates and prints the value of an expression of the language your |
| program is written in (see <a href="Languages.html#Languages">Using <span class="sc">gdb</span> with Different Languages</a>). It may also print the expression using a |
| Python-based pretty-printer (see <a href="Pretty-Printing.html#Pretty-Printing">Pretty Printing</a>). |
| |
| <dl> |
| <dt><code>print </code><var>expr</var><dt><code>print /</code><var>f</var> <var>expr</var><dd><var>expr</var> is an expression (in the source language). By default the |
| value of <var>expr</var> is printed in a format appropriate to its data type; |
| you can choose a different format by specifying ‘<samp><span class="samp">/</span><var>f</var></samp>’, where |
| <var>f</var> is a letter specifying the format; see <a href="Output-Formats.html#Output-Formats">Output Formats</a>. |
| |
| <br><dt><code>print</code><dt><code>print /</code><var>f</var><dd><a name="index-reprint-the-last-value-620"></a>If you omit <var>expr</var>, <span class="sc">gdb</span> displays the last value again (from the |
| <dfn>value history</dfn>; see <a href="Value-History.html#Value-History">Value History</a>). This allows you to |
| conveniently inspect the same value in an alternative format. |
| </dl> |
| |
| <p>A more low-level way of examining data is with the <code>x</code> command. |
| It examines data in memory at a specified address and prints it in a |
| specified format. See <a href="Memory.html#Memory">Examining Memory</a>. |
| |
| <p>If you are interested in information about types, or about how the |
| fields of a struct or a class are declared, use the <code>ptype </code><var>exp</var> |
| command rather than <code>print</code>. See <a href="Symbols.html#Symbols">Examining the Symbol Table</a>. |
| |
| <p><a name="index-exploring-hierarchical-data-structures-621"></a><a name="index-explore-622"></a>Another way of examining values of expressions and type information is |
| through the Python extension command <code>explore</code> (available only if |
| the <span class="sc">gdb</span> build is configured with <code>--with-python</code>). It |
| offers an interactive way to start at the highest level (or, the most |
| abstract level) of the data type of an expression (or, the data type |
| itself) and explore all the way down to leaf scalar values/fields |
| embedded in the higher level data types. |
| |
| <dl> |
| <dt><code>explore </code><var>arg</var><dd><var>arg</var> is either an expression (in the source language), or a type |
| visible in the current context of the program being debugged. |
| </dl> |
| |
| <p>The working of the <code>explore</code> command can be illustrated with an |
| example. If a data type <code>struct ComplexStruct</code> is defined in your |
| C program as |
| |
| <pre class="smallexample"> struct SimpleStruct |
| { |
| int i; |
| double d; |
| }; |
| |
| struct ComplexStruct |
| { |
| struct SimpleStruct *ss_p; |
| int arr[10]; |
| }; |
| </pre> |
| <p class="noindent">followed by variable declarations as |
| |
| <pre class="smallexample"> struct SimpleStruct ss = { 10, 1.11 }; |
| struct ComplexStruct cs = { &ss, { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } }; |
| </pre> |
| <p class="noindent">then, the value of the variable <code>cs</code> can be explored using the |
| <code>explore</code> command as follows. |
| |
| <pre class="smallexample"> (gdb) explore cs |
| The value of `cs' is a struct/class of type `struct ComplexStruct' with |
| the following fields: |
| |
| ss_p = <Enter 0 to explore this field of type `struct SimpleStruct *'> |
| arr = <Enter 1 to explore this field of type `int [10]'> |
| |
| Enter the field number of choice: |
| </pre> |
| <p class="noindent">Since the fields of <code>cs</code> are not scalar values, you are being |
| prompted to chose the field you want to explore. Let's say you choose |
| the field <code>ss_p</code> by entering <code>0</code>. Then, since this field is a |
| pointer, you will be asked if it is pointing to a single value. From |
| the declaration of <code>cs</code> above, it is indeed pointing to a single |
| value, hence you enter <code>y</code>. If you enter <code>n</code>, then you will |
| be asked if it were pointing to an array of values, in which case this |
| field will be explored as if it were an array. |
| |
| <pre class="smallexample"> `cs.ss_p' is a pointer to a value of type `struct SimpleStruct' |
| Continue exploring it as a pointer to a single value [y/n]: y |
| The value of `*(cs.ss_p)' is a struct/class of type `struct |
| SimpleStruct' with the following fields: |
| |
| i = 10 .. (Value of type `int') |
| d = 1.1100000000000001 .. (Value of type `double') |
| |
| Press enter to return to parent value: |
| </pre> |
| <p class="noindent">If the field <code>arr</code> of <code>cs</code> was chosen for exploration by |
| entering <code>1</code> earlier, then since it is as array, you will be |
| prompted to enter the index of the element in the array that you want |
| to explore. |
| |
| <pre class="smallexample"> `cs.arr' is an array of `int'. |
| Enter the index of the element you want to explore in `cs.arr': 5 |
| |
| `(cs.arr)[5]' is a scalar value of type `int'. |
| |
| (cs.arr)[5] = 4 |
| |
| Press enter to return to parent value: |
| </pre> |
| <p>In general, at any stage of exploration, you can go deeper towards the |
| leaf values by responding to the prompts appropriately, or hit the |
| return key to return to the enclosing data structure (the <i>higher</i> |
| level data structure). |
| |
| <p>Similar to exploring values, you can use the <code>explore</code> command to |
| explore types. Instead of specifying a value (which is typically a |
| variable name or an expression valid in the current context of the |
| program being debugged), you specify a type name. If you consider the |
| same example as above, your can explore the type |
| <code>struct ComplexStruct</code> by passing the argument |
| <code>struct ComplexStruct</code> to the <code>explore</code> command. |
| |
| <pre class="smallexample"> (gdb) explore struct ComplexStruct |
| </pre> |
| <p class="noindent">By responding to the prompts appropriately in the subsequent interactive |
| session, you can explore the type <code>struct ComplexStruct</code> in a |
| manner similar to how the value <code>cs</code> was explored in the above |
| example. |
| |
| <p>The <code>explore</code> command also has two sub-commands, |
| <code>explore value</code> and <code>explore type</code>. The former sub-command is |
| a way to explicitly specify that value exploration of the argument is |
| being invoked, while the latter is a way to explicitly specify that type |
| exploration of the argument is being invoked. |
| |
| <dl> |
| <dt><code>explore value </code><var>expr</var><dd><a name="index-explore-value-623"></a>This sub-command of <code>explore</code> explores the value of the |
| expression <var>expr</var> (if <var>expr</var> is an expression valid in the |
| current context of the program being debugged). The behavior of this |
| command is identical to that of the behavior of the <code>explore</code> |
| command being passed the argument <var>expr</var>. |
| |
| <br><dt><code>explore type </code><var>arg</var><dd><a name="index-explore-type-624"></a>This sub-command of <code>explore</code> explores the type of <var>arg</var> (if |
| <var>arg</var> is a type visible in the current context of program being |
| debugged), or the type of the value/expression <var>arg</var> (if <var>arg</var> |
| is an expression valid in the current context of the program being |
| debugged). If <var>arg</var> is a type, then the behavior of this command is |
| identical to that of the <code>explore</code> command being passed the |
| argument <var>arg</var>. If <var>arg</var> is an expression, then the behavior of |
| this command will be identical to that of the <code>explore</code> command |
| being passed the type of <var>arg</var> as the argument. |
| </dl> |
| |
| <ul class="menu"> |
| <li><a accesskey="1" href="Expressions.html#Expressions">Expressions</a>: Expressions |
| <li><a accesskey="2" href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous Expressions</a>: Ambiguous Expressions |
| <li><a accesskey="3" href="Variables.html#Variables">Variables</a>: Program variables |
| <li><a accesskey="4" href="Arrays.html#Arrays">Arrays</a>: Artificial arrays |
| <li><a accesskey="5" href="Output-Formats.html#Output-Formats">Output Formats</a>: Output formats |
| <li><a accesskey="6" href="Memory.html#Memory">Memory</a>: Examining memory |
| <li><a accesskey="7" href="Auto-Display.html#Auto-Display">Auto Display</a>: Automatic display |
| <li><a accesskey="8" href="Print-Settings.html#Print-Settings">Print Settings</a>: Print settings |
| <li><a accesskey="9" href="Pretty-Printing.html#Pretty-Printing">Pretty Printing</a>: Python pretty printing |
| <li><a href="Value-History.html#Value-History">Value History</a>: Value history |
| <li><a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>: Convenience variables |
| <li><a href="Convenience-Funs.html#Convenience-Funs">Convenience Funs</a>: Convenience functions |
| <li><a href="Registers.html#Registers">Registers</a>: Registers |
| <li><a href="Floating-Point-Hardware.html#Floating-Point-Hardware">Floating Point Hardware</a>: Floating point hardware |
| <li><a href="Vector-Unit.html#Vector-Unit">Vector Unit</a>: Vector Unit |
| <li><a href="OS-Information.html#OS-Information">OS Information</a>: Auxiliary data provided by operating system |
| <li><a href="Memory-Region-Attributes.html#Memory-Region-Attributes">Memory Region Attributes</a>: Memory region attributes |
| <li><a href="Dump_002fRestore-Files.html#Dump_002fRestore-Files">Dump/Restore Files</a>: Copy between memory and a file |
| <li><a href="Core-File-Generation.html#Core-File-Generation">Core File Generation</a>: Cause a program dump its core |
| <li><a href="Character-Sets.html#Character-Sets">Character Sets</a>: Debugging programs that use a different |
| character set than GDB does |
| <li><a href="Caching-Target-Data.html#Caching-Target-Data">Caching Target Data</a>: Data caching for targets |
| <li><a href="Searching-Memory.html#Searching-Memory">Searching Memory</a>: Searching memory for a sequence of bytes |
| <li><a href="Value-Sizes.html#Value-Sizes">Value Sizes</a>: Managing memory allocated for values |
| </ul> |
| |
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