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| </div> |
| <hr> |
| <a name="Values-From-Inferior-In-Guile-1"></a> |
| <h4 class="subsubsection">23.3.3.5 Values From Inferior In Guile</h4> |
| <a name="index-values-from-inferior_002c-in-guile"></a> |
| <a name="index-guile_002c-working-with-values-from-inferior"></a> |
| |
| <a name="index-_003cgdb_003avalue_003e"></a> |
| <p><small>GDB</small> provides values it obtains from the inferior program in |
| an object of type <code><gdb:value></code>. <small>GDB</small> uses this object |
| for its internal bookkeeping of the inferior’s values, and for |
| fetching values when necessary. |
| </p> |
| <p><small>GDB</small> does not memoize <code><gdb:value></code> objects. |
| <code>make-value</code> always returns a fresh object. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) guile (eq? (make-value 1) (make-value 1)) |
| $1 = #f |
| (gdb) guile (equal? (make-value 1) (make-value 1)) |
| $1 = #t |
| </pre></div> |
| |
| <p>A <code><gdb:value></code> that represents a function can be executed via |
| inferior function call with <code>value-call</code>. |
| Any arguments provided to the call must match the function’s prototype, |
| and must be provided in the order specified by that prototype. |
| </p> |
| <p>For example, <code>some-val</code> is a <code><gdb:value></code> instance |
| representing a function that takes two integers as arguments. To |
| execute this function, call it like so: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define result (value-call some-val 10 20)) |
| </pre></div> |
| |
| <p>Any values returned from a function call are <code><gdb:value></code> objects. |
| </p> |
| <p>Note: Unlike Python scripting in <small>GDB</small>, |
| inferior values that are simple scalars cannot be used directly in |
| Scheme expressions that are valid for the value’s data type. |
| For example, <code>(+ (parse-and-eval "int_variable") 2)</code> does not work. |
| And inferior values that are structures or instances of some class cannot |
| be accessed using any special syntax, instead <code>value-field</code> must be used. |
| </p> |
| <p>The following value-related procedures are provided by the |
| <code>(gdb)</code> module. |
| </p> |
| <dl> |
| <dt><a name="index-value_003f"></a>Scheme Procedure: <strong>value?</strong> <em>object</em></dt> |
| <dd><p>Return <code>#t</code> if <var>object</var> is a <code><gdb:value></code> object. |
| Otherwise return <code>#f</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-make_002dvalue"></a>Scheme Procedure: <strong>make-value</strong> <em>value <span class="roman">[</span>#:type type<span class="roman">]</span></em></dt> |
| <dd><p>Many Scheme values can be converted directly to a <code><gdb:value></code> |
| with this procedure. If <var>type</var> is specified, the result is a value |
| of this type, and if <var>value</var> can’t be represented with this type |
| an exception is thrown. Otherwise the type of the result is determined from |
| <var>value</var> as described below. |
| </p> |
| <p>See <a href="Architectures-In-Guile.html#Architectures-In-Guile">Architectures In Guile</a>, for a list of the builtin |
| types for an architecture. |
| </p> |
| <p>Here’s how Scheme values are converted when <var>type</var> argument to |
| <code>make-value</code> is not specified: |
| </p> |
| <dl compact="compact"> |
| <dt>Scheme boolean</dt> |
| <dd><p>A Scheme boolean is converted the boolean type for the current language. |
| </p> |
| </dd> |
| <dt>Scheme integer</dt> |
| <dd><p>A Scheme integer is converted to the first of a C <code>int</code>, |
| <code>unsigned int</code>, <code>long</code>, <code>unsigned long</code>, |
| <code>long long</code> or <code>unsigned long long</code> type |
| for the current architecture that can represent the value. |
| </p> |
| <p>If the Scheme integer cannot be represented as a target integer |
| an <code>out-of-range</code> exception is thrown. |
| </p> |
| </dd> |
| <dt>Scheme real</dt> |
| <dd><p>A Scheme real is converted to the C <code>double</code> type for the |
| current architecture. |
| </p> |
| </dd> |
| <dt>Scheme string</dt> |
| <dd><p>A Scheme string is converted to a string in the current target |
| language using the current target encoding. |
| Characters that cannot be represented in the current target encoding |
| are replaced with the corresponding escape sequence. This is Guile’s |
| <code>SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE</code> conversion strategy |
| (see <a href="http://www.gnu.org/software/guile/manual/html_node/Strings.html#Strings">Strings</a> in <cite>GNU Guile Reference Manual</cite>). |
| </p> |
| <p>Passing <var>type</var> is not supported in this case, |
| if it is provided a <code>wrong-type-arg</code> exception is thrown. |
| </p> |
| </dd> |
| <dt><code><gdb:lazy-string></code></dt> |
| <dd><p>If <var>value</var> is a <code><gdb:lazy-string></code> object (see <a href="Lazy-Strings-In-Guile.html#Lazy-Strings-In-Guile">Lazy Strings In Guile</a>), then the <code>lazy-string->value</code> procedure is called, and |
| its result is used. |
| </p> |
| <p>Passing <var>type</var> is not supported in this case, |
| if it is provided a <code>wrong-type-arg</code> exception is thrown. |
| </p> |
| </dd> |
| <dt>Scheme bytevector</dt> |
| <dd><p>If <var>value</var> is a Scheme bytevector and <var>type</var> is provided, |
| <var>value</var> must be the same size, in bytes, of values of type <var>type</var>, |
| and the result is essentially created by using <code>memcpy</code>. |
| </p> |
| <p>If <var>value</var> is a Scheme bytevector and <var>type</var> is not provided, |
| the result is an array of type <code>uint8</code> of the same length. |
| </p></dd> |
| </dl> |
| </dd></dl> |
| |
| <a name="index-optimized-out-value-in-guile"></a> |
| <dl> |
| <dt><a name="index-value_002doptimized_002dout_003f"></a>Scheme Procedure: <strong>value-optimized-out?</strong> <em>value</em></dt> |
| <dd><p>Return <code>#t</code> if the compiler optimized out <var>value</var>, |
| thus it is not available for fetching from the inferior. |
| Otherwise return <code>#f</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002daddress"></a>Scheme Procedure: <strong>value-address</strong> <em>value</em></dt> |
| <dd><p>If <var>value</var> is addressable, returns a |
| <code><gdb:value></code> object representing the address. |
| Otherwise, <code>#f</code> is returned. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dtype"></a>Scheme Procedure: <strong>value-type</strong> <em>value</em></dt> |
| <dd><p>Return the type of <var>value</var> as a <code><gdb:type></code> object |
| (see <a href="Types-In-Guile.html#Types-In-Guile">Types In Guile</a>). |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002ddynamic_002dtype"></a>Scheme Procedure: <strong>value-dynamic-type</strong> <em>value</em></dt> |
| <dd><p>Return the dynamic type of <var>value</var>. This uses C<tt>++</tt> run-time |
| type information (<acronym>RTTI</acronym>) to determine the dynamic type of the |
| value. If the value is of class type, it will return the class in |
| which the value is embedded, if any. If the value is of pointer or |
| reference to a class type, it will compute the dynamic type of the |
| referenced object, and return a pointer or reference to that type, |
| respectively. In all other cases, it will return the value’s static |
| type. |
| </p> |
| <p>Note that this feature will only work when debugging a C<tt>++</tt> program |
| that includes <acronym>RTTI</acronym> for the object in question. Otherwise, |
| it will just return the static type of the value as in <kbd>ptype foo</kbd>. |
| See <a href="Symbols.html#Symbols">ptype</a>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dcast"></a>Scheme Procedure: <strong>value-cast</strong> <em>value type</em></dt> |
| <dd><p>Return a new instance of <code><gdb:value></code> that is the result of |
| casting <var>value</var> to the type described by <var>type</var>, which must |
| be a <code><gdb:type></code> object. If the cast cannot be performed for some |
| reason, this method throws an exception. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002ddynamic_002dcast"></a>Scheme Procedure: <strong>value-dynamic-cast</strong> <em>value type</em></dt> |
| <dd><p>Like <code>value-cast</code>, but works as if the C<tt>++</tt> <code>dynamic_cast</code> |
| operator were used. Consult a C<tt>++</tt> reference for details. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dreinterpret_002dcast"></a>Scheme Procedure: <strong>value-reinterpret-cast</strong> <em>value type</em></dt> |
| <dd><p>Like <code>value-cast</code>, but works as if the C<tt>++</tt> <code>reinterpret_cast</code> |
| operator were used. Consult a C<tt>++</tt> reference for details. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002ddereference"></a>Scheme Procedure: <strong>value-dereference</strong> <em>value</em></dt> |
| <dd><p>For pointer data types, this method returns a new <code><gdb:value></code> object |
| whose contents is the object pointed to by <var>value</var>. For example, if |
| <code>foo</code> is a C pointer to an <code>int</code>, declared in your C program as |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">int *foo; |
| </pre></div> |
| |
| <p>then you can use the corresponding <code><gdb:value></code> to access what |
| <code>foo</code> points to like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define bar (value-dereference foo)) |
| </pre></div> |
| |
| <p>The result <code>bar</code> will be a <code><gdb:value></code> object holding the |
| value pointed to by <code>foo</code>. |
| </p> |
| <p>A similar function <code>value-referenced-value</code> exists which also |
| returns <code><gdb:value></code> objects corresonding to the values pointed to |
| by pointer values (and additionally, values referenced by reference |
| values). However, the behavior of <code>value-dereference</code> |
| differs from <code>value-referenced-value</code> by the fact that the |
| behavior of <code>value-dereference</code> is identical to applying the C |
| unary operator <code>*</code> on a given value. For example, consider a |
| reference to a pointer <code>ptrref</code>, declared in your C<tt>++</tt> program |
| as |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">typedef int *intptr; |
| ... |
| int val = 10; |
| intptr ptr = &val; |
| intptr &ptrref = ptr; |
| </pre></div> |
| |
| <p>Though <code>ptrref</code> is a reference value, one can apply the method |
| <code>value-dereference</code> to the <code><gdb:value></code> object corresponding |
| to it and obtain a <code><gdb:value></code> which is identical to that |
| corresponding to <code>val</code>. However, if you apply the method |
| <code>value-referenced-value</code>, the result would be a <code><gdb:value></code> |
| object identical to that corresponding to <code>ptr</code>. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define scm-ptrref (parse-and-eval "ptrref")) |
| (define scm-val (value-dereference scm-ptrref)) |
| (define scm-ptr (value-referenced-value scm-ptrref)) |
| </pre></div> |
| |
| <p>The <code><gdb:value></code> object <code>scm-val</code> is identical to that |
| corresponding to <code>val</code>, and <code>scm-ptr</code> is identical to that |
| corresponding to <code>ptr</code>. In general, <code>value-dereference</code> can |
| be applied whenever the C unary operator <code>*</code> can be applied |
| to the corresponding C value. For those cases where applying both |
| <code>value-dereference</code> and <code>value-referenced-value</code> is allowed, |
| the results obtained need not be identical (as we have seen in the above |
| example). The results are however identical when applied on |
| <code><gdb:value></code> objects corresponding to pointers (<code><gdb:value></code> |
| objects with type code <code>TYPE_CODE_PTR</code>) in a C/C<tt>++</tt> program. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dreferenced_002dvalue"></a>Scheme Procedure: <strong>value-referenced-value</strong> <em>value</em></dt> |
| <dd><p>For pointer or reference data types, this method returns a new |
| <code><gdb:value></code> object corresponding to the value referenced by the |
| pointer/reference value. For pointer data types, |
| <code>value-dereference</code> and <code>value-referenced-value</code> produce |
| identical results. The difference between these methods is that |
| <code>value-dereference</code> cannot get the values referenced by reference |
| values. For example, consider a reference to an <code>int</code>, declared |
| in your C<tt>++</tt> program as |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">int val = 10; |
| int &ref = val; |
| </pre></div> |
| |
| <p>then applying <code>value-dereference</code> to the <code><gdb:value></code> object |
| corresponding to <code>ref</code> will result in an error, while applying |
| <code>value-referenced-value</code> will result in a <code><gdb:value></code> object |
| identical to that corresponding to <code>val</code>. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define scm-ref (parse-and-eval "ref")) |
| (define err-ref (value-dereference scm-ref)) ;; error |
| (define scm-val (value-referenced-value scm-ref)) ;; ok |
| </pre></div> |
| |
| <p>The <code><gdb:value></code> object <code>scm-val</code> is identical to that |
| corresponding to <code>val</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dfield"></a>Scheme Procedure: <strong>value-field</strong> <em>value field-name</em></dt> |
| <dd><p>Return field <var>field-name</var> from <code><gdb:value></code> object <var>value</var>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dsubscript"></a>Scheme Procedure: <strong>value-subscript</strong> <em>value index</em></dt> |
| <dd><p>Return the value of array <var>value</var> at index <var>index</var>. |
| The <var>value</var> argument must be a subscriptable <code><gdb:value></code> object. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dcall"></a>Scheme Procedure: <strong>value-call</strong> <em>value arg-list</em></dt> |
| <dd><p>Perform an inferior function call, taking <var>value</var> as a pointer |
| to the function to call. |
| Each element of list <var>arg-list</var> must be a <gdb:value> object or an object |
| that can be converted to a value. |
| The result is the value returned by the function. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002d_003ebool"></a>Scheme Procedure: <strong>value->bool</strong> <em>value</em></dt> |
| <dd><p>Return the Scheme boolean representing <code><gdb:value></code> <var>value</var>. |
| The value must be “integer like”. Pointers are ok. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002d_003einteger"></a>Scheme Procedure: <strong>value->integer</strong></dt> |
| <dd><p>Return the Scheme integer representing <code><gdb:value></code> <var>value</var>. |
| The value must be “integer like”. Pointers are ok. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002d_003ereal"></a>Scheme Procedure: <strong>value->real</strong></dt> |
| <dd><p>Return the Scheme real number representing <code><gdb:value></code> <var>value</var>. |
| The value must be a number. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002d_003ebytevector"></a>Scheme Procedure: <strong>value->bytevector</strong></dt> |
| <dd><p>Return a Scheme bytevector with the raw contents of <code><gdb:value></code> |
| <var>value</var>. No transformation, endian or otherwise, is performed. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002d_003estring"></a>Scheme Procedure: <strong>value->string</strong> <em>value <span class="roman">[</span>#:encoding encoding<span class="roman">]</span> <span class="roman">[</span>#:errors errors<span class="roman">]</span> <span class="roman">[</span>#:length length<span class="roman">]</span></em></dt> |
| <dd><p>If <var>value></var> represents a string, then this method |
| converts the contents to a Guile string. Otherwise, this method will |
| throw an exception. |
| </p> |
| <p>Values are interpreted as strings according to the rules of the |
| current language. If the optional length argument is given, the |
| string will be converted to that length, and will include any embedded |
| zeroes that the string may contain. Otherwise, for languages |
| where the string is zero-terminated, the entire string will be |
| converted. |
| </p> |
| <p>For example, in C-like languages, a value is a string if it is a pointer |
| to or an array of characters or ints of type <code>wchar_t</code>, <code>char16_t</code>, |
| or <code>char32_t</code>. |
| </p> |
| <p>If the optional <var>encoding</var> argument is given, it must be a string |
| naming the encoding of the string in the <code><gdb:value></code>, such as |
| <code>"ascii"</code>, <code>"iso-8859-6"</code> or <code>"utf-8"</code>. It accepts |
| the same encodings as the corresponding argument to Guile’s |
| <code>scm_from_stringn</code> function, and the Guile codec machinery will be used |
| to convert the string. If <var>encoding</var> is not given, or if |
| <var>encoding</var> is the empty string, then either the <code>target-charset</code> |
| (see <a href="Character-Sets.html#Character-Sets">Character Sets</a>) will be used, or a language-specific encoding |
| will be used, if the current language is able to supply one. |
| </p> |
| <p>The optional <var>errors</var> argument is one of <code>#f</code>, <code>error</code> or |
| <code>substitute</code>. <code>error</code> and <code>substitute</code> must be symbols. |
| If <var>errors</var> is not specified, or if its value is <code>#f</code>, then the |
| default conversion strategy is used, which is set with the Scheme function |
| <code>set-port-conversion-strategy!</code>. |
| If the value is <code>'error</code> then an exception is thrown if there is any |
| conversion error. If the value is <code>'substitute</code> then any conversion |
| error is replaced with question marks. |
| See <a href="http://www.gnu.org/software/guile/manual/html_node/Strings.html#Strings">Strings</a> in <cite>GNU Guile Reference Manual</cite>. |
| </p> |
| <p>If the optional <var>length</var> argument is given, the string will be |
| fetched and converted to the given length. |
| The length must be a Scheme integer and not a <code><gdb:value></code> integer. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002d_003elazy_002dstring"></a>Scheme Procedure: <strong>value->lazy-string</strong> <em>value <span class="roman">[</span>#:encoding encoding<span class="roman">]</span> <span class="roman">[</span>#:length length<span class="roman">]</span></em></dt> |
| <dd><p>If this <code><gdb:value></code> represents a string, then this method |
| converts <var>value</var> to a <code><gdb:lazy-string</code> (see <a href="Lazy-Strings-In-Guile.html#Lazy-Strings-In-Guile">Lazy Strings In Guile</a>). Otherwise, this method will throw an exception. |
| </p> |
| <p>If the optional <var>encoding</var> argument is given, it must be a string |
| naming the encoding of the <code><gdb:lazy-string</code>. Some examples are: |
| <code>"ascii"</code>, <code>"iso-8859-6"</code> or <code>"utf-8"</code>. If the |
| <var>encoding</var> argument is an encoding that <small>GDB</small> does not |
| recognize, <small>GDB</small> will raise an error. |
| </p> |
| <p>When a lazy string is printed, the <small>GDB</small> encoding machinery is |
| used to convert the string during printing. If the optional |
| <var>encoding</var> argument is not provided, or is an empty string, |
| <small>GDB</small> will automatically select the encoding most suitable for |
| the string type. For further information on encoding in <small>GDB</small> |
| please see <a href="Character-Sets.html#Character-Sets">Character Sets</a>. |
| </p> |
| <p>If the optional <var>length</var> argument is given, the string will be |
| fetched and encoded to the length of characters specified. If |
| the <var>length</var> argument is not provided, the string will be fetched |
| and encoded until a null of appropriate width is found. |
| The length must be a Scheme integer and not a <code><gdb:value></code> integer. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dlazy_003f"></a>Scheme Procedure: <strong>value-lazy?</strong> <em>value</em></dt> |
| <dd><p>Return <code>#t</code> if <var>value</var> has not yet been fetched |
| from the inferior. |
| Otherwise return <code>#f</code>. |
| <small>GDB</small> does not fetch values until necessary, for efficiency. |
| For example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define myval (parse-and-eval "somevar")) |
| </pre></div> |
| |
| <p>The value of <code>somevar</code> is not fetched at this time. It will be |
| fetched when the value is needed, or when the <code>fetch-lazy</code> |
| procedure is invoked. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-make_002dlazy_002dvalue"></a>Scheme Procedure: <strong>make-lazy-value</strong> <em>type address</em></dt> |
| <dd><p>Return a <code><gdb:value></code> that will be lazily fetched from the |
| target. The object of type <code><gdb:type></code> whose value to fetch is |
| specified by its <var>type</var> and its target memory <var>address</var>, which |
| is a Scheme integer. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dfetch_002dlazy_0021"></a>Scheme Procedure: <strong>value-fetch-lazy!</strong> <em>value</em></dt> |
| <dd><p>If <var>value</var> is a lazy value (<code>(value-lazy? value)</code> is <code>#t</code>), |
| then the value is fetched from the inferior. |
| Any errors that occur in the process will produce a Guile exception. |
| </p> |
| <p>If <var>value</var> is not a lazy value, this method has no effect. |
| </p> |
| <p>The result of this function is unspecified. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-value_002dprint"></a>Scheme Procedure: <strong>value-print</strong> <em>value</em></dt> |
| <dd><p>Return the string representation (print form) of <code><gdb:value></code> |
| <var>value</var>. |
| </p></dd></dl> |
| |
| <hr> |
| <div class="header"> |
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