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23.3.3.5 Values From Inferior In Guile

gdb provides values it obtains from the inferior program in an object of type <gdb:value>. gdb uses this object for its internal bookkeeping of the inferior's values, and for fetching values when necessary.

gdb does not memoize <gdb:value> objects. make-value always returns a fresh object. 

     (gdb) guile (eq? (make-value 1) (make-value 1))
     $1 = #f
     (gdb) guile (equal? (make-value 1) (make-value 1))
     $1 = #t

A <gdb:value> that represents a function can be executed via inferior function call with value-call. Any arguments provided to the call must match the function's prototype, and must be provided in the order specified by that prototype.

For example, some-val is a <gdb:value> instance representing a function that takes two integers as arguments. To execute this function, call it like so: 

     (define result (value-call some-val 10 20))

Any values returned from a function call are <gdb:value> objects.

Note: Unlike Python scripting in gdb, inferior values that are simple scalars cannot be used directly in Scheme expressions that are valid for the value's data type. For example, (+ (parse-and-eval "int_variable") 2) does not work. And inferior values that are structures or instances of some class cannot be accessed using any special syntax, instead value-field must be used.

The following value-related procedures are provided by the (gdb) module.

— Scheme Procedure: value? object

Return #t if object is a <gdb:value> object. Otherwise return #f.

— Scheme Procedure: make-value value [#:type type]

Many Scheme values can be converted directly to a <gdb:value> with this procedure. If type is specified, the result is a value of this type, and if value can't be represented with this type an exception is thrown. Otherwise the type of the result is determined from value as described below.

See Architectures In Guile, for a list of the builtin types for an architecture.

Here's how Scheme values are converted when type argument to make-value is not specified:

Scheme boolean
A Scheme boolean is converted the boolean type for the current language.
Scheme integer
A Scheme integer is converted to the first of a C int, unsigned int, long, unsigned long, long long or unsigned long long type for the current architecture that can represent the value.

If the Scheme integer cannot be represented as a target integer an out-of-range exception is thrown.

Scheme real
A Scheme real is converted to the C double type for the current architecture.
Scheme string
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 SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE conversion strategy (see Strings).

Passing type is not supported in this case, if it is provided a wrong-type-arg exception is thrown.

<gdb:lazy-string>
If value is a <gdb:lazy-string> object (see Lazy Strings In Guile), then the lazy-string->value procedure is called, and its result is used.

Passing type is not supported in this case, if it is provided a wrong-type-arg exception is thrown.

Scheme bytevector
If value is a Scheme bytevector and type is provided, value must be the same size, in bytes, of values of type type, and the result is essentially created by using memcpy.

If value is a Scheme bytevector and type is not provided, the result is an array of type uint8 of the same length.

— Scheme Procedure: value-optimized-out? value

Return #t if the compiler optimized out value, thus it is not available for fetching from the inferior. Otherwise return #f.

— Scheme Procedure: value-address value

If value is addressable, returns a <gdb:value> object representing the address. Otherwise, #f is returned.

— Scheme Procedure: value-type value

Return the type of value as a <gdb:type> object (see Types In Guile).

— Scheme Procedure: value-dynamic-type value

Return the dynamic type of value. This uses C++ run-time type information (RTTI) 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.

Note that this feature will only work when debugging a C++ program that includes RTTI for the object in question. Otherwise, it will just return the static type of the value as in ptype foo. See ptype.

— Scheme Procedure: value-cast value type

Return a new instance of <gdb:value> that is the result of casting value to the type described by type, which must be a <gdb:type> object. If the cast cannot be performed for some reason, this method throws an exception.

— Scheme Procedure: value-dynamic-cast value type

Like value-cast, but works as if the C++ dynamic_cast operator were used. Consult a C++ reference for details.

— Scheme Procedure: value-reinterpret-cast value type

Like value-cast, but works as if the C++ reinterpret_cast operator were used. Consult a C++ reference for details.

— Scheme Procedure: value-dereference value

For pointer data types, this method returns a new <gdb:value> object whose contents is the object pointed to by value. For example, if foo is a C pointer to an int, declared in your C program as 

          int *foo;

then you can use the corresponding <gdb:value> to access what foo points to like this: 

          (define bar (value-dereference foo))

The result bar will be a <gdb:value> object holding the value pointed to by foo.

A similar function value-referenced-value exists which also returns <gdb:value> objects corresonding to the values pointed to by pointer values (and additionally, values referenced by reference values). However, the behavior of value-dereference differs from value-referenced-value by the fact that the behavior of value-dereference is identical to applying the C unary operator * on a given value. For example, consider a reference to a pointer ptrref, declared in your C++ program as 

          typedef int *intptr;
          ...
          int val = 10;
          intptr ptr = &val;
          intptr &ptrref = ptr;

Though ptrref is a reference value, one can apply the method value-dereference to the <gdb:value> object corresponding to it and obtain a <gdb:value> which is identical to that corresponding to val. However, if you apply the method value-referenced-value, the result would be a <gdb:value> object identical to that corresponding to ptr. 

          (define scm-ptrref (parse-and-eval "ptrref"))
          (define scm-val (value-dereference scm-ptrref))
          (define scm-ptr (value-referenced-value scm-ptrref))

The <gdb:value> object scm-val is identical to that corresponding to val, and scm-ptr is identical to that corresponding to ptr. In general, value-dereference can be applied whenever the C unary operator * can be applied to the corresponding C value. For those cases where applying both value-dereference and value-referenced-value 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 <gdb:value> objects corresponding to pointers (<gdb:value> objects with type code TYPE_CODE_PTR) in a C/C++ program.

— Scheme Procedure: value-referenced-value value

For pointer or reference data types, this method returns a new <gdb:value> object corresponding to the value referenced by the pointer/reference value. For pointer data types, value-dereference and value-referenced-value produce identical results. The difference between these methods is that value-dereference cannot get the values referenced by reference values. For example, consider a reference to an int, declared in your C++ program as 

          int val = 10;
          int &ref = val;

then applying value-dereference to the <gdb:value> object corresponding to ref will result in an error, while applying value-referenced-value will result in a <gdb:value> object identical to that corresponding to val. 

          (define scm-ref (parse-and-eval "ref"))
          (define err-ref (value-dereference scm-ref))      ;; error
          (define scm-val (value-referenced-value scm-ref)) ;; ok

The <gdb:value> object scm-val is identical to that corresponding to val.

— Scheme Procedure: value-field value field-name

Return field field-name from <gdb:value> object value.

— Scheme Procedure: value-subscript value index

Return the value of array value at index index. The value argument must be a subscriptable <gdb:value> object.

— Scheme Procedure: value-call value arg-list

Perform an inferior function call, taking value as a pointer to the function to call. Each element of list arg-list 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.

— Scheme Procedure: value->bool value

Return the Scheme boolean representing <gdb:value> value. The value must be “integer like”. Pointers are ok.

— Scheme Procedure: value->integer

Return the Scheme integer representing <gdb:value> value. The value must be “integer like”. Pointers are ok.

— Scheme Procedure: value->real

Return the Scheme real number representing <gdb:value> value. The value must be a number.

— Scheme Procedure: value->bytevector

Return a Scheme bytevector with the raw contents of <gdb:value> value. No transformation, endian or otherwise, is performed.

— Scheme Procedure: value->string value [#:encoding encoding] [#:errors errors] [#:length length]

If value> represents a string, then this method converts the contents to a Guile string. Otherwise, this method will throw an exception.

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.

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 wchar_t, char16_t, or char32_t.

If the optional encoding argument is given, it must be a string naming the encoding of the string in the <gdb:value>, such as "ascii", "iso-8859-6" or "utf-8". It accepts the same encodings as the corresponding argument to Guile's scm_from_stringn function, and the Guile codec machinery will be used to convert the string. If encoding is not given, or if encoding is the empty string, then either the target-charset (see Character Sets) will be used, or a language-specific encoding will be used, if the current language is able to supply one.

The optional errors argument is one of #f, error or substitute. error and substitute must be symbols. If errors is not specified, or if its value is #f, then the default conversion strategy is used, which is set with the Scheme function set-port-conversion-strategy!. If the value is 'error then an exception is thrown if there is any conversion error. If the value is 'substitute then any conversion error is replaced with question marks. See Strings.

If the optional length argument is given, the string will be fetched and converted to the given length. The length must be a Scheme integer and not a <gdb:value> integer.

— Scheme Procedure: value->lazy-string value [#:encoding encoding] [#:length length]

If this <gdb:value> represents a string, then this method converts value to a <gdb:lazy-string (see Lazy Strings In Guile). Otherwise, this method will throw an exception.

If the optional encoding argument is given, it must be a string naming the encoding of the <gdb:lazy-string. Some examples are: "ascii", "iso-8859-6" or "utf-8". If the encoding argument is an encoding that gdb does not recognize, gdb will raise an error.

When a lazy string is printed, the gdb encoding machinery is used to convert the string during printing. If the optional encoding argument is not provided, or is an empty string, gdb will automatically select the encoding most suitable for the string type. For further information on encoding in gdb please see Character Sets.

If the optional length argument is given, the string will be fetched and encoded to the length of characters specified. If the length 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 <gdb:value> integer.

— Scheme Procedure: value-lazy? value

Return #t if value has not yet been fetched from the inferior. Otherwise return #f. gdb does not fetch values until necessary, for efficiency. For example: 

          (define myval (parse-and-eval "somevar"))

The value of somevar is not fetched at this time. It will be fetched when the value is needed, or when the fetch-lazy procedure is invoked.

— Scheme Procedure: make-lazy-value type address

Return a <gdb:value> that will be lazily fetched from the target. The object of type <gdb:type> whose value to fetch is specified by its type and its target memory address, which is a Scheme integer.

— Scheme Procedure: value-fetch-lazy! value

If value is a lazy value ((value-lazy? value) is #t), then the value is fetched from the inferior. Any errors that occur in the process will produce a Guile exception.

If value is not a lazy value, this method has no effect.

The result of this function is unspecified.

— Scheme Procedure: value-print value

Return the string representation (print form) of <gdb:value> value.