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Debugging with GDB

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24.14 GDB/MI Variable Objects

Motivation for Variable Objects in GDB/MI

For the implementation of a variable debugger window (locals, watched expressions, etc.), we are proposing the adaptation of the existing code used by Insight.

The two main reasons for that are:

  1. It has been proven in practice (it is already on its second generation).

  2. It will shorten development time (needless to say how important it is now).

The original interface was designed to be used by Tcl code, so it was slightly changed so it could be used through GDB/MI. This section describes the GDB/MI operations that will be available and gives some hints about their use.

Note: In addition to the set of operations described here, we expect the GUI implementation of a variable window to require, at least, the following operations:

Introduction to Variable Objects in GDB/MI

The basic idea behind variable objects is the creation of a named object to represent a variable, an expression, a memory location or even a CPU register. For each object created, a set of operations is available for examining or changing its properties.

Furthermore, complex data types, such as C structures, are represented in a tree format. For instance, the struct type variable is the root and the children will represent the struct members. If a child is itself of a complex type, it will also have children of its own. Appropriate language differences are handled for C, C++ and Java.

When returning the actual values of the objects, this facility allows for the individual selection of the display format used in the result creation. It can be chosen among: binary, decimal, hexadecimal, octal and natural. Natural refers to a default format automatically chosen based on the variable type (like decimal for an int, hex for pointers, etc.).

The following is the complete set of GDB/MI operations defined to access this functionality:

Operation Description
-var-create create a variable object
-var-delete delete the variable object and its children
-var-set-format set the display format of this variable
-var-show-format show the display format of this variable
-var-info-num-children tells how many children this object has
-var-list-children return a list of the object's children
-var-info-type show the type of this variable object
-var-info-expression print what this variable object represents
-var-show-attributes is this variable editable? does it exist here?
-var-evaluate-expression get the value of this variable
-var-assign set the value of this variable
-var-update update the variable and its children

In the next subsection we describe each operation in detail and suggest how it can be used.

Description And Use of Operations on Variable Objects

The -var-create Command

Synopsis

 
 -var-create {name | "-"}
    {frame-addr | "*"} expression

This operation creates a variable object, which allows the monitoring of a variable, the result of an expression, a memory cell or a CPU register.

The name parameter is the string by which the object can be referenced. It must be unique. If `-' is specified, the varobj system will generate a string "varNNNNNN" automatically. It will be unique provided that one does not specify name on that format. The command fails if a duplicate name is found.

The frame under which the expression should be evaluated can be specified by frame-addr. A `*' indicates that the current frame should be used.

expression is any expression valid on the current language set (must not begin with a `*'), or one of the following:

Result

This operation returns the name, number of children and the type of the object created. Type is returned as a string as the ones generated by the GDB CLI:

 
 name="name",numchild="N",type="type"

The -var-delete Command

Synopsis

 
 -var-delete name

Deletes a previously created variable object and all of its children.

Returns an error if the object name is not found.

The -var-set-format Command

Synopsis

 
 -var-set-format name format-spec

Sets the output format for the value of the object name to be format-spec.

The syntax for the format-spec is as follows:

 
 format-spec ==>
 {binary | decimal | hexadecimal | octal | natural}

The -var-show-format Command

Synopsis

 
 -var-show-format name

Returns the format used to display the value of the object name.

 
 format ==>
 format-spec

The -var-info-num-children Command

Synopsis

 
 -var-info-num-children name

Returns the number of children of a variable object name:

 
 numchild=n

The -var-list-children Command

Synopsis

 
 -var-list-children name

Returns a list of the children of the specified variable object:

 
 numchild=n,children={{name=name,
 numchild=n,type=type},(repeats N times)}

The -var-info-type Command

Synopsis

 
 -var-info-type name

Returns the type of the specified variable name. The type is returned as a string in the same format as it is output by the GDB CLI:

 
 type=typename

The -var-info-expression Command

Synopsis

 
 -var-info-expression name

Returns what is represented by the variable object name:

 
 lang=lang-spec,exp=expression

where lang-spec is {"C" | "C++" | "Java"}.

The -var-show-attributes Command

Synopsis

 
 -var-show-attributes name

List attributes of the specified variable object name:

 
 status=attr [ ( ,attr )* ]

where attr is { { editable | noneditable } | TBD }.

The -var-evaluate-expression Command

Synopsis

 
 -var-evaluate-expression name

Evaluates the expression that is represented by the specified variable object and returns its value as a string in the current format specified for the object:

 
 value=value

The -var-assign Command

Synopsis

 
 -var-assign name expression

Assigns the value of expression to the variable object specified by name. The object must be `editable'.

The -var-update Command

Synopsis

 
 -var-update {name | "*"}

Update the value of the variable object name by evaluating its expression after fetching all the new values from memory or registers. A `*' causes all existing variable objects to be updated.


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