Debugging with GDB Manual The GNU Source-Level Debugger (769148-001, March 2014)

GDB/MI variable objects
Motivation for variable objects in GDB/MI
For the implementation of a variable debugger window (locals, watched expressions, and so on.),
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:
• -gdb-show output-radix
• -stack-list-arguments
• -stack-list-locals
• -stack-select-frame
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, and so on.).
The following is the complete set of GDB/MI operations defined to access this functionality:
Table 33 GDB/MI Operations
DescriptionOperation
create a variable object-var-create
delete the variable object and its children-var-delete
set the display format of this variable-var-set-format
show the display format of this variable-var-show-format
tells how many children this object has-var-info-num-children
return a list of the object children-var-list-children
show the type of this variable object-var-info-type
print what this variable object represents-var-info-expression
is this variable editable? does it exist here?-var-show-attributes
get the value of this variable-var-evaluate-expression
272 The GDB/MI Interface