Chapter 3: Inspecting and setting variables.
Our Main Example Code
The main example is an implementation of a binary tree.
Advanced Inspection and Setting of Variables
Instead of using three print commands as in
p tmp->val
p tmp->left
p tmp->right
you can print a struct in its entirety using
p *tmp
Even better is using GDB's
display or
disp command,
which tells GDB to automatically print the specified item each time there is a
pause in execution (due to a breakpoint, the
next or
step commands,...)
You can also use GDB's commands command to execute several
commands (like print) when a breakpoint is hit. Next to the
print command, the
printf() command can be helpful for this. Its formatting is similar to that
of its C-language namesake.
The command set for a given breakpoint can be modified dynamically, or simply
canceled by redefining an empty set:
commands 1
end
If your code contains functions to print certain data-structures, you can
call these from GDB using the call command:
call printtree(root)
In DDD, the Right-Click / Display option is very useful for displaying linked
data-structures! To follow a link, right-click on the link and select the
`Display *()' choice. To remove something, right-click and choose the Undisplay
option. To avoid windows cluttered with data, you can run DDD
with the --separate option, which will create three separate
windows for Source Code, Console and Data.
Eclipse has its Variables view where you can inspect the value of variables. By
default, this view does not show global variables. You can add those by
right-clicking within the Variables view and selecting 'Add Global Variables'.
For printing values of dynamically created arrays (e.g. int *x;),
you can use for example:
p x
p *x
p x[5]
p *x@25
p (int [25]) *x
In DDD it's probably best to also use these commands in the Console. In
Eclipse, you select the variabe in the Variables view by right-clicking and
selecting `Display As Array'.
For C++ code, the same GDB commands work, but with somewhat different output.
Also, GDB needs you to specify variables according to the same scope rules that
C++ uses. For example, don't use
p *root
but specify
root via its full name as
p *node::root
GDB's ptype command is handy to get a quick review of the structure
of a class or struct. In DDD, you can right-click the class or variable name
and choose `What Is' to get the same information. Eclipse has its Outline view
for this.
With the info locals command in GDB, you can get a list of values
of all the local variables in the current stack frame. In DDD, you can display
these by clicking Data | Display Local Variables. Eclipse displays the local
variables in the Variables view.
To examine memory at a given address directly, GDB provides the x
(examine) command. In DDD one selects Data | Memory. Eclipse has a Memory
view, in which you can create Memory Monitors. These things are mainly useful
in assembly language contexts.
The print and display commands allow you to specify
alternative formats for hex, character, string and floating point:
p/x myvar
p/c myvar
p/s myvar
p/f myvar
To temporarily disable, re-enable and delete a display item, do:
info disp
dis disp 1
enable disp 1
undisp 1
Setting Variables from Within GDB/DDD/Eclipse
In GDB, you can very easily set values with for example:
set x = 12
There is no mouse-based way to do this in DDD, so do it from the Console.
In Eclipse, right-click the variable in the Variables view and select Change
Value.
Note also that the changes you make to
argv[1],
argv[2] etc. with the
set args command will
not occur until the next time you issue the
run command.
GDB has the info args command to check the arguments of the current
function. DDD provides this when you click Data | Display Arguments.
GDB's Own Variables
Output values from GDB's print command are labeled $1,
$2, and so on, and are collectively called the value
history. They can be used to produce shortcuts in future print
commands, for example:
(gdb) p tmp->left
$1 = (struct node *) 0x80496a8
(gdb) p *$1
There is also a special value history variable
$.
You can also store certain values in convenience variables. For example
set $q = p
p $*q
The variable
$q here is called a
convenience variables.
These variables can change values according to C rules.
