Evaluating Variables and Expressions

This section shows how to evaluate variables and expressions.

Verifying Which Variable dbx Uses

If you are not sure which variable dbx is evaluating, use the which command to see the fully qualified name dbx is using.

To see other functions and files in which a variable name is defined, use the whereis command.

Variables Outside the Scope of the Current Function

When you want to evaluate or monitor a variable outside the scope of the current function:

• Qualify the name of the function

• Visit the function by changing the current function.

• Print the value of a variable or expression

An expression should follow current language syntax, with the exception of the meta syntax that dbx introduces to deal with scope and arrays.

To evaluate a variable or expression:

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print expression

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Printing C++

In C++ an object pointer has two types, its static type, what is defined in the source code, and its dynamic type. dbx can sometimes provide you with the information about the dynamic type of an object.

In general, when an object has a virtual function table, a vtable, in it, dbx can use the information in the vtable to correctly figure out what an object's type is.

You can use the commands print or display with the --r (recursive) option. dbx displays all the data members directly defined by a class and those inherited from a base class.

These commands also take a -d or +d option that toggles the default behavior of the dbxenv output_derived_type.

Using the --d flag or setting the dbxenv output_dynamic_type to on when there is no process running generates a "program is not active" error message because it is not possible to access dynamic information when there is no process. An "illegal cast on class pointers" error message is generated if you try to find a dynamic type through a virtual inheritance (casting from a virtual baseclass to a derived class is not legal in C++).

Evaluating Unnamed Arguments in C++ Programs

C++ allows you to define functions with unnamed arguments. For example:

---

void tester(int)
{
};
main(int, char **)
{
   tester(1);
};

---

Though you cannot use unnamed arguments elsewhere in a program, dbx encodes unnamed arguments in a form that allows you to evaluate them. The form is:

---

_ARG_%n_

---

where dbx assigns an integer to %n.

To obtain an assigned argument name from dbx, issue the whatis command with the function name as its target:

---

(dbx) whatis tester
void tester(int _ARG_0_);
(dbx) whatis main
int main(int _ARG_1_, char **_ARG_2_);

---

To evaluate (or display) an unnamed function argument,

---

(dbx) print _ARG_1_
_ARG_1_ = 4

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Dereferencing Pointers

When you dereference a pointer, you ask for the contents of the container the pointer points to.

To dereference a pointer, dbx prints the evaluation in the command pane; in this case, the value pointed to by t:

---

(dbx) print *t
*t = {
a = 4
}

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Monitoring Expressions

Monitoring the value of an expression each time the program stops is an effective technique for learning how and when a particular expression or variable changes. The display command instructs dbx to monitor one or more specified expressions or variables. Monitoring continues until you turn it off with the undisplay command.

To display the value of a variable or expression each time the program stops:

---

display expression,
...     

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You can monitor more than one variable at a time. The display command used with no options prints a list of all expressions being displayed:

---

display

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Turning Off Display (Undisplay)

dbx continues to display the value of a variable you are monitoring until you turn off display with the undisplay command. You can turn off the display of a specified expression or turn off the display of all expressions currently being monitored.

To turn off the display of a particular variable or expression:

---

undisplay expression

---

To turn off the display of all currently monitored variables:

---

undisplay 0

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