TotalView Users Guide - CI Wiki

Why is There an Expression System?
invocation and polymorphism. (This is discussed in “Using C++” on
page 384.)
Calling Functions: Problems and Issues
Unfortunately, calling functions in the expression system can cause problems.
Some of these problems are:
� What happens if the function has a side-effect. For example, suppose
you have enter my_var[cntr] in one row in an Expression List Window, followed
by my_var[++cntr] in another? If cntr equals 3, you’ll be seeing
the values of my_var[3] and my_var[4]. However, since cntr now equals 4,
the first entry is no longer correct.
� What happens when the function crashes (after all you are trying to debug
problems), doesn’t return, returns the wrong value, or hits a breakpoint?
� What does calling functions do to your debugging interaction if evaluation
takes an excessive amount of time?
� What happens if a function creates processes and threads? Or worse,
kills them?
In general, there are some protections in the code. For example, if you’re
displaying items in an Expression List Window, TotalView avoids being in an
infinite loop by only evaluating items once. This does mean that the information
is only accurate at the time at which TotalView made the evaluation.
In most other cases, you’re basically on your own. If there’s a problem,
you’ll get an error message. If something takes too long, you can press the
Halt button. But if a function alters memory values or starts or stops processes
or threads and you can’t live with it, you’ll need to restart your program.
However, if an error occurs while using the Evaluate Window, pressing
the Stop button pops the stack, leaving your program in the state it was in
before you used the Evaluate command. However, changes made to heap
variables will, of course, not be undone.
Expressions in Eval Points and the Evaluate Window
Expression evaluation is not limited to a Variable Window or an Expression
List Window. You can use expressions within eval points and in the Tools >
Evaluate Window. The expressions you type here also let you use programming
language constructs. For example, here’s a trivial example of code
that can execute within the Evaluate Window:
int i, j, k;
j = k = 10;
for (i=0; i< 20; i++)
{
j = j + access_func(i, k);
}
j;
This code fragment declares a couple of variables, runs them through a for
loop, then displays the value of j. In all cases, the programming language
constructs being interpreted or compiled within TotalView are based on
TotalView Users Guide: version 8.7 383