CODEWITHSUNDEEP
c++c
user180574
user180574

Reputation: 6094

trace variable access (c/c++)?

Not sure if feasible... I am reading a large software code, and sometimes feel lost when tracing how certain variable is accessed.

It would be great if there is tool/method to trace that.

More specifically, suppose I have the following class.

class A
{
  public:
    int _a;
};

...

and whenever

... = ...->_a;

during runtime, I would like to print out FILE and LINE. Thank you for the suggestion.

P.S., I could grep all "->_a" appearances, but that is also pretty hard to sort out.

Upvotes: 0

Views: 1600

Answers (3)

Bryan
Bryan

Reputation: 12200

You can use a tool like OpenGrok or Source Navigator to find all occurrences. This is static analysis, not runtime, but easier than grep.

Upvotes: 0

Deepanjan Mazumdar
Deepanjan Mazumdar

Reputation: 1507

One of the most powerful tools to debug C code is GDB. For your specific need, you can make use of watch/awatch/rwatch commands in GDB to identify points when your variable is accessed (read/write). Once program execution stops at those points, you could use print to either check or set the value of that variable.

Refer to this link for further clarifications.

Upvotes: 0

coelhudo
coelhudo

Reputation: 5080

Instead of:

class A
{
  public:
    int _a;
};

You could do something like this:

#include <iostream>

class Integer
{
public:
    Integer(int value) : value(value)
    {
    }

    operator int()
    {
        std::cout << "Integer called\n";
        return value;
    }
private:
    int value;
};

class A
{
public:
    A() : _a(5) {}
    Integer _a;
};

int main(int argc, char *argv[])
{
    A a;
    std::cout << a._a << std::endl;

    return 0;
}

Following your example:

  • Create a class and provide an operator for implicit conversion;

  • Substitute your member variable for this new class.

The implicit conversion operator will do the work.

It will print:

Integer called

5

A tip: never let a member variable be public. Always provide an accessor method.

Edit

Sorry, I forgot about FILE and LINE:

I didn't find a way to inject LINE or FILE because the implicit cast will be done at runtime while macro substitution under preprocessing.

What you can do it is to put a breakpoint at implicit conversion function, as pointed by @veer, and manually trace each calling.

Edit 2

At least it is possible to see the call stack at runtime.

On Linux this can be done through execinfo and on Windows through StackWalker.

Upvotes: 3

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