How do you code the perfect fuzzing function for some other function?

Question

Here is a snippet of code:

struct somedata {
    char mychar;
    int myint;
    unsigned short myushort;
    string mystring;
};

void some_func(somedata *data) {
  /* does something with data */
}

How do you write a perfect fuzzing function to test the correct functionality, security, and robustness of this code?

By perfect I mean complete tests that cover all cases (if possible). Say: out of range values, different data types, etc..

You do not have source code for some_func.

Answer 1

It seems that I got fuzzing wrong.

Fuzz testing is a simple technique that can have a profound effect on your code quality. In fuzzing we inject random bad data into an application to see what breaks

Because Fuzzing != Testing, we do not build perfect test-cases or test all possible cases, instead we just generate random bad data and insert it into the application.

A Typical fuzzing function for the mentioned code would be:

void fuzzTesting ()
{
    //create somedata
    somedata data;

    //generate a random vector
    srand(time(NULL)); //seed = current_time

    //bufferOverflow
    int i= 200000
    while(i>=0)
    {
        r = rand()
        data.mychar = r;
        data.myint = r;
        data.myushort = r;
        some_func(&data);
        --i;
    }

    //Format String
    int i= 200000
    while(i>=0)
    {
        r = rand()
        data.mychar = '%s' + r;
        data.myint = '%s' + r;
        data.myushort = '%s' + r;
        some_func(&data);
        --i;
    }

    //Integer overflow
    int i= 200000
    while(i>=0)
    {
        r = rand()
        data.mychar = r + 0xffffffff;
        data.myint = r + 0xffffffff;
        data.myushort = r + 0xffffffff;
        some_func(&data);
        --i;
    }
}

for more fuzz testing vectors and details see this wikipage