Question

I have this code:

typedef struct {
    string fName;
    string str; 

}t;

//-------Other functions------//
void BeginTh()
{
    string arg = "yes";
    t *arglist;
    arglist = (t*)malloc(sizeof(t));
    arglist->fName = "comBomber";
    arglist->str = arg;
    _beginthread(startOver, 0, (void*)arglist);
    free(arglist);
}

And at ‘arglist->fName = “comBomber”;’ i get this error:

An unhandled exception of type 'System.AccessViolationException' occurred in <appname>

Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

Anyone can help me ? How solve this problem ?

Thanks.

Answer 1

One problem is that your t instance is not properly initialized. You can fix that by it by using new instead of malloc Your struct holds a string, whose constructor needs to be called. Calling new ensures that the t object gets constructed properly.

 t* arglist = new t;

then “free” the memory by calling delete:

delete arglist;

This points to the second problem, which is that your t instance must be guaranteed to be alive during the whole execution of the thread. You should not de-allocate its memory until the thread is finished. This is a C++ example where the t object is guaranteed to outlive the thread:

#include <thread>

int main()
{
  t arglist = whatever;
  std::thread t(startover, &whatever); // launches thread which runs startover(&arglist)

  // do other stuff

  t.join(); // wait for thread execution to finish

}

In general, Instead of using raw pointers to dynamically allocated objects, you should use a smart pointer.

As an aside, the typedef syntax for declaring a struct looks pretty strange in C++. Normally, you would do this:

struct t {
    string fName;
    string str; 
};