Access Violation on char array

Question

I'm getting an access violation on a char array I just created using new .

DispatchCommand(char* cmdStr)
        {
            // Dispatch
            for(int i = 0; i < sizeof(_lpCommands); i++)
            {
                const int len = strlen(_lpCommands[i].szCommand);
                char* cmdblip = new char[len + 1];
                memcpy(&cmdblip, cmdStr, len);
                cmdblip[len] = '\0';  // Access Violation

                if(strcmp(cmdblip, _lpCommands[i].szCommand) == 0)
                {
                    if(strlen(cmdStr) > strlen(_lpCommands[i].szCommand))
                        (*_lpCommands[i].cbCallback)(&cmdStr[strlen(_lpCommands[i].szCommand)]);
                    else
                        (*_lpCommands[i].cbCallback)("");

                    delete cmdblip;
                    return;
                }

                delete cmdblip;
            }

            // Error and return
            *Out::ServerInfo<<"Command not found!"<<ENDL;
        }

_lpCommands is an array of Command structures:

struct Command
{
    char* szCommand;
    CommandCallback cbCallback;
};

The produced error message is:

Unhandled exception at 0x012219cf in Program.exe: 0xC0000005: Access violation writing location 0x66647366.

This was a rewrite of similar code which was using memcmp , which ended up giving me an access violation as well without be doing a memcpy .

What gives?

Answer 1

Don't pass &cmdblip to memcpy . You should pass a pointer to the destination buffer, not a pointer to that pointer. Pass cmdblip instead.

Edit: I agree that in general, std::string should be used in C++. Still, the technical reason this code crashes is that memcpy corrupts the cmdblip pointer, making it point on a memory location that is actually made of the first 4 bytes of the copied string. Then, cmdblip[len] results in a memory location that is not within the allocated buffer (or any other legally allocated buffer), hence the crash. So, if you want to write better code, use C++ classes. And if you want to understand why the given code crashed, consider the above.

Answer 2

The only possible helpful answer to this question is "Use std::string ". The specific problem you are having now will simply re-occur, or an identical one, every time you modify this function or write another like it. The only way to solve the problem in the general case is to move to a class-based solution, which is kindly provided for you as Standard. For example, your current code is exception unsafe, on top of whatever is giving you an access violation, not to mention that it's unreadable and begging for a number of other errors, such as off-by-one, not properly NULL terminating, double deletes, and memory leaks. Oh, and UB because you delete what you new[] .