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.