Question

For class I’m creating a shell in C++. It’s been a while since I’ve worked with C++ so I’m having a bit of trouble. One of the requirements for the project is that I must use the read() system call.

I’m required to keep a command history (similar to bash’s if you press the up arrow) that stores the most recent 20 commands. I feel the best way to do this is to use an array of pointers to previous statements. I’m running into a problem where no matter what I do, the string that contains the user input is always stored in the same location in memory. To clarify, this means, if a user inputs 5 statements and then views his/her history, they will see the most recent statement 5 times. My code looks a little something like this (I have to cut some things out because there is a lot of error handling in the middle):

char *history[20];
int historyCounter = 0;

while(true){
  char currLine[65];
  int charsRead = read(0,currLine,65);

  char tmp[charsRead];
  strcpy(tmp,currLine); //This is my attempt to ensure the char[] is stored int a 
                        //unique location every time, but this attempt failed.

  history[historycounter] = tmp;
  historycounter++;
}

Just to note, in my source, I do handle the case when historycounter > 19. Just not in this snippet.

If any more clarification is required I’d be happy to provide it. This is my first time posting on stack overflow so I apologize in advance if I’m making any sort of rookie mistakes. I also apologize if the solution is painfully obvious. I’ve been looking at this for a while and it is entirely possible that I’m just not thinking straight.

Answer 1

In this case, tmp is most likely to be in the same location, on stack. Its allocated every iteration, and deallocated when the iteration ends.

The line

history[historycounter] = tmp;

Will lead to undefined behavior when you use it because you’ll be using an address of a local variable outside of its scope.

If you want to ensure unique address (and solve the UB problem) – use new to allocate the memory, and don’t use delete until you’re done. Make sure to keep track of all the allocated pointers and delete them when done.