Deleting a character array after using strcat causes a seg fault

Question

In my code I am using system() to call a function that exists only on the raspberry pi's shell, specifically "raspistill"

When doing this, I use a char[] and strcat() as follows:

#include <stdlib.h> 

int main(int argc, char * argv[]){
  if(argc != 2) return -1;
  char command[] = "raspistill -o ";
  strcat(command, argv[1]);
  system(command);
  delete[] command;
  return 0;
}

I get a warning saying that the length to be deleted is 15, the initial length when initializing command, instead of the new length after the strcat function.

This problem goes away when I use a C++ string with append() instead. Why does this cause a seg fault and how can I avoid it?

Answer 1

This isn't really C++. If you replace the

delete[] command;

with

free(command);

then you will get a C program. It will still be a buggy program however - my point is that there is very little C++ in your program.

The problem would be solved if you did fully embrace C++. In modern C++, you don't use [] arrays for anything (except in certain very special cases). If you want a collection of characters, where you can add and remove characters at will, then just use a string .

#include <string>
using std::string; 

int main(int argc, char * argv[]){
  if(argc != 2) return -1;
  string command = "raspistill -o ";
  command.append(argv[1]);
  system(command.c_str()); // unfortunately, system() doesn't accept string
                           // directly, must convert to pointer with .c_str()
  return 0;
}

You don't need to delete in well written (modern) C++. In fact, it would be a serious bug to attempt to delete command (or delete &command ) here.

Don't use pointers (except std::shared_ptr and std::unique_ptr where you really want the 'action-at-a-distance' behaviour). Don't use new , and don't use delete .

Sorry if this doesn't answer your original question directly. But I feel that just jumping straight to C++ is the best approach in situations like this.

Answer 2

You are trying to invoke strcat on a char[] to which you assigned a constant string literal. This will fail. Also, there is no reason for you to delete[] the aforementioned command , as you are not dynamically allocating it on the free store (eg using new[], malloc, etc.)

Edit: The following was relevant to a previous revision of the question

You're missing the new[] to go with the delete[] . The code you just posted shouldn't even be compiling, as you do not define name anywhere.

Answer 3

Buffer overrun I presume. command is not long enough. You're corrupting the stack. And worse you're using delete on memory that wasn't allocated on the heap.