Cstring in C++ - using standard C functions leading to a segmentation fault

Question

for certain functions i want to create a copy of the string within the function and then manipulate this - for some strange reason, i cant get strcpy to work (gives me a segmentation fault) - i've also tried passing the arg as a string, this doesnt work either (g++ throws an error saying it expect a char*)

#include <iostream>
#include <cstring>

using namespace std;
void copy_string(char* stri);

int main ()
{
  copy_string("sample string");

  return 0;
}

void copy_string(char* stri) {
  char* stri_copy;

  strcpy(stri_copy, stri);

  cout << "String: " << stri_copy;

}

im not sure i understand why this is happening.

so my two questions are:

1. why this is occuring - is there an easy fix?
2. whats the simplest / most efficient way of creating a local copy of a string passed to a function?

thanks!

Answer 1

 char* stri_copy;

 stri_copy = (char*)malloc(strlen(stri) * sizeof(char) + 1); 
 strcpy(stri_copy, stri);

You aren't allocating space for stri_copy.

Answer 2

The pointer to stri_copy has not been malloc'd. Using strdup below will solve the problem and allocate the memory accordingly for the value of stri.

char* stri_copy;
stri_copy = strdup(stri);

Hope this helps, Best regards, Tom.

Answer 3

I don't know if this is what you're looking for but I got it to work with strings.

#include <iostream>
#include <cstring>
#include <string.h>
using namespace std;
void copy_string(string);

int main ()
{  
  copy_string("sample string");

  return 0;
}

void copy_string(string stri) {
  string cpy = stri;
  cout << "String: " << cpy;

}

Answer 4

char* stri_copy;
strcpy(stri_copy, stri);

The problem is that stri_copy does not point to valid memory. strcpy first parameter expects the proper memory location.

int len = strlen(stri);
char* stri_copy = new char[ len + 1];
strncpy(stri_copy, stri, len );
stri_copy[len] = '\0';

Answer 5

To use strcpy, you need a buffer of allocated memory as a target. Your stri_copy pointer is not pointing to such a buffer.

Answer 6

You have a segmentation fault because stri_copy does not point to valid memory.

If you can use the STL, then you can have a way to do this:

void copy_string(const std::string& stri) {
  char* stri_copy= stri.c_str();

  // work with the copy of the string

  std::cout << "String: " << stri_copy;

}

The std::string makes a copy of the string parameter and disposes of the copy for you after you've finished with it.

Edit: Used const std::string& as the parameter type.

Answer 7

strcpy does not allocate any storage to hold the result. You're using a "random" pointer as the destination which is why you are seeing segfaults. A very simple implementation of strcpy would look like

void naivestrcpy(char* destination, const char* source) {
   while(*source) *destination++ = *source++;
   *destination = 0;
}

strcpy does exactly what it says on the tin, it copies. It does not ensure you've loaded the right size of paper into the xerox tray.