Modify the content of char* str

Question

I'm trying to reverse the string via void function, but segmentation fault occurs for "str" it's stored read-only memory (as I known from searching existing thread in the forum).

I tried to use strcpy(copy, str) in "reverse" function but it's just reversing the newly created char array "copy" which cannot be returned to main function. I don't want to use printf("%s", copy) in the reverse function to print the reversed string.

Is there anyway to reverse str

1. Without changing it to char str[] = "Hello"
2. Without changing the code in main() function
3. Keep the function type of reverse() as void?

void reverse(char* str)
{
  int l = strlen(str);
  char temp;
  int j = length - 1;

  for (int i = 0; i < j; ++i)
  {
     temp = str[i];
     str[i] = str[j];
     str[j] = temp;
     --j;
  }
}

int main(int argc, char* argv[])
{
   char* str = "Hello";
   reverse(str);
   printf("%s\n", str);
   return 0;
}

Answer 1

The problem is you cannot modify string literals.

Try like this:

char str[] = "Hello";
reverse(str);

above, str is no longer a string literal, it's an array initialized to {'H', 'e', 'l', 'l', 'o', '\0'}, so you can modify it's contents if you want.

Or you can use malloc()

char *str = malloc(6);
if (str != NULL)
 {
    strcpy(str, "Hello");
    reverse(str);
    fprintf(stdout, "%s\n", str);
    /* And never forget to `free' */ 
    free(str);
 }

When defining string literals use const to protect it from being modified, at least from accidentally doing so.

There is no way to satisfy these requirements

1. Without changing it to char str[] = "Hello"

   Because you cannot modify string literals

2. Without changing the code in main() function

   Because that would require the program to modify a string literal and YOU CAN'T

The third requirement, does not impose any problem at all.

Answer 2

everytime you use a pointer that you have to use a malloc() in C or new() in C++ which could probably be the issue. but you can always cast de-reference anything to its address back to a pointer like i did below

DEMO: http://ideone.com/LnOtDL

#include <string.h>

void reverse(char* str)
{
  int length = strlen(str);
  char temp;
  int j = length - 1;

  for (int i = 0; i < j; ++i)
  {
     temp = str[i];
     str[i] = str[j];
     str[j] = temp;
     --j;
  }
}

int main(int argc, char* argv[])
{
   char str[] = "Hello";
   reverse(&str[0]);
   printf("%s\n", str);
   return 0;
}