C : reverse a string in place. I used the prog answered in an earlier question. It compiles ok but crashes with segmenation fault

Question

I call the function provided by Chris Conway (http://stackoverflow.com/questions/198199/how-do-you-reverse-a-string-in-place-in-c-or-c) from main (C code). When I run this program using cygwin, program crashes when it is in while loop (commented the lines where it breaks). Could you please explain what is going wrong here. Thanks

#include <stdio.h>
#include <string.h>

void strrev(char* z);

int main()
{
        char *a;
    printf("before reverse: %s\n", a);
        strrev(a); // function provided by Chris Conway
    printf("after reverse: %s\n", a);
    return 0;
}

void strrev(char *str) {
  char temp, *end_ptr;

  /* If str is NULL or empty, do nothing */
  if( str == NULL || !(*str) )
    return;

  end_ptr = str + strlen(str) - 1;

  /* Swap the chars */
  while( end_ptr > str ) {
    temp = *str;
    *str = *end_ptr;  //crashes here (cygwin gives segmentation fault)
    *end_ptr = temp;  //for testing, if I comment out line above, it crashes here
    str++;
    end_ptr--;
  }
}

Answer 1

The function is fine, but your main() does not appear to initialize the string a.

Try:

int main() {
  char a[1024];
  strcpy(a, "Some string");
  printf("before reverse: %s\n", a);
  strrev(a); // function provided by Chris Conway
  printf("after reverse: %s\n", a);
  return 0;
}

Note that I create a copy of "Some string" inside a (instead of directly assigning char* a = "Some String") because trying to alter a constant string directly will not compile. And if you did manage to compile (e.g. lax compiler, or you forced the constness away via cast/const_cast) then you run a very high risk of crashing your program, because "Some string" is actually in a portion of memory that is read-only on some systems, which is why a copy must be made inside a local variable (i.e. on the stack) or a new variable (allocated with new or malloc, i.e. on the heap.)