Converting Strings using pointer to pointer

[VietMan04]

#include <stdio.h>

void cat_convert(char **);
int main() {
char dog[] = "Dog";
char **ptr;

*ptr = dog;
cat_convert(ptr);
printf("%s\n",dog);
return 0;
}

void cat_convert(char **str) {
*str = "Cat";
}

-My program is fairly straightforward, converting "Dog" to "cat" via a helper function, however I receive a Segmentation fault when I run it. I traced the error to the line " *ptr = dog ", however I don't know why that would give such an error. Help please?

[Abramus]

You're dereferencing uninitialized pointer ('ptr' must point somewhere before you can do '*ptr').

[kbw]

If you think of ptr as the address of a pointer to the start of a string, then *ptr = dog is dereferencing an unitialised pointer. You're then overwriting the memory at that unknown location.

A better way to do what you've attempted is:

#include <stdio.h>

void cat_convert(char **);

int main() {
  char dog[] = "Dog";
  char *ptr;

  ptr = dog;
  cat_convert(&ptr);
  printf("%s\n",dog);
  return 0;
}

void cat_convert(char **str) {
  *str = "Cat";
}