What is the difference between char **ptr and char *ptr[] in C?

Question

#include<stdio.h>
void display(int n, char *str[])
{
  int i=0;
  while(i<n) printf("%s ",str[i++]);
}
int main()
{
  display(1,"Hello");return 0;
}

when I run this above code I get warnings as

    arr.c: In function 'main':
    arr.c:11:12: warning: passing argument 2 of 'display' from incompatible         pointer
    type
    display(1,"hello");
            ^
    arr.c:3:6: note: expected 'char **' but argument is of type 'char *'
    void display(int n,char *str[])

But then how is it different from

    int main(int argc, char * argv[])

And what is the difference between

    char **argv and char *argv[] 

I am strictly not asking about something like char *argv[100]

Answer 1

As a parameter of a function both char **ptr and char *ptr[] are equivalent, otherwise they are different. Former is a pointer to pointer to a char while latter is an array of pointers to char .

When a string literal is passed to a function, then pointer to its first character ( char * type) is passed. You need to change the function's second parameter to char *str .

void display(int n, char *str)  
{
    // Function body
}

Answer 2

Well, char **ptr is a double pointer (pointer to a pointer) of char while char *ptr[] is an open array of pointer to char.

According to cdel , char **p; gives the result of "declare p as pointer to pointer to char" while char *p[]; gives "declare p as array of pointer to char."

Character strings are always arrays, and arrays are generally always pointers, so the two are generally equivalent which means that **ptr = *ptr[] . What you end up with is an array of an array of chars, or an array of strings. Take main() for instance:

int main(int argc, char **argv)
  {
    int i;

    for (i = 0; i < argc; i++)
      {
        if (strcmp(argv[i], "some string") == 0) do something;
      }

    Do some stuff;

    return(0);
  }

Another way to declare it is

int main(int argc, char *argv[])

Programmatically, it's easier to understand *ptr[] than **ptr .