C/Pointers

A helper for people starting out with pointers:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
 
#define P(expr) printf("%10s: 0x%02X\n", #expr, expr)
 
int main(void) {
	char *s = "cba";
	char **p = &s;
 
	P(      s      );
	P(     &s      );
 
	P(      s[0]   );
	P(     *s      );
 
	P(      p      );
	P(     &p      );
	P(      p[0]   );
	P(     *p      );
 
	P(      p[1]   );
 
	P(     *p[1]   );
	P(    (*p)[1]  );
	P(    *(p[1])  );
 
	P(    (*p)     );
	P(    (*p)++   );
	P(    (*p)[0]  );
	P(    (*p)     );
	P(  ++(*p)     );
	P(    (*p)[0]  );
	P(    (*p)     );
 
	return 0;
}

Output:

$ gcc test.c -o test && ./test
         s: 0x8048734      <-- the address of the 'c'
        &s: 0xBF893E8C     <-- the address of the variable s
      s[0]: 0x63           <-- the 'c'
        *s: 0x63           <-- the 'c'
         p: 0xBF893E8C     <-- the address of the variable s
        &p: 0xBF893E88     <-- the address of the variable p
      p[0]: 0x8048734      <-- the address of the 'c'
        *p: 0x8048734      <-- the address of the 'c'
      p[1]: 0x8048680      <-- INVALID - the value stored just after the variable p (the program may segfault here)
     *p[1]: 0x55           <-- INVALID - the value stored at location 0x8048680 (again, the program may segfault here)
   (*p)[1]: 0x62           <-- the 'b'
   *(p[1]): 0x55           <-- INVALID - the same as *p[1]
      (*p): 0x8048734      <-- the address of the 'c'
    (*p)++: 0x8048734      <-- the address of the 'c' - the pointer is POST incremented
   (*p)[0]: 0x62           <-- the 'b'
      (*p): 0x8048735      <-- the address of the 'b'
    ++(*p): 0x8048736      <-- the address of the 'a' - the pointer is PRE incremented
   (*p)[0]: 0x61           <-- the 'a'
      (*p): 0x8048736      <-- the address of the 'a'