C/Pointers

A helper for people starting out with pointers:

test.c

#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'

Memory Map

...	...	...	...
style="color:red"| 0x08		INVALID (MSB)	This will probably be different when you run the program
0xBF893E92	0x04		INVALID	This will probably be different when you run the program
0xBF893E91	0x86		INVALID	This will probably be different when you run the program
0xBF893E90	0x80		INVALID (LSB)	This will probably be different when you run the program
0xBF893E8F	0x08		s (MSB)
0xBF893E8E	0x04		s
0xBF893E8D	0x87		s
0xBF893E8C	0x34		s (LSB)
0xBF893E8B	0xBF		p (MSB)
0xBF893E8A	0x89		p
0xBF893E89	0x3E		p
0xBF893E88	0x8C		p (LSB)
...	...	...	...
0x08048737	0x00	'\0'
0x08048736	0x63	'a'
0x08048735	0x62	'b'
0x08048734	0x63	'c'
...	...	...	...
0x08048680	0x55			This will probably be different when you run the program