C pointers

February 12, 2021 | c, programming

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For understanding how pointers work, consider the following simplified memory layout. the memory address starts in 0x1 and goes up to address 0xA. The value row holds the current data stored in that memory cell. Address 0x1 holds a pointer to address 0x6, which in turn 0x6 holds the value s.

Address	0x1	0x2	0x3	0x4	0x5	0x6	0x7	0x8	0x9	0xA
Value	`0x6`					s	e	d	s	\0
Variable name	`hostname`					`hostname`	`hostname + 1`	`hostname + 2`	`hostname + 3`	`hostname +4`

.. which we can translate to:

int main() {
  char *hostname = "seds"; /* hostname points to 0x6 */
  printf("&hostname is '%p', *hostname is '%c' and hostname is '%s'\n",
         &hostname, *hostname, hostname);

  printf("hostname address is '%p' and hostname[0] address is %p\n\n", hostname,
         &hostname[0]);
  printf("hostname + 1 address is '%p' and hostname[1] address is %p\n",
         hostname + 1, &hostname[1]);

  printf("hostname + 2 address is '%p' and hostname[2] address is %p\n",
         hostname + 2, &hostname[2]);

  printf("hostname + 3 address is '%p' and hostname[3] address is %p\n",
         hostname + 3, &hostname[3]);

  printf("hostname + 4 address is '%p' and hostname[3] address is %p\n",
         hostname + 4, &hostname[4]);
}

&hostname is '0x16b666398', *hostname is 's' and hostname is 'seds'
hostname address is '0x10479be3c' and hostname[0] address is 0x10479be3c

hostname + 1 address is '0x10479be3d' and hostname[1] address is 0x10479be3d
hostname + 2 address is '0x10479be3e' and hostname[2] address is 0x10479be3e
hostname + 3 address is '0x10479be3f' and hostname[3] address is 0x10479be3f
hostname + 4 address is '0x10479be40' and hostname[3] address is 0x10479be40

int main() {
  char *hostname = "seds"; /* hostname points to 0x6 */

  printf("Hostname is '%s'\n", hostname);
  printf("Hostname first char '%c'\n", *hostname);
  printf("Hostname second char '%c'\n", *(hostname + 1));
  printf("Hostname third char '%c'\n", *(hostname + 2));
  printf("Hostname fourth char '%c'\n", *(hostname + 3));
  printf("Hostname last char '%d'\n", *(hostname + 4));
  printf("Out of bound (garbage) '%c'\n", *(hostname + 5));
}

Hostname is 'seds'
Hostname first char 's'
Hostname second char 'e'
Hostname third char 'd'
Hostname fourth char 's'
Hostname last char '0'
Out of bound (garbage) 'H'

int main() {
  char *hostname = "seds"; /* hostname points to 0x6 */

  while (*hostname) {
    printf("hostname is '%c', at address %p \n", *hostname, hostname);
    hostname++;
  }
  printf("Hostname is out of bound: '%s', at address %p\n", hostname, hostname);
}

hostname is 's', at address 0x104b87f50
hostname is 'e', at address 0x104b87f51
hostname is 'd', at address 0x104b87f52
hostname is 's', at address 0x104b87f53
Hostname is out of bound: '', at address 0x104b87f54

C pointers

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