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 '0x7ffeed0e2a28', *hostname is 's' and hostname is 'seds'
hostname address is '0x102b1fe42' and hostname[0] address is 0x102b1fe42

hostname + 1 address is '0x102b1fe43' and hostname[1] address is 0x102b1fe43
hostname + 2 address is '0x102b1fe44' and hostname[2] address is 0x102b1fe44
hostname + 3 address is '0x102b1fe45' and hostname[3] address is 0x102b1fe45
hostname + 4 address is '0x102b1fe46' and hostname[3] address is 0x102b1fe46

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 0x10c2cdf5a
hostname is 'e', at address 0x10c2cdf5b
hostname is 'd', at address 0x10c2cdf5c
hostname is 's', at address 0x10c2cdf5d
Hostname is out of bound: '', at address 0x10c2cdf5e

C pointers

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