C pointers

C pointers

February 12, 2021
C, programming

tags
Programming Computer Science

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.

Address0x10x20x30x40x50x60x70x80x90xA
Value0x6seds\0
Variable namehostnamehostnamehostname + 1hostname + 2hostname + 3hostname +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 '0x7ffee5873288', *hostname is 's' and hostname is 'seds'
hostname address is '0x10a38fe42' and hostname[0] address is 0x10a38fe42

hostname + 1 address is '0x10a38fe43' and hostname[1] address is 0x10a38fe43
hostname + 2 address is '0x10a38fe44' and hostname[2] address is 0x10a38fe44
hostname + 3 address is '0x10a38fe45' and hostname[3] address is 0x10a38fe45
hostname + 4 address is '0x10a38fe46' and hostname[3] address is 0x10a38fe46
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 0x107f8ef5a
hostname is 'e', at address 0x107f8ef5b
hostname is 'd', at address 0x107f8ef5c
hostname is 's', at address 0x107f8ef5d
Hostname is out of bound: '', at address 0x107f8ef5e