codebeaker-example.c

#include <stdio.h>
#include <stdlib.h>

struct config_struct {
  int port;
  char *hostname;
};

typedef struct config_struct config;

void setup(config*);
void change(config*);
void set_hostname(config*, char*);
void get_hostname_into(config*, char**);
void teardown(config*);
void inspect(config*);

enum Property { Port, Hostname };
void get_property(config*, enum Property, void*);
void get_property(config* c, enum Property p, void* target) {
  switch(p) {
    case Port:
      {
        int *port;
        port = (int *) target;
        *port = c->port;
      }
      break;
    case Hostname:
      {
        char *hostname;
        hostname = (char *) target;
        *hostname = c->hostname;
      }
      break;
  }
}

int main() {

  char* hostname;
  config* c;
  c = calloc( 1, sizeof(config));

  setup(c);
  inspect(c);

  change(c);
  inspect(c);

  set_hostname(c, "test.com");
  inspect(c);

  get_hostname_into(c, &hostname);
  inspect(c);  
  printf("retrieved hostname is %s (%p)\n", hostname, &hostname);

  int get_port_into_here = 0;
  get_property(c, Port, &get_port_into_here);
  printf("genericly retrieved port is %d (%p)\n", get_port_into_here, &get_port_into_here);

  char *get_hostname_into_here;
  get_property(c, Hostname, &get_hostname_into_here);
  printf("genericly retrieved hostname is %s (%p)\n", get_hostname_into_here, &get_hostname_into_here);

  teardown(c);
  printf("retrieved hostname is %s (%p) (after teardown)\n", hostname, &hostname);

  return EXIT_SUCCESS;
}

void setup(config* c) {
  c->port     = 9933;
  c->hostname = "localhost";
}

void change(config* c) {
  c->port     = 12345;
  c->hostname = "example.com";
}

void set_hostname(config* c, char* new_hostname) {
  c->hostname = new_hostname;
}

void get_hostname_into(config* c, char** where) {
  *where = c->hostname;
}

void teardown(config* c) {
  free(c);
}

void inspect(config* c) {
  printf("c is at %p\n", c);
  printf("c is %ld bytes\n", sizeof(*c));
  printf("c->port is %d (%p)\n", c->port, &(c->port));
  printf("c->hostname is %s (%p)\n", c->hostname, &(c->port));
}

output.log

untitled: In function ‘get_property’:
untitled:33: warning: assignment makes integer from pointer without a cast
c is at 0x1040008c0
c is 16 bytes
c->port is 9933 (0x1040008c0)
c->hostname is localhost (0x1040008c0)
c is at 0x1040008c0
c is 16 bytes
c->port is 12345 (0x1040008c0)
c->hostname is example.com (0x1040008c0)
c is at 0x1040008c0
c is 16 bytes
c->port is 12345 (0x1040008c0)
c->hostname is test.com (0x1040008c0)
c is at 0x1040008c0
c is 16 bytes
c->port is 12345 (0x1040008c0)
c->hostname is test.com (0x1040008c0)
retrieved hostname is test.com (0x7fff63b704c0)
genericly retrieved port is 12345 (0x7fff63b704b4)
genericly retrieved hostname is (null) (0x7fff63b704a8)
retrieved hostname is test.com (0x7fff63b704c0) (after teardown)