At the end it was quite easy. I just used strace to find out how iwconfig gets the information about my current wireless settings. Below you can find the relevant part.

$ strace -v iwconfig wlan0
...
socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 3
ioctl(3, SIOCGIWNAME, 0x7fff88f9b130)   = 0
ioctl(3, SIOCGIWNWID, 0x7fff88f9b130)   = -1 EOPNOTSUPP (Operation not supported)
ioctl(3, SIOCGIWFREQ, 0x7fff88f9b130)   = 0
ioctl(3, SIOCGIWENCODE, 0x7fff88f9b130) = -1 EPERM (Operation not permitted)
ioctl(3, SIOCGIWESSID, 0x7fff88f9b130)  = 0
...

So, that is the ioctl call you have to invoke to get the information (and actually a bunch of other interface related information). Then, looking for the ioctl I found a great article here on the topic (http://www.linux-mag.com/id/6206/). Well, and just in case you get here actually looking for that snippet and you want to save one click to the article linked above... here it is the code.

#include <stdio.h>  // printf
#include <string.h> // strdup prototype
#include <stdlib.h> // free protype

/* socket systemcall */
#include <sys/types.h>
#include <sys/socket.h>


#include 

/* Wireless monitoring */
#include 

char *
get_essid (char *iface)
{
   int           fd;
   struct iwreq  w;
   char          essid[IW_ESSID_MAX_SIZE];

   if (!iface) return NULL;

   fd = socket(AF_INET, SOCK_DGRAM, 0);

   strncpy (w.ifr_ifrn.ifrn_name, iface, IFNAMSIZ);
   memset (essid, 0, IW_ESSID_MAX_SIZE);
   w.u.essid.pointer = (caddr_t *) essid;
   w.u.data.length = IW_ESSID_MAX_SIZE;
   w.u.data.flags = 0;

   ioctl (fd, SIOCGIWESSID, &w);
   close (fd);

   return strdup (essid);
}

int
main (int argc, char *argv[])
{
  char *essid = NULL;

  printf ("ESSID: %s\n", (essid = get_essid ("wlan0")));
  free (essid); // Remember you have to free the memory.... or change get_essid implementation :)

  return 0;
}

Hope it helps CU
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