You should set length properly first before using werq,
check this,
int sockfd;
char * id;
id = new char[IW_ESSID_MAX_SIZE+1];
struct iwreq wreq;
memset(&wreq, 0, sizeof(struct iwreq));
wreq.u.essid.length = IW_ESSID_MAX_SIZE+1;
sprintf(wreq.ifr_name, IW_INTERFACE);
if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
fprintf(stderr, "Cannot open socket \n");
fprintf(stderr, "errno = %d \n", errno);
fprintf(stderr, "Error description is : %s\n",strerror(errno));
exit(1);
}
printf("\nSocket opened successfully \n");
wreq.u.essid.pointer = id;
if (ioctl(sockfd,SIOCGIWESSID, &wreq) == -1) {
fprintf(stderr, "Get ESSID ioctl failed \n");
fprintf(stderr, "errno = %d \n", errno);
fprintf(stderr, "Error description : %s\n",strerror(errno));
exit(2);
}
printf("IOCTL Successfull\n");
printf("ESSID is %s\n", (char *)wreq.u.essid.pointer);