liblockdown is a front end to communicate with the lockdown server (lockdownd). This front end can query, update or remove values in lockdownd. These values include system values such as the IMEI, UDID, ECID, etc.
Example
LockdownConnectionRef connection = lockdown_connect();
CFNumberRef totalBytes = lockdown_copy_value(connection, kLockdownDiskUsageDomainKey, kLockdownTotalDiskCapacityKey);
lockdown_disconnect(connection);
float totalBytes_float;
CFNumberGetValue(totalBytes, kCFNumberFloatType, &totalBytes_float);
printf("Size of your device = %.2g GB", totalBytes_float*1.0e-9);
References