/* Allocate buffer for control data. The buffer will be of size zero, however,   to minimise number of (re)allocations you should specify the capacity in   such a way as o fit all the ancillary data. Returns 0 in case of success,   -1 if the buffer cannot be allocated. */void msg_control_init (struct msghdr *hdr, size_t capacity);/* Deallocate control data. */void msg_control_term (struct msghdr *hdr);/* Return property specified by level and type. If the size of the property   is known in advance, 'size' may be set to NULL. Returns 0 in case of success,   -1 if the property in question is not present. */int msg_control_get (struct msghdr *hdr,     int level, int type, void **data, size_t *size);/* Add the property specified by level and type to the buffer. Returns 0 in case   of success, -1 if additional space in the buffer cannot be allocated. */int msg_control_set (struct msghdr *hdr,     int level, int type, void *data, size_t size);/* Remove the property specified by level and type from the buffer. Returns 0   in case of success, -1 if the property in question is not present. */int msg_control_rm (struct msghdr *hdr, int level, int type);/* Use this function instead of sendmsg(2). */ssize_t msg_control_send (int fd, struct msghdr *hdr, int flags);/* Use this function instead of recvmsg(2). */ssize_t msg_control_recv (int fd, struct msghdr *hdr, int *flags);