libdill: Structured Concurrency for C


udp_recvl - receives an UDP packet


#include <libdill.h>

ssize_t udp_recvl(
    int s,
    struct ipaddr* addr,
    struct iolist* first,
    struct iolist* last,
    int64_t deadline);


UDP is an unreliable message-based protocol defined in RFC 768. The size of the message is limited. The protocol has no initial or terminal handshake. A single socket can be used to different destinations.

This function receives a single UDP packet.

This function accepts a linked list of I/O buffers instead of a single buffer. Argument first points to the first item in the list, last points to the last buffer in the list. The list represents a single, fragmented message, not a list of multiple messages. Structure iolist has the following members:

void *iol_base;          /* Pointer to the buffer. */
size_t iol_len;          /* Size of the buffer. */
struct iolist *iol_next; /* Next buffer in the list. */
int iol_rsvd;            /* Reserved. Must be set to zero. */

When receiving, iol_base equal to NULL means that iol_len bytes should be skipped.

The function returns EINVAL error in the case the list is malformed:

The list (but not the buffers themselves) can be temporarily modified while the function is in progress. However, once the function returns the list is guaranteed to be the same as before the call.

s: Handle of the UDP socket.

addr: Out parameter. IP address of the sender of the packet. Can be NULL.

first: Pointer to the first item of a linked list of I/O buffers.

last: Pointer to the last item of a linked list of I/O buffers.

deadline: A point in time when the operation should time out, in milliseconds. Use the now function to get your current point in time. 0 means immediate timeout, i.e., perform the operation if possible or return without blocking if not. -1 means no deadline, i.e., the call will block forever if the operation cannot be performed.

This function is not available if libdill is compiled with --disable-sockets option.


In case of success the function returns size of the received message, in bytes. In case of error it returns -1 and sets errno to one of the values below.



struct ipaddr local;
ipaddr_local(&local, NULL, 5555, 0);
struct ipaddr remote;
ipaddr_remote(&remote, "", 5555, 0, -1);
int s = udp_open(&local, &remote);
udp_send(s1, NULL, "ABC", 3);
char buf[2000];
ssize_t sz = udp_recv(s, NULL, buf, sizeof(buf), -1);


mrecv(3) mrecvl(3) msend(3) msendl(3) now(3) udp_open(3) udp_open_mem(3) udp_recv(3) udp_send(3) udp_sendl(3)