read() from UDP socket drops data and blocks unexpectedly in C - c

Question

I'm trying to read data from a UDP socket, but after reading the first 255 bytes, read() seems to drop the rest of the data on the socket and block until another data-gram comes in.
Here's the network code I'm using:
int sock;
struct sockaddr_in remote_addr, self_addr;
uint8_t network_init(uint16_t port)
{
memset((char *) &remote_addr, 0, sizeof(remote_addr));
remote_addr.sin_family = AF_INET;
remote_addr.sin_addr.s_addr = inet_addr("192.168.1.22");
remote_addr.sin_port = htons(3001);
memset((char *) &self_addr, 0, sizeof(self_addr));
self_addr.sin_family = AF_INET;
self_addr.sin_addr.s_addr = INADDR_ANY;
self_addr.sin_port = htons(3001);
if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
{
fprintf(stderr, "Could not create socket.");
return 1;
}
else if (bind(sock, (struct sockaddr *) &self_addr, sizeof(self_addr)) != 0)
{
fprintf(stderr, "Could not bind to socket.");
return 1;
}
return 0;
}
void network_send(uint8_t *data, uint8_t len)
{
sendto(sock, data, len, 0, (struct sockaddr *) &remote_addr, sizeof(remote_addr));
}
void read_data()
{
int len = 0;
ioctl(sock, FIONREAD, &len);
// We have data
if (len > 0)
{
char *buffer = (char *) malloc(256);
uint8_t buflen;
printf("==== %d | Data:\n", len);
while (len > 0)
{
buflen = min(255, len);
len = len - buflen;
buffer[buflen] = '\0';
printf("len: %d, buflen: %d,\n",len, buflen);
read(sock, buffer, buflen);
printf("%s\n", buffer);
}
printf("\n");
}
}
Here's the command I'm using to send data:
echo -n '12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567' | nc -u localhost 3001
And here's the output:
==== 257 | Data:
len: 2, buflen: 255,
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345
len: 0, buflen: 2,
^C
Also, after performing this read, ioctl(sock, FIONREAD, &len); produces a length result of 0. My suspicion is that for some reason, read() is clearing out the rest of the data before it has a chance to be read, but I can't seem to find any reference to this behaviour in any documentation.
I'm developing on an Ubuntu linux machine (x86_64).

Answer 1

With UDP sockets, each call to read() reads a whole datagram out of the kernel. If the read buffer isn't big enough for the entire datagram, the rest of it will be discarded. It's not like a stream socket, where you can keep calling until you get everything.
Since FIONREAD tells you the number of bytes in the message, you should use that as the size to malloc() rather than using 256:
if (len > 0) {
char *buffer = malloc(len);
...
P.S. Do I cast the result of malloc?