Question

i want a full example in C to using flags MSG_TRUNC , MSG_CTRUNC in UDP socket
and some explanation to these flags

recvmsg(udpSocket, &msg, flags);
if (msg.msg_flags & MSG_TRUNC)
printf("MSG_TRUNC\n");

thanks

Answer 1

MSG_TRUNC indicates that the buffer space provided for receiving was insufficient, so that some of the packet data were lost.
This flag is used when you want to discover how much data was truncated. The recvmsg function will set this flag (msg_flags) for the structure msghdr (used for sending UDP packets)

struct msghdr {
       void                  *msg_name;
       int                   msg_namelen;
       struct iovec          *msg_iov;
       __kernel_size_t       msg_iovlen;
       void                  *msg_control;
       __kernel_size_t       msg_controllen;
       unsigned              msg_flags;
};

You need to add this structs to your code:

struct msghdr msg;
struct iovec iov;

And a your receiving buffer:

char buffer[BUFFER_SIZE];

Init all this stuff:

iov.iov_base = buffer;
iov.iov_len = sizeof(buffer);

msg.msg_iov = iov;

With the maximum size of the buffer set and the flag MSG_TRUNC set, when you call:

recvmsg(udpSocket, &msg, flags);

The struct header will be populated after this call, and you can check if your received buffer was truncated or not with this:

if (msg.msg_flags & MSG_TRUNC)
printf("MSG_TRUNC\n");

EDIT : Using recvfrom

This is a sample on how to use recvfrom with flags:

char *buffer[BUFFER_MAX_SIZE]; 
struct sockaddr from;
socklen_t from_len;
int available_data;
int socket;

socket = socket(AF_INET, SOCK_DGRAM, 0);

available_data = recvfrom(socket, (char*)buffer,0, MSG_TRUNC, &from, &fromlen);
if(available_data > 0)
{
    if (available_data > BUFFER_MAX_SIZE) 
    {
        fprintf(stderr, "UDP Packet is bigger than expected\n"); 

        exit(EXIT_FAILURE);
    }

    else 
    { 
        /* Process receiving data */ 
    }
}
else
{
    /* Handle error */
}