Understanding the msghdr structure from sys/socket.h

I’m trying to understand the following members of the msghdr structure of the sys/socket.h lib.

• struct iovec *msg_iov scatter/gather array
• void *msg_control ancillary data, see below

It states below:

Ancillary data consists of a sequence of pairs, each consisting of a cmsghdr structure followed by a data array. The data array contains the ancillary data message, and the cmsghdr structure contains descriptive information that allows an application to correctly parse the data.

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I’m assuming the msghdr struct, contains the protocol-header information? if so… *msg_iov is the input/output “vector” of parameters in the request/response? and the *msg_control contains the response messages?

Answer

msg_iov is an array of input/output buffers with length msg_iovlen. Each member of this array contains a pointer to a data buffer and the size of the buffer. This is where the data to read/write lives. It allows you to read/write to an array of buffers which are not necessarily in contiguous memory regions.

msg_control points to a buffer of size msg_controllen that contains additional information about the packet. To read this field, you first need to declare a struct cmsghdr * (let’s call it cmhdr). You populate this by calling CMSG_FIRSTHDR() the first time, passing it the address of the msghdr struct, and CMSG_NXTHDR() each subsequent time, passing it the address of the msghdr struct and the current value of cmhdr.

From the msg_control, you can find interesting things like the destination IP of the packet (useful for multicast) and the contents of the TOS/DSCP byte in the IP header (useful for custom congestion control protocols), among others. In most cases, you’ll need to make a setsockopt call to enable receiving this data. In the examples given, the IP_PKTINFO and IP_TOS options need to be enabled.

See the cmsg(3) manpage for more details.

The source IP and port, are not in msg_control, but are in msg_name which expects a pointer to a struct sockaddr with length msg_namelen.

Here’s an example of how to use this:

struct msghdr mhdr;
struct iovec iov[1];
struct cmsghdr *cmhdr;
char control[1000];
struct sockaddr_in sin;
char databuf[1500];
unsigned char tos;

mhdr.msg_name = &sin
mhdr.msg_namelen = sizeof(sin);
mhdr.msg_iov = iov;
mhdr.msg_iovlen = 1;
mhdr.msg_control = &control;
mhdr.msg_controllen = sizeof(control);
iov[0].iov_base = databuf;
iov[0].iov_len = sizeof(databuf);
memset(databuf, 0, sizeof(databuf));
if ((*len = recvmsg(sock, &mhdr, 0)) == -1) {
    perror("error on recvmsg");
    exit(1);
} else {
    cmhdr = CMSG_FIRSTHDR(&mhdr);
    while (cmhdr) {
        if (cmhdr->cmsg_level == IPPROTO_IP && cmhdr->cmsg_type == IP_TOS) {
            // read the TOS byte in the IP header
            tos = ((unsigned char *)CMSG_DATA(cmhdr))[0];
        }
        cmhdr = CMSG_NXTHDR(&mhdr, cmhdr);
    }
    printf("data read: %s, tos byte = %02Xn", databuf, tos); 
}