CMSG_FIRSTHDR is a Linux command primarily used to extract the first ancillary data header from a message buffer. It’s particularly useful for retrieving information from network communications, such as socket options or firewall and routing information.


#include <sys/socket.h>
struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgp);




Extract and print the first ancillary data header from a received message:

#include <sys/socket.h>
#include <stdio.h>

int main() {
  struct msghdr msg;
  struct cmsghdr *cmhdr;

  // Receive a message
  recvmsg(sock, &msg, 0);

  // Extract the first ancillary data header
  cmhdr = CMSG_FIRSTHDR(&msg);

  if (cmhdr != NULL) {
    printf("Ancillary data header type: %d\n", cmhdr->cmsg_type);
    printf("Ancillary data header length: %d\n", cmhdr->cmsg_len);
  } else {
    printf("No ancillary data headers found\n");

  return 0;

Common Issues

No ancillary data header found: This could occur if there is no ancillary data associated with the received message.

Accessing invalid memory: Ensure that the message buffer is valid and that the ancillary data header is within the buffer’s bounds.


With CMSG_NXTHDR: Use CMSG_NXTHDR to iterate through subsequent ancillary data headers in the message buffer.

With getsockopt() and setsockopt(): Use CMSG_FIRSTHDR with getsockopt() or setsockopt() to retrieve or set ancillary data on a socket.

Related Commands

  • getsockopt()
  • setsockopt()
  • recvmsg()