CMSG_FIRSTHDR - Linux
Overview
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.
Syntax
#include <sys/socket.h>
struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgp);
Options/Flags
None.
Examples
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.
Integration
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()
- CMSG_NXTHDR