While the existing documentation is good, there are a couple things that have changed since it was originally written, and I had to make a couple minor adjustments to get it to work. Here’s the code to send a message. The receive part should work as originally published; what is important is the set of headers. I built and ran this on an AARCH64 platform running Fedora 38.

I split the code into a header and a .c file for reasons I will address in another article.

spdmcli.h

#include <stdint.h>

#define MCTP_TAG_OWNER 0x08

struct mctp_addr {
    uint8_t             s_addr;
};

struct sockaddr_mctp {
    uint16_t            smctp_family;
    uint32_t            smctp_network;
    struct mctp_addr    smctp_addr;
    uint8_t             smctp_type;
    uint8_t             smctp_tag;
}

I also coded this for spdm, not MCTP control messages. The difference is the value for addr.smctp_type

EDIT: I also added the recvfrom call to make this complete.

spdmcli.c

#include <linux/mctp.h>
#endif
#include <linux/if_link.h>
#include <linux/rtnetlink.h>

#include "stdio.h"
#include "spdmcli.h"

int main(void)
{
    struct sockaddr_mctp addr = { 0 };
    char buf[] = "hello, world!";
    int sd, rc;

    /* create the MCTP socket */
    sd = socket(AF_MCTP, SOCK_DGRAM, 0);
    if (sd < 0)
        err(EXIT_FAILURE, "socket() failed");

    /* populate the remote address information */
    addr.smctp_family = AF_MCTP;  /* we're using the MCTP family */
    addr.smctp_addr.s_addr = 0x32;   /* send to remote endpoint ID 32 */
    addr.smctp_type = 5;          /* encapsulated protocol type iSPDM = 5 */
    addr.smctp_tag = MCTP_TAG_OWNER; /* we own the tag, and so the kernel
                                        will allocate one for us */

    addr.smctp_network = 1 ;
    /* send the MCTP message */
    rc = sendto(sd, buf, sizeof(buf), 0,
                (struct sockaddr *)&addr, sizeof(addr));

    if (rc != sizeof(buf))
        err(EXIT_FAILURE, "sendto() failed");

   //EDIT: The following code recieves a message from the network and prints it to the console.
        if (rc < 0){
        err(EXIT_FAILURE, "recvfrom() failed");
    }

    printf("recv rc = %d\n",rc);        

    for (int i =0; i< rc; i++){
        printf("%x", rxbuf[i]); 

    }



    return EXIT_SUCCESS;
}