example_msg_ctrunc.c

#include <sys/socket.h>
#include <linux/memfd.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>

void broadcaster( int socket )
{

    unsigned counter = 0;
    for( ;; )
    {
        int fd = memfd_create( "test_fd", MFD_CLOEXEC );

        char buffer[512];

        struct iovec iov =
        {
            .iov_base = &counter,
            .iov_len  = sizeof( counter )
        };

        struct msghdr hdr =
        {
            .msg_name = NULL,
            .msg_namelen = 0,
            .msg_iov = &iov,
            .msg_iovlen = 1,
            .msg_control = buffer,
            .msg_controllen = sizeof( buffer )
        };

        struct cmsghdr *cmsg = CMSG_FIRSTHDR( &hdr );
        cmsg->cmsg_level = SOL_SOCKET;
        cmsg->cmsg_type = SCM_RIGHTS;
        cmsg->cmsg_len = CMSG_LEN( sizeof( int ) );
        void *p_data = CMSG_DATA( cmsg );
        memcpy( p_data, &fd, sizeof( int ) );

        hdr.msg_controllen = cmsg->cmsg_len;

        ssize_t size = sendmsg( socket, &hdr, MSG_EOR );

        printf( "[BROADCAST] sent %dth file descriptor, which was %d\n",
                counter, fd );

        close( fd );

        counter ++;
    }
}

void receiver( int socket )
{
    unsigned counter = 0;
    unsigned packet_counter = 0;
    for( ;; )
    {
        char buffer[512];

        struct iovec iov =
        {
            .iov_base = &counter,
            .iov_len  = sizeof( counter )
        };

        struct msghdr hdr =
        {
            .msg_name = NULL,
            .msg_namelen = 0,
            .msg_iov = &iov,
            .msg_iovlen = 1,
            .msg_control = buffer,
            .msg_controllen = sizeof( buffer )
        };

        ssize_t size = recvmsg(socket, &hdr, 0);

        if( hdr.msg_flags & MSG_CTRUNC )
        {
            printf( "[RECEIVER] got MSG_CTRUNC at %dth packet\n",
                    packet_counter );
        }

        struct cmsghdr *cmsg = CMSG_FIRSTHDR( &hdr );

        while( cmsg != NULL )
        {

            if( cmsg->cmsg_level == SOL_SOCKET
                    && cmsg->cmsg_type == SCM_RIGHTS )
            {

                int fd;
                memcpy( &fd, CMSG_DATA( cmsg ),
                        sizeof( int ) );
                printf( "[RECEIVER] received fd in %dth packet, "
                        "marked %dth, with value %d\n",
                        packet_counter, counter, fd );
            }
            cmsg = CMSG_NXTHDR( &hdr, cmsg );
        }

        packet_counter ++;
    }
}

int main( int argc, char** argv )
{
    int sockets[2];

    {
        int ret = socketpair( AF_UNIX, SOCK_SEQPACKET, 0, sockets);
        if( ret != 0 )
            return -1;
    }

    {
        int ret = fork();
        if( ret > 0 )
            receiver(sockets[0]);
        else if( ret == 0 )
            broadcaster(sockets[1]);
        else
            return -1;
    }

    return 0;
}