sendmsg

#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>


ssize_t read_fd(int fd, void *ptr, size_t nbytes, int *recvfd)
{
    struct msghdr   msg;
    struct iovec    iov[1];
    ssize_t         n;

    union
    {
      struct cmsghdr    cm;
      char              control[CMSG_SPACE(sizeof(int))];
    } control_un;
    struct cmsghdr  *cmptr;

    msg.msg_control = control_un.control;
    msg.msg_controllen = sizeof(control_un.control);

    msg.msg_name = NULL;
    msg.msg_namelen = 0;

    iov[0].iov_base = ptr;
    iov[0].iov_len = nbytes;
    msg.msg_iov = iov;
    msg.msg_iovlen = 1;

    if ( (n = recvmsg(fd, &msg, 0)) <= 0)
        return(n);

    if( (cmptr = CMSG_FIRSTHDR(&msg)) != NULL && cmptr->cmsg_len == CMSG_LEN(sizeof(int)) )
    {
        if (cmptr->cmsg_level != SOL_SOCKET)
            return -1;
        if (cmptr->cmsg_type != SCM_RIGHTS)
            return -1;
        *recvfd = *((int *) CMSG_DATA(cmptr));
    }
    else
    {
        *recvfd = -1;       /* descriptor was n ./server/read_fd.c ./server/write_fd.cot passed */
    }

    return(n);
}

ssize_t write_fd(int fd, void *ptr, size_t nbytes, int sendfd)
{
    struct msghdr msg;
    struct iovec iov[1];

    union
    {
        struct cmsghdr cm;
        char control[CMSG_SPACE(sizeof(int))];
    } control_un;
    struct cmsghdr* cmptr;

    msg.msg_control = control_un.control;
    msg.msg_controllen = sizeof(control_un.control);

    cmptr = CMSG_FIRSTHDR(&msg);
    cmptr->cmsg_len = CMSG_LEN(sizeof(int));
    cmptr->cmsg_level = SOL_SOCKET;
    cmptr->cmsg_type = SCM_RIGHTS;
    *((int *) CMSG_DATA(cmptr)) = sendfd;

    msg.msg_name = NULL;
    msg.msg_namelen = 0;

    iov[0].iov_base = ptr;
    iov[0].iov_len = nbytes;
    msg.msg_iov = iov;
    msg.msg_iovlen = 1;

    return(sendmsg(fd, &msg, 0));
}


int main()
{
    int socks[2];
    char dummy = 0;
    int myfd = -1;

    socketpair(AF_LOCAL, SOCK_STREAM, 0, socks);

    if (fork())
    {
        myfd = STDIN_FILENO;
        if (write_fd(*socks, &dummy, 1, myfd) < 1)
        {
            printf("epic fail a\n");
            abort();
        }
    }
    else
    {
        for (;;)
        {
            myfd = -1;
            if (read_fd(socks[1], &dummy, 1, &myfd) < 1)
            {
                printf("epic fail b\n");
                abort();
            }
            printf("myfd received: %i\n", myfd);
        }
    }

    return 0;
}