scm_rights

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/wait.h>

#define pexit(msg) do { perror(msg); exit(1); } while (0)

static int sendfd(int sockfd, int fd)
{
    struct msghdr msg = { 0 };
    struct cmsghdr *cmsg;
    int err;
    char iobuf;
    struct iovec io = {
        .iov_base = &iobuf,
        .iov_len = sizeof(iobuf),
    };
    union {
        char buf[CMSG_SPACE(sizeof(fd))];
        struct cmsghdr align;
    } u;

    msg.msg_iov = &io;
    msg.msg_iovlen = 1;
    msg.msg_control = u.buf;
    msg.msg_controllen = sizeof(u.buf);
    cmsg = CMSG_FIRSTHDR(&msg);
    cmsg->cmsg_level = SOL_SOCKET;
    cmsg->cmsg_type = SCM_RIGHTS;
    cmsg->cmsg_len = CMSG_LEN(sizeof(fd));

    /* Send the file descriptor with an empty message
     * containing only the ancillary data. */
    msg.msg_iov = NULL;
    msg.msg_iovlen = 0;

    memcpy(CMSG_DATA(cmsg), &fd, sizeof(fd));

    err = sendmsg(sockfd, &msg, 0);
    if (err < 0)
        pexit("sendmsg");

    if (err != msg.msg_iovlen)
        pexit("sendmsg");

    return 0;
}

static int recvfd(int sockfd)
{
    struct msghdr msg = { 0 };
    struct cmsghdr *cmsg;
    int err;
    int fd;
    union {
        char buf[CMSG_SPACE(sizeof(fd))];
        struct cmsghdr align;
    } u;

    msg.msg_control = u.buf;
    msg.msg_controllen = sizeof(u.buf);

    err = recvmsg(sockfd, &msg, 0);
    if (err < 0)
        pexit("recvmsg");

    cmsg = CMSG_FIRSTHDR(&msg);
    if (!cmsg ||
        cmsg->cmsg_len != CMSG_LEN(sizeof(fd)) ||
        cmsg->cmsg_level != SOL_SOCKET ||
        cmsg->cmsg_type != SCM_RIGHTS)
        pexit("Invalid cmsg");

    memcpy(&fd, CMSG_DATA(cmsg), sizeof(fd));

    return fd;
}

int main(void)
{
    int scm_fds[2], r;

    r = socketpair(AF_UNIX, SOCK_STREAM, 0, scm_fds);
    if (r < 0)
        pexit("socketpair");

    if (fork()) {
        int fd = open("/", O_PATH | O_DIRECTORY);

        if (fd < 0)
            pexit("open");

        r = sendfd(scm_fds[0], fd);
        if (r < 0)
            pexit("sendfd");

        puts("sent fd");

    } else {
        int fd = recvfd(scm_fds[1]);

        if (fd < 0)
            pexit("recvfd");

        puts("received fd");

        exit(0);
    }

    wait(NULL);
    puts("done");

    return 0;
}