scm_rights.c

#define _GNU_SOURCE
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static void send_fd(int sock, int fd)
{
    char buf[1] = {0};
    struct iovec iov = { .iov_base = buf, .iov_len = 1 };

    union {
        struct cmsghdr cm;
        char           ctrl[CMSG_SPACE(sizeof(int))];
    } cmsg_buf;

    struct msghdr msg = {
        .msg_iov        = &iov,
        .msg_iovlen     = 1,
        .msg_control    = cmsg_buf.ctrl,
        .msg_controllen = sizeof(cmsg_buf.ctrl),
    };

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

    if (sendmsg(sock, &msg, 0) < 0) { perror("sendmsg"); exit(1); }
}

static int recv_fd(int sock)
{
    char buf[1];
    struct iovec iov = { .iov_base = buf, .iov_len = 1 };

    union {
        struct cmsghdr cm;
        char           ctrl[CMSG_SPACE(sizeof(int))];
    } cmsg_buf;

    struct msghdr msg = {
        .msg_iov        = &iov,
        .msg_iovlen     = 1,
        .msg_control    = cmsg_buf.ctrl,
        .msg_controllen = sizeof(cmsg_buf.ctrl),
    };

    if (recvmsg(sock, &msg, 0) < 0) { perror("recvmsg"); exit(1); }

    struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
    if (!cmsg || cmsg->cmsg_type != SCM_RIGHTS) {
        fprintf(stderr, "no fd received\n"); exit(1);
    }
    int fd;
    memcpy(&fd, CMSG_DATA(cmsg), sizeof(int));
    return fd;
}

int main(void)
{
    int sv[2];
    if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) < 0) {
        perror("socketpair"); return 1;
    }

    int orig = open("/etc/hostname", O_RDONLY);
    if (orig < 0) { perror("open"); return 1; }

    pid_t pid = fork();
    if (pid < 0) { perror("fork"); return 1; }

    if (pid == 0) {
        /* child: receive the fd and read from it, no knowledge of path */
        close(sv[0]);
        close(orig);
        int fd = recv_fd(sv[1]);
        printf("[child] received fd %d\n", fd);

        char rbuf[128] = {0};
        ssize_t n = read(fd, rbuf, sizeof(rbuf) - 1);
        if (n > 0) printf("[child] content: %s", rbuf);
        close(fd);
        close(sv[1]);
        return 0;
    }

    /* parent: send the fd then close its own copy */
    close(sv[1]);
    send_fd(sv[0], orig);
    printf("[parent] sent fd %d (/etc/hostname)\n", orig);
    close(orig);
    close(sv[0]);

    int status;
    waitpid(pid, &status, 0);
    return 0;
}