Sending file descriptors over unix domain sockets

receiver.c

#include <err.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <sys/un.h>
#include <unistd.h>

int main() {
    int sock = socket(AF_UNIX, SOCK_SEQPACKET, 0);
    if (sock == -1)
        err(1, "socket() failed");

    struct sockaddr_un addr;
    socklen_t addr_len = sizeof addr;
    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, "send_fd.sock", sizeof addr.sun_path);
    if (connect(sock, (struct sockaddr *)&addr, addr_len) == -1)
        err(1, "connect() failed");

    int fd;
    char cmsg_buf[CMSG_SPACE(sizeof fd)];

    struct msghdr msg;
    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_iov = NULL;
    msg.msg_iovlen = 0;
    msg.msg_control = cmsg_buf;
    msg.msg_controllen = sizeof cmsg_buf;
    msg.msg_flags = 0;
    if (recvmsg(sock, &msg, 0) == -1)
        err(1, "recvmsg() failed");

    struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
    unsigned char *cmsg_data = CMSG_DATA(cmsg);
    fd = *(int *)cmsg_data;

    char second_half[] = "world!\n";
    if (write(fd, second_half, strlen(second_half)) == -1)
        err(1, "write() failed");

    exit(0);
}

sender.c

#include <err.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <sys/un.h>
#include <unistd.h>

int main() {
    int fd = open("file.txt", O_WRONLY|O_CREAT|O_TRUNC, 0644);
    if (fd == -1)
        err(1, "open() failed");

    char first_half[] = "Hello, ";
    if (write(fd, first_half, strlen(first_half)) == -1)
        err(1, "write() failed");

    int sock = socket(AF_UNIX, SOCK_SEQPACKET, 0);
    if (sock == -1)
        err(1, "socket() failed");

    unlink("send_fd.sock");
    struct sockaddr_un addr;
    socklen_t addr_len = sizeof addr;
    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, "send_fd.sock", sizeof addr.sun_path);
    if (bind(sock, (struct sockaddr *)&addr, addr_len) == -1)
        err(1, "bind() failed");

    if (listen(sock, 5) == -1)
        err(1, "listen() falied");

    struct sockaddr_un raddr;
    socklen_t raddr_len = sizeof raddr;
    int client = accept(sock, (struct sockaddr *)&raddr, &raddr_len);
    if (client == -1)
        err(1, "accept() failed");

    struct msghdr msg;
    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_iov = NULL;
    msg.msg_iovlen = 0;
    msg.msg_flags = 0;

    char cmsg_buf[CMSG_SPACE(sizeof fd)];
    msg.msg_control = cmsg_buf;
    msg.msg_controllen = sizeof cmsg_buf;

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

    unsigned char *cmsg_data = CMSG_DATA(cmsg);
    *(int *)cmsg_data = fd;

    msg.msg_controllen = cmsg->cmsg_len;

    if (sendmsg(client, &msg, 0) == -1)
        err(1, "sendmsg() failed");

    unlink("send_fd.sock");

    exit(0);
}