This is sockets/scm_rights_send.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is a supplementary file for Chapter 61.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


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/* scm_rights_send.c

   Used in conjunction with scm_rights_recv.c to demonstrate passing of
   file descriptors via a UNIX domain socket.

   This program sends a file descriptor to a UNIX domain socket.

   Usage is as shown in the usageErr() call below.

   File descriptors can be exchanged over stream or datagram sockets. This
   program uses stream sockets by default; the "-d" command-line option
   specifies that datagram sockets should be used instead.

   This program is Linux-specific.

   See also scm_multi_recv.c.
#include "scm_rights.h"
main(int argc, char *argv[])

    /* Allocate a char array of suitable size to hold the ancillary data.
       However, since this buffer is in reality a 'struct cmsghdr', use a
       union to ensure that it is aligned as required for that structure.
       Alternatively, we could allocate the buffer using malloc(), which
       returns a buffer that satisfies the strictest alignment
       requirements of any type. */

    union {
        char   buf[CMSG_SPACE(sizeof(int))];
                        /* Space large enough to hold an 'int' */
        struct cmsghdr align;
    } controlMsg;

    /* Parse command-line options */

    bool useDatagramSocket = false;
    int opt;

    while ((opt = getopt(argc, argv, "d")) != -1) {
        switch (opt) {
        case 'd':
            useDatagramSocket = true;

            usageErr("%s [-d] file\n"
                     "        -d    use datagram socket\n", argv[0]);

    /* 'optind' is a global variable that is updated by getopt(). Upon
       completion of option processing, it contains the index of the next word
       in the command line following the options and option arguments. */

    if (argc != optind + 1)
        usageErr("%s [-d] file\n", argv[0]);

    /* Open the file named on the command line */

    int fd = open(argv[optind], O_RDONLY);
    if (fd == -1)

    /* The 'msg_name' field can be used to specify the address of the
       destination socket when sending a datagram. However, we do not
       need to use this field because we use connect() below, which sets
       a default outgoing address for datagrams. */

    struct msghdr msgh;
    msgh.msg_name = NULL;
    msgh.msg_namelen = 0;

    /* On Linux, we must transmit at least 1 byte of real data in
       order to send ancillary data */

    struct iovec iov;
    int data = 12345;

    msgh.msg_iov = &iov;
    msgh.msg_iovlen = 1;
    iov.iov_base = &data;
    iov.iov_len = sizeof(data);
    printf("Sending data = %d\n", data);

    /* Set 'msgh' fields to describe the ancillary data buffer */

    msgh.msg_control = controlMsg.buf;
    msgh.msg_controllen = sizeof(controlMsg.buf);

    /* The control message buffer must be zero-initialized in order
       for the CMSG_NXTHDR() macro to work correctly. Although we
       don't need to use CMSG_NXTHDR() in this example (because
       there is only one block of ancillary data), we show this
       step to demonstrate best practice */

    memset(controlMsg.buf, 0, sizeof(controlMsg.buf));

    /* Set message header to describe the ancillary data that
       we want to send */

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

    /* Connect to the peer socket */

    int sfd = unixConnect(SOCK_PATH,
                          useDatagramSocket ? SOCK_DGRAM : SOCK_STREAM);
    if (sfd == -1)

    printf("Sending FD %d\n", fd);

    /* Send real plus ancillary data */

    ssize_t ns = sendmsg(sfd, &msgh, 0);
    if (ns == -1)

    printf("sendmsg() returned %zd\n", ns);

    /* Once the file descriptor has been sent, it is no longer necessary
       to keep it open in the sending process */

    if (close(fd) == -1)



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