This is sockets/scm_rights_recv.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 2022, 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.


Download sockets/scm_rights_recv.c

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/* scm_rights_recv.c

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

   This program receives a file descriptor sent 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"

#define BUF_SIZE 100
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]\n"
                     "        -d    use datagram socket\n", argv[0]);

    /* Create socket bound to a well-known address. In the case where
       we are using stream sockets, also make the socket a listening
       socket and accept a connection on the socket. */

    if (remove(SOCK_PATH) == -1 && errno != ENOENT)
        errExit("remove-%s", SOCK_PATH);

    int sfd;
    if (useDatagramSocket) {
        sfd = unixBind(SOCK_PATH, SOCK_DGRAM);
        if (sfd == -1)

    } else {
        int lfd = unixBind(SOCK_PATH, SOCK_STREAM);
        if (lfd == -1)

        if (listen(lfd, 5) == -1)

        sfd = accept(lfd, NULL, NULL);
        if (sfd == -1)

    /* The 'msg_name' field can be set to point to a buffer where the
       kernel will place the address of the peer socket. However, we don't
       need the address of the peer, so we set this field to NULL. */

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

    /* Set fields of 'msgh' to point to a buffer used to receive
       the (real) data read by recvmsg() */

    struct iovec iov;
    int data;

    msgh.msg_iov = &iov;
    msgh.msg_iovlen = 1;
    iov.iov_base = &data;
    iov.iov_len = sizeof(data);

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

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

    /* Receive real plus ancillary data */

    ssize_t nr = recvmsg(sfd, &msgh, 0);
    if (nr == -1)

    printf("recvmsg() returned %zd\n", nr);

    if (nr > 0)
        printf("Received data = %d\n", data);

    /* Get the address of the first 'cmsghdr' in the received
       ancillary data */

    struct cmsghdr *cmsgp = CMSG_FIRSTHDR(&msgh);

    /* Check the validity of the 'cmsghdr' */

    if (cmsgp == NULL || cmsgp->cmsg_len != CMSG_LEN(sizeof(int)))
        fatal("bad cmsg header / message length");
    if (cmsgp->cmsg_level != SOL_SOCKET)
        fatal("cmsg_level != SOL_SOCKET");
    if (cmsgp->cmsg_type != SCM_RIGHTS)
        fatal("cmsg_type != SCM_RIGHTS");

    /* The data area of the 'cmsghdr' is an 'int' (a file descriptor);
       copy that integer to a local variable. (The received file descriptor
       is typically a different file descriptor number than was used in the
       sending process.) */

    int fd;
    memcpy(&fd, CMSG_DATA(cmsgp), sizeof(int));
    printf("Received FD %d\n", fd);

    /* Having obtained the file descriptor, read the file's contents and
       print them on standard output */

    for (;;) {
        char buf[BUF_SIZE];
        ssize_t numRead;

        numRead = read(fd, buf, BUF_SIZE);
        if (numRead == -1)

        if (numRead == 0)

        write(STDOUT_FILENO, buf, numRead);



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