This is sockets/scm_cred_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 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_cred_send.c

  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* scm_cred_send.c

   Used in conjunction with scm_cred_recv.c to demonstrate passing of
   process credentials via a UNIX domain socket.

   This program sends credentials to a UNIX domain socket.

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

   Credentials 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_send.c.
#include "scm_cred.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(struct ucred))];
                        /* Space large enough to hold a ucred structure */
        struct cmsghdr align;
    } controlMsg;

    /* Parse command-line options */

    bool useDatagramSocket = false;
    bool noExplicitCreds = false;
    int opt;

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

        case 'n':
            noExplicitCreds = true;

            usageErr("%s [-d] [-n] [data [PID [UID [GID]]]]\n"
                    "        -d    use datagram socket\n"
                    "        -n    don't construct explicit "
                                  "credentials structure\n", argv[0]);

    /* 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 */

    int data = (argc > optind) ? atoi(argv[optind]) : 12345;
                    /* Data is optionally taken from command line */
    fprintf(stderr, "Sending data = %d\n", data);

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

    if (noExplicitCreds) {

        /* Don't construct an explicit credentials structure. (It is not
           necessary to do so, if we just want the receiver to receive
           our real credentials.) */

        printf("Not explicitly sending a credentials structure\n");
        msgh.msg_control = NULL;
        msgh.msg_controllen = 0;

    } else {

        /* 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(struct ucred));
        cmsgp->cmsg_level = SOL_SOCKET;
        cmsgp->cmsg_type = SCM_CREDENTIALS;

        /* Use sender's own PID, real UID, and real GID, unless
           alternate values were supplied on the command line */

        struct ucred creds; = getpid();
        if (argc > optind + 1 && strcmp(argv[optind + 1], "-") != 0)
   = atoi(argv[optind + 1]);

        creds.uid = getuid();
        if (argc > optind + 2 && strcmp(argv[optind + 2], "-") != 0)
            creds.uid = atoi(argv[optind + 2]);

        creds.gid = getgid();
        if (argc > optind + 3 && strcmp(argv[optind + 3], "-") != 0)
            creds.gid = atoi(argv[optind + 3]);

        printf("Send credentials pid=%ld, uid=%ld, gid=%ld\n",
                (long), (long) creds.uid, (long) creds.gid);

        /* Copy 'ucred' structure into data field in the 'cmsghdr' */

        memcpy(CMSG_DATA(cmsgp), &creds, sizeof(struct ucred));

    /* Connect to the peer socket */

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

    /* Send real plus ancillary data */

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

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



Download sockets/scm_cred_send.c

Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.

Valid XHTML 1.1