This is sockets/scm_multi_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_multi_recv.c

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/* scm_multi_recv.c

   Used in conjunction with scm_multi_send.c to demonstrate passing of
   ancillary data containing multiple 'msghdr' structures on a UNIX
   domain socket.

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

   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.
#define _GNU_SOURCE
#include "scm_multi.h"

#define BUF_SIZE 100
main(int argc, char *argv[])
    /* Allocate a buffer of suitable size to hold the ancillary data.
       This buffer is in reality treated as a 'struct cmsghdr',
       and so needs to be suitably aligned: malloc() provides a block
       with suitable alignment. */

    size_t controlMsgSize = CMSG_SPACE(sizeof(int[MAX_FDS])) +
                            CMSG_SPACE(sizeof(struct ucred));
    char *controlMsg = malloc(controlMsgSize);
    if (controlMsg == NULL)

    /* Parse command-line options */

    bool useDatagramSocket = false;
    int optControlMsgSize = -1;
    int opt;

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

        case 'n':
            optControlMsgSize = atoi(optarg);

            usageErr("%s [-d]\n"
                     "        -d           use datagram socket\n"
                     "        -n nbytes    limit on size of received "
                                          "ancillary data\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)

    /* We must set the SO_PASSCRED socket option in order to receive
       credentials */

    int optval = 1;
    if (setsockopt(sfd, SOL_SOCKET, SO_PASSCRED, &optval, sizeof(optval)) == -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.

       The 'optControlMsgSize' value (specified as a command-line option)
       can be used to artificially limit the size of the received ancillary
       data. This can be used to demonstrate that when the buffer size is
       too small, the list of received file descriptors is truncated, and
       the excess file descriptors are automatically closed. */

    msgh.msg_control = controlMsg;
    msgh.msg_controllen = (optControlMsgSize == -1) ?
                          controlMsgSize : optControlMsgSize;

    /* 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);

    if (optControlMsgSize != -1) {
        char cbuf[1000];

        /* Display this process's set of open file descriptors via
           /proc/PID/fd */

        snprintf(cbuf, sizeof(cbuf), "ls -l /proc/%ld/fd", (long) getpid());

    /* Check to see if the ancillary data was truncated */

    if (msgh.msg_flags & MSG_CTRUNC)
        printf("********** Ancillary data was truncated!!! **********\n");

    /* Walk through the series of headers in the ancillary data */

    for (struct cmsghdr *cmsgp = CMSG_FIRSTHDR(&msgh);
             cmsgp != NULL;
             cmsgp = CMSG_NXTHDR(&msgh, cmsgp)) {

        printf("cmsg_len: %ld\n", (long) cmsgp->cmsg_len);

        /* Check that 'cmsg_level' is as expected */

        if (cmsgp->cmsg_level != SOL_SOCKET)
            fatal("cmsg_level != SOL_SOCKET");

        switch (cmsgp->cmsg_type) {

        case SCM_RIGHTS:        /* Header containing file descriptors */

            printf("SCM_RIGHTS: ");

            /* The number of file descriptors is the size of the control
               message block minus the size that would be allocated for
               a zero-length data block (i.e., the size of the 'cmsghdr'
               structure plus padding), divided by the size of a file
               descriptor */

            int fdCnt = (cmsgp->cmsg_len - CMSG_LEN(0)) / sizeof(int);
            printf("received %d file descriptors\n", fdCnt);

            /* Allocate an array to hold the received file descriptors,
               and copy file descriptors from cmsg into array */

            int *fdList;
            size_t fdAllocSize = sizeof(int) * fdCnt;
            fdList = malloc(fdAllocSize);
            if (fdList == NULL)

            memcpy(fdList, CMSG_DATA(cmsgp), fdAllocSize);

            /* For each of the received file descriptors, display the file
               descriptor number and read and display the file content */

            for (int j = 0; j < fdCnt; j++) {
                printf("--- [%d] Received FD %d\n", j, fdList[j]);

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

                    numRead = read(fdList[j], buf, BUF_SIZE);
                    if (numRead == -1)

                    if (numRead == 0)

                    write(STDOUT_FILENO, buf, numRead);

                if (close(fdList[j]) == -1)

        case SCM_CREDENTIALS:   /* Header containing credentials */

            /* Check validity of the 'cmsghdr' */

            if (cmsgp->cmsg_len != CMSG_LEN(sizeof(struct ucred)))
                fatal("cmsg data has incorrect size");

            /* The data in this control message block is a 'struct ucred' */

            struct ucred creds;
            memcpy(&creds, CMSG_DATA(cmsgp), sizeof(struct ucred));
            printf("SCM_CREDENTIALS: pid=%ld, uid=%ld, gid=%ld\n",
                        (long), (long) creds.uid, (long) creds.gid);

            fatal("Bad cmsg_type (%d)", cmsgp->cmsg_type);



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