This is sockets/ud_ucase_sv.c (Listing 57-6, page 1172), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* ud_ucase_sv.c

   A server that uses a UNIX domain datagram socket to receive datagrams,
   convert their contents to uppercase, and then return them to the senders.

   See also ud_ucase_cl.c.
#include "ud_ucase.h"
main(int argc, char *argv[])
    struct sockaddr_un svaddr, claddr;
    int sfd, j;
    ssize_t numBytes;
    socklen_t len;
    char buf[BUF_SIZE];

    sfd = socket(AF_UNIX, SOCK_DGRAM, 0);       /* Create server socket */
    if (sfd == -1)

    /* Construct well-known address and bind server socket to it */

    /* For an explanation of the following check, see the erratum note for
       page 1168 at */

    if (strlen(SV_SOCK_PATH) > sizeof(svaddr.sun_path) - 1)
        fatal("Server socket path too long: %s", SV_SOCK_PATH);

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

    memset(&svaddr, 0, sizeof(struct sockaddr_un));
    svaddr.sun_family = AF_UNIX;
    strncpy(svaddr.sun_path, SV_SOCK_PATH, sizeof(svaddr.sun_path) - 1);

    if (bind(sfd, (struct sockaddr *) &svaddr, sizeof(struct sockaddr_un)) == -1)

    /* Receive messages, convert to uppercase, and return to client */

    for (;;) {
        len = sizeof(struct sockaddr_un);
        numBytes = recvfrom(sfd, buf, BUF_SIZE, 0,
                            (struct sockaddr *) &claddr, &len);
        if (numBytes == -1)

        printf("Server received %ld bytes from %s\n", (long) numBytes,

        for (j = 0; j < numBytes; j++)
            buf[j] = toupper((unsigned char) buf[j]);

        if (sendto(sfd, buf, numBytes, 0, (struct sockaddr *) &claddr, len) !=


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