This is sockets/i6d_ucase_sv.c (Listing 59-3, page 1208), 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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/* i6d_ucase_sv.c

   A server that receives datagrams, converts their contents to uppercase, and
   then returns them to the senders.

   See also i6d_ucase_cl.c.
#include "i6d_ucase.h"
main(int argc, char *argv[])
    struct sockaddr_in6 svaddr, claddr;
    int sfd, j;
    ssize_t numBytes;
    socklen_t len;
    char buf[BUF_SIZE];
    char claddrStr[INET6_ADDRSTRLEN];

    /* Create a datagram socket bound to an address in the IPv6 domain */

    sfd = socket(AF_INET6, SOCK_DGRAM, 0);
    if (sfd == -1)

    memset(&svaddr, 0, sizeof(struct sockaddr_in6));
    svaddr.sin6_family = AF_INET6;
    svaddr.sin6_addr = in6addr_any;                     /* Wildcard address */
    svaddr.sin6_port = htons(PORT_NUM);

    if (bind(sfd, (struct sockaddr *) &svaddr,
                sizeof(struct sockaddr_in6)) == -1)

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

    for (;;) {
        len = sizeof(struct sockaddr_in6);
        numBytes = recvfrom(sfd, buf, BUF_SIZE, 0,
                            (struct sockaddr *) &claddr, &len);
        if (numBytes == -1)

        /* Display address of client that sent the message */

        if (inet_ntop(AF_INET6, &claddr.sin6_addr, claddrStr,
                    INET6_ADDRSTRLEN) == NULL)
            printf("Couldn't convert client address to string\n");
            printf("Server received %ld bytes from (%s, %u)\n",
                    (long) numBytes, claddrStr, ntohs(claddr.sin6_port));
            /*FIXME: above: should use %zd here, and remove (long) cast */

        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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