This is sockets/is_seqnum_sv.c (Listing 59-6, page 1221), 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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/* is_seqnum_sv.c

   A simple Internet stream socket server. Our service is to provide
   unique sequence numbers to clients.

   Usage:  is_seqnum_sv [init-seq-num]
                        (default = 0)

   See also is_seqnum_cl.c.
#define _BSD_SOURCE             /* To get definitions of NI_MAXHOST and
                                   NI_MAXSERV from <netdb.h> */
#include <netdb.h>
#include "is_seqnum.h"

#define BACKLOG 50
main(int argc, char *argv[])
    uint32_t seqNum;
    char reqLenStr[INT_LEN];            /* Length of requested sequence */
    char seqNumStr[INT_LEN];            /* Start of granted sequence */
    struct sockaddr_storage claddr;
    int lfd, cfd, optval, reqLen;
    socklen_t addrlen;
    struct addrinfo hints;
    struct addrinfo *result, *rp;
    char addrStr[ADDRSTRLEN];
    char host[NI_MAXHOST];
    char service[NI_MAXSERV];

    if (argc > 1 && strcmp(argv[1], "--help") == 0)
        usageErr("%s [init-seq-num]\n", argv[0]);

    seqNum = (argc > 1) ? getInt(argv[1], 0, "init-seq-num") : 0;

    /* Ignore the SIGPIPE signal, so that we find out about broken connection
       errors via a failure from write(). */

    if (signal(SIGPIPE, SIG_IGN) == SIG_ERR)    errExit("signal");

    /* Call getaddrinfo() to obtain a list of addresses that
       we can try binding to */

    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_canonname = NULL;
    hints.ai_addr = NULL;
    hints.ai_next = NULL;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_family = AF_UNSPEC;        /* Allows IPv4 or IPv6 */
    hints.ai_flags = AI_PASSIVE | AI_NUMERICSERV;
                        /* Wildcard IP address; service name is numeric */

    if (getaddrinfo(NULL, PORT_NUM, &hints, &result) != 0)

    /* Walk through returned list until we find an address structure
       that can be used to successfully create and bind a socket */

    optval = 1;
    for (rp = result; rp != NULL; rp = rp->ai_next) {
        lfd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
        if (lfd == -1)
            continue;                   /* On error, try next address */

        if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval))
                == -1)

        if (bind(lfd, rp->ai_addr, rp->ai_addrlen) == 0)
            break;                      /* Success */

        /* bind() failed: close this socket and try next address */


    if (rp == NULL)
        fatal("Could not bind socket to any address");

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


    for (;;) {                  /* Handle clients iteratively */

        /* Accept a client connection, obtaining client's address */

        addrlen = sizeof(struct sockaddr_storage);
        cfd = accept(lfd, (struct sockaddr *) &claddr, &addrlen);
        if (cfd == -1) {

        if (getnameinfo((struct sockaddr *) &claddr, addrlen,
                    host, NI_MAXHOST, service, NI_MAXSERV, 0) == 0)
            snprintf(addrStr, ADDRSTRLEN, "(%s, %s)", host, service);
            snprintf(addrStr, ADDRSTRLEN, "(?UNKNOWN?)");
        printf("Connection from %s\n", addrStr);

        /* Read client request, send sequence number back */

        if (readLine(cfd, reqLenStr, INT_LEN) <= 0) {
            continue;                   /* Failed read; skip request */

        reqLen = atoi(reqLenStr);
        if (reqLen <= 0) {              /* Watch for misbehaving clients */
            continue;                   /* Bad request; skip it */

        snprintf(seqNumStr, INT_LEN, "%d\n", seqNum);
        if (write(cfd, seqNumStr, strlen(seqNumStr)) != strlen(seqNumStr))
            fprintf(stderr, "Error on write");

        seqNum += reqLen;               /* Update sequence number */

        if (close(cfd) == -1)           /* Close connection */


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