#define ECHOMAX         255     /* Longest string to echo */
#define TIMEOUT_SECS    2       /* Seconds between retransmits */
#define MAXTRIES        5       /* Tries before giving up */
int tries=0;   /* Count of times sent - GLOBAL for signal-handler access */
void CatchAlarm(int ignored)     /* Handler for SIGALRM */
{
    tries += 1;
}
static void DieWithError(char *errorMessage)
{
    perror(errorMessage);
    exit(1);
}
void TCP () {
    int sock;                        /* Socket descriptor */
    struct sockaddr_in echoServAddr; /* Echo server address */
    struct sockaddr_in fromAddr;     /* Source address of echo */
    unsigned short echoServPort;     /* Echo server port */
    unsigned int fromSize;           /* In-out of address size for recvfrom() */
    struct sigaction myAction;       /* For setting signal handler */
    char *servIP;                    /* IP address of server */
    char *echoString;                /* String to send to echo server */
    char echoBuffer[ECHOMAX+1];      /* Buffer for echo string */
    int echoStringLen;               /* Length of string to echo */
    int respStringLen;               /* Size of received datagram */

//The IP is invalid since I need to test the function about timeout!!
    servIP = "192.168.1.254";           /* First arg:  server IP address (dotted quad) */
    echoString = "abc";       /* Second arg: string to echo */

    if ((echoStringLen = strlen(echoString)) > ECHOMAX)
        DieWithError("Echo word too long");

        echoServPort = atoi("12345");  /* Use given port, if any */

    /* Create a best-effort datagram socket using UDP */
    if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
        DieWithError("socket() failed");

    /* Set signal handler for alarm signal */
    myAction.sa_handler = CatchAlarm;
    if (sigfillset(&myAction.sa_mask) < 0) /* block everything in handler */
        DieWithError("sigfillset() failed");
    myAction.sa_flags = 0;

      if (sigaction(SIGALRM, &myAction, 0) < 0)
        DieWithError("sigaction() failed for SIGALRM");

    /* Construct the server address structure */
    memset(&echoServAddr, 0, sizeof(echoServAddr));    /* Zero out structure */
    echoServAddr.sin_family = AF_INET;
    echoServAddr.sin_addr.s_addr = inet_addr(servIP);  /* Server IP address */
    echoServAddr.sin_port = htons(echoServPort);       /* Server port */

    /* Send the string to the server */
    if (sendto(sock, echoString, echoStringLen, 0, (struct sockaddr *)
               &echoServAddr, sizeof(echoServAddr)) != echoStringLen)
        DieWithError("sendto() sent a different number of bytes than expected");

    /* Get a response */
    fromSize = sizeof(fromAddr);
    alarm(TIMEOUT_SECS);        /* Set the timeout */
  When I call this function from a thread, it is blocking here, and no response anymore!
    while ((respStringLen = recvfrom(sock, echoBuffer, ECHOMAX, 0,
           (struct sockaddr *) &fromAddr, &fromSize)) < 0) {
        if (errno == EINTR)     /* Alarm went off  */
        {
            if (tries < MAXTRIES)      /* incremented by signal handler */
            {
                printf("timed out, %d more tries...\n", MAXTRIES-tries);
                if (sendto(sock, echoString, echoStringLen, 0, (struct sockaddr *)
                            &echoServAddr, sizeof(echoServAddr)) != echoStringLen)
                    DieWithError("sendto() failed");
                alarm(TIMEOUT_SECS);
            }
            else
                DieWithError("No Response");
        }
        else
            DieWithError("recvfrom() failed");
    }

    /* recvfrom() got something --  cancel the timeout */
    alarm(0);

    /* null-terminate the received data */
    echoBuffer[respStringLen] = '\0';
    printf("Received: %s\n", echoBuffer);    /* Print the received data */

    close(sock);
    exit(0);
}

static void* testRun () {
    TCP();
    pthread_exit(NULL);
    return NULL;
}
int main ( void ) {
    pthread_t thread1;
    assert( pthread_create( &thread1, NULL, &testRun, NULL) == 0 );
    assert( pthread_join( thread1, NULL) == 0 );
    while ( 1 ) {}
}

 
int main ( void ) {
 TCP();
 while ( 1 ) {}
}