Unable to create a UDP client from thread?

[sehang]

I try to initial a UDP client from threading, but it doesn't work? why?


These codes from the textbook

#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);
}

These code does not work!

static void* testRun () {
    TCP();
    return NULL;
}
int main ( void ) {
    pthread_t tid;
    pthread_attr_t tx;
    pthread_attr_setdetachstate( &tx, PTHREAD_CREATE_DETACHED );
    pthread_attr_init( &tx );
    if ( pthread_create( &tid, &tx, testRun, NULL ) )
        exit( -1 );
  while ( 1 ) {}
  return 0;
}

The following codes work!!! WHY? WHY? I ready have no idea!

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

[Codeplug]

Returning from main() kills your process, including any threads.

You can join with the thread (and not create it detached). Or call pthread_exit() at the end of main().

gg

[sehang]

Returning from main() kills your process, including any threads.

You can join with the thread (and not create it detached). Or call pthread_exit() at the end of main().

gg

Sorry, I forgot put the code of forever loop at the main function, it should be here; and I will try to modify the thread by using join.

[Codeplug]

You should initialize "tx" before using it.

"testRun" has an incorrect signature.

gg

[sehang]

You should initialize "tx" before using it.

"testRun" has an incorrect signature.

gg

What does incorrect signature means? Thanks.

[sehang]

It still does not work! The following codes are modified, but no effect!

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 ) {}
    return 0;
}

[sehang]

It properly about the function sigaction() with threading, is does not work, why signal and threading is conflict here?

[Codeplug]

>> What does incorrect signature means?
A thread function returns a void* and takes a single void*.
http://pubs.opengroup.org/onlinepubs...ad_create.html

>> ... sigaction() with threading, ...
Read this and the example code: http://pubs.opengroup.org/onlinepubs...d_sigmask.html
Additional information: http://pubs.opengroup.org/onlinepubs...html#tag_15_04

gg

[sehang]

Hi, I still have no idea what does 'A thread function returns a void* and takes a single void*.'.

1.
Is it means, I should use

pthread_create( &thread1, NULL, &testRun, NULL) == 0

instead of

pthread_create( &thread1, NULL, testRun, NULL) == 0

or

Using

void* testRun () {
     ...
    return (void*)NULL;
}

2. Where should I put the signalwait function, as the example you gave, every thread should has own signalwait function, put when I using UDP and call recvfrom(...), is the signalwait function should be placed before the recvfrom(...) function? That is,

    int       sig_caught;    /* signal caught       */
    int       rc;            /* returned code       */


    rc = sigwait (&signal_mask, &sig_caught);
    if (rc != 0) {
        /* handle error */
    }
    switch (sig_caught)
    {
    case SIGINT:     /* process SIGINT  */
        ...
        break;
    case SIGTERM:    /* process SIGTERM */
        ...
        break;
    default:         /* should normally not happen */
        fprintf (stderr, "\nUnexpected signal %d\n", sig_caught);
        break;
    }

    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");
    }
...

[Codeplug]

>> A thread function returns a void* and takes a single void*.
"void* testRun(void*)"

Just forget about using signals at all. Learn how to use use select() instead.
http://beej.us/guide/bgnet/

gg