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How to use sigmask in order to make signals can be processed by a thread

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# 1  
Old 03-06-2011
How to use sigmask in order to make signals can be processed by a thread


I have a UDP server and client program, and they must run within a program, so I decided two threads, one for UDP server and another for UDP client.

The simple architecture is shown in attachment.

However, I can't send the packets out on the UDP client, no any time message and sigaction is triggered. Is any sigmask needs? If it is, then where should I put the mask?

The following are the source codes

The UDP server
static void SIGIOHandler( int signalType ) {
    struct sockaddr_in clntAddr;
    unsigned int clntLen;
    int recvMsgSize;
    UINT8 buf[ MSG_SIZE ];
    do {
        clntLen = sizeof( clntAddr );
        if ( ( recvMsgSize = recvfrom( srvSock, buf, MSG_SIZE, 0, ( struct sockaddr* )&clntAddr,&clntLen ) ) < 0 ) {
            if ( errno != EWOULDBLOCK ) {
                      ...Error handler
        } else {
            printf( "Handling client %s\n", inet_ntoa( clntAddr.sin_addr ) );
            if ( Sendto( srvSock, buf, recvMsgSize, 0, ( struct sockaddr* )&clntAddr, sizeof( clntAddr ),
            __FILE__, __LINE__ ) != recvMsgSize ) {
               ...Error handler
    }  while ( recvMsgSize >= 0 );
    printf( "%s completed\n", inet_ntoa( clntAddr.sin_addr ) );

void Server_Constructor () {
    char buf[ 128 ];
    struct sockaddr_in echoServAddr;
    struct sigaction handler;

    /* Initiating socket. */
    srvSock = socket( PF_INET, SOCK_DGRAM, IPPROTO_UDP );
    memset( &echoServAddr, 0, sizeof( echoServAddr ) );
    echoServAddr.sin_family = AF_INET;
    echoServAddr.sin_addr.s_addr = htonl( INADDR_ANY );
    echoServAddr.sin_port = htons( ( unsigned short )23456 );
    Bind( srvSock, ( struct sockaddr* )&echoServAddr, sizeof( echoServAddr ) );

    /* Initiating signal handler. */
    handler.sa_handler = SIGIOHandler;
    if ( sigfillset( &handler.sa_mask ) < 0 ) {
      ...Error handler
    handler.sa_flags = 0;   /* No flags */
    if ( sigaction( SIGIO, &handler, 0 ) < 0 ) {
      ...Error handler
    if ( fcntl( srvSock, F_SETOWN, getpid() ) < 0 ) {
      ...Error handler
    if ( fcntl ( srvSock, F_SETFL, O_NONBLOCK | FASYNC ) < 0 ) {
      ...Error handler

    /* Wait forever. */

The UDP client
static void CatchAlarm ( int ignored ) {
    tries += 1;

void Client_Sendto ( const struct sockaddr* dest_addr, const void* message ) {
    int sock;                        /* Socket descriptor */
    struct sockaddr_in echoServAddr; /* Echo server address */
    struct sockaddr_in fromAddr;     /* Source address of echo */
    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[ MSG_SIZE ] = "123";                /* String to send to echo server */
    char recvbuf[ MSG_SIZE ];
    int respStringLen;               /* Size of received datagram */

    servIP = "";           /* First arg:  server IP address (dotted quad) */

    sock = Socket( PF_INET, SOCK_DGRAM, IPPROTO_UDP, __FILE__, __LINE__ );

    /* 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( ( unsigned short )UDP_PORT );       /* Server port */

The UDP client unable send packet out, I think it can't receive the signal in the thread, so how to do that? 

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

    /* Get a response */
    fromSize = sizeof( fromAddr );
    alarm( TIMEOUT_SECS );  /* Set the timeout */
    while ((respStringLen = recvfrom(sock, recvbuf, MSG_SIZE, 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, MSG_SIZE, 0, (struct sockaddr *)
                            &echoServAddr, sizeof(echoServAddr)) != MSG_SIZE)
                    DieWithError("sendto() failed");
                DieWithError("No Response");
            DieWithError("recvfrom() failed");
    alarm( 0 ); /* Cancel the timeout. */
    Close( sock, __FILE__, __LINE__ );
    printf( "Received: %s\n", recvbuf );    /* Print the received data */

static void* SENDER ( void* null ) {
    do {
        Client_Sendto( NULL, NULL );
        sleep( 1 );
    } while ( 1 );
    return NULL;

void Client_DebugThread () {
    pthread_t tid;              /* Thread ID. */
    pthread_attr_t rx;          /* Parameter of thread. */
    pthread_attr_init( &rx );   /* Free resources immediately if a thread is terminated. */
    pthread_attr_setdetachstate( &rx, PTHREAD_CREATE_DETACHED );
    pthread_create( &tid, &rx, SENDER, NULL, __FILE__, __LINE__ ); 

How to use sigmask in order to make signals can be processed by a thread-udppng
# 2  
Old 03-06-2011
First, never, ever, ever make calls like "printf()" within a signal handler. The only system calls you can safely make from within a signal handler are those listes as async-signal safe. On Linux. see the man page for "signal" for a list.

Calling a non-async-signal-safe call from within a signal handler results in "undefined" behavior. In practice that usually means a deadlock because your signal handler will attempt to acquire a lock already held by the thread that the signal interrupted.

Second, don't call sigfillset() on your sigaction structure. If anything, call sigemptyset().
# 3  
Old 03-06-2011
Worse, you can't even count on being able to safely set the value of a variable in a signal handler unless it can be done atomically (such as with volatile sig_atomic_t). You really want to do as little as possible in a sighandler. If possible, set an atomic flag and get out.


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