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Simplified use of the select( ) function

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• A simplified usage of select( )
  • In general:

    The fd_set variable used in select( ) is: a bit array of 1024 bits

    Manipulating (set/reset) the bits in such a large bit array requires the use of macros

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  • However:

    you can use smaller of bit array variables in select( ) !!!!

    Example:

    An int variable is a bit array of 32 bits

    I will first show you how to use select( ) with an int (32 bit) bit array.

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• Testing read-readiness of a socket
  • Recall:

    An int is an array of 32 bits

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  • We can clear all bits in an int using:

    int s = 0; // Clear all bits

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  • We can set the bit position i using:

    s = s | (1 << i);

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  • How to test the read (receive) readiness of a socket using select( ):

    int fdset; // bit array of 32 bits int s; // socket // Assume s is initialized and bound to some port... /* =========================================== Set up timeout value =========================================== */ struct timeval timeout; timeout.tv_sec = 0; timeout.tv_usec = 50; // 50 micro sec time out /* ============================================= Prepare file descriptors for select( ) ============================================= */ fdset = 0; // Clear bit array fdset = fdset | (1 << s); // s = an initialized socket variable /* =================================================== Check for message arrival with in timeout =================================================== */ if ( select( 32, &fdset, NULL, NULL, &timeout) != 0 ) { // s is read for reading } else { // timeout occured }

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• Example program on using select( )
  • Example program to demonstrate the effect of select( ):

    // I need to create a scoket and bind it first.... /* --- Create a socket --- */ s = socket(PF_INET, SOCK_DGRAM, 0); /* --- Set up socket end-point info for binding --- */ in_addr.sin_family = AF_INET; /* Protocol family */ in_addr.sin_addr.s_addr = htonl(INADDR_ANY); /* Wildcard IP address */ in_addr.sin_port = htons(atoi(argv[1])); /* Use any UDP port */ /* --- Bind socket s --- */ bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr)) /* ================================== How to use select ================================== */ int recv_fds; /* I can use a simple int */ struct timeval timeout; /* Time value for time out */ /* =============================== Select the bit for socket s =============================== */ recv_fds = 0; // Clear all bits in recv_fds recv_fds = recv_fds | ( 1 << s ); // Set bit for socket s /* =================================== Set timeout value to 5 sec =================================== */ timeout.tv_sec = 5; // Timeout = 5 sec + 0 micro sec timeout.tv_usec = 0; // number of micro seconds int result ; printf("Now send a UDP message to port %s before %d sec !!!....\n", argv[1], timeout.tv_sec ); result = select(32, (fd_set *)&recv_fds, NULL, NULL, &timeout); printf("select( ) returns: %d\n", result);

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  • Example Program: (Demo above code)                                                
    • The select( ) test Prog file: click here
    • Use this sender to send a message to the select program: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile the select1 test prog:   gcc -o select1 select1.c To compile the udp4-s3 test prog:   gcc -o udp4-s3 udp4-s3.c -lnsl To run:          select1 8000 on machine X To send a msg to select prog:   udp4-s3 X 8000