---

Intro to the select( ) function

---

• Header files for select
  • In order to use the select( ) function in C, you must include this header file:

    #include <sys/select.h>

  ---

  ---

  ---

  ---

• Usage of the select( ) function
  • Usage of the select( ) function:

    select( ) allow a program to check/test and/or wait until: one or more file descriptors/sockets is "ready" for reading (receiving) or writing (sending)

    ---

    ---

  • "Ready" for reading/writing:

    Recall that: A read/recvfrom operation will block if: There is no data in the receive buffer A socket is ready for some operation (read/write) if: The operation (read/write) will not block

  ---

  ---

  ---

  ---

• The select system call
  • Syntax:

    #include <sys/select.h> #include <sys/time.h> /* Header files */ int select( int nfds, fd_set * readfds, // a bit array fd_set * writefds, // a bit array fd_set * errorfds, // a bit array struct timeval * timeout ); The data types used: fd_set = a bit array type (an int array interpreted as bit array) struct timeval { unsigned tv_sec; /* # seconds in the timeout value */ unsigned tv_usec; /* # microseconds */ }

    Meaning of the parameters:

    nfds = number of bits used in readfds, writefds and errorfds readfds = descriptor bit array to test for read readiness (NULL means empty set) The ith bit in the bit array corresponds to the file descriptor i writefds = descriptor bit array to test for write readiness (NULL means empty set) errorfds = descriptor bit array to test for error conditions (NULL means empty set) timeout = maximum wait duration (time out value) (NULL means wait indefinitely)

    ---

    ---

    ---

  • Effect of the select( ) function:

    The select( ) function examines the file (or sockets) descriptor sets in the variables: readfds for read readiness (= has data) writefds for write readiness (= has buffer space) errorfds for error          for a maximum duration given by the timeout variable If some of these fds/sockets are ready before the timeout expires: The select( ) call will return immediately         Return values: The function's return value is equal to: the number of file descriptors/sockets that are ready The bit array variables readfds, writefds and errorfds will contains: the file descriptors that are ready for the corresponding operation (I.e., the bit position of the file descriptors in the bit array = 1) The timeout variable will be: reduced by the amount of time spent inside the select( ) call (I.e.: the timeout variable will contain the remaining waiting time before the timeout expires) Otherwise (nothing is ready after waiting for timeout amount of time): select( ) will return (the function value) 0