select(S)                      6 January 1993                      select(S)


 Name

    select - synchronous I/O multiplexing

 Syntax


    cc  . . .  -lc


    #include  <sys/types.h>
    #include  <sys/times.h>
    #include  <sys/select.h>

    int select(nfds, readfds, writefds, exceptfds, timeout)
    int nfds;
    fd_set *readfds, *writefds, *exceptfds;
    struct timeval *timeout;

    FD_CLR(fd, &fdset)
    int fd;
    fd_set fdset;

    FD_ISSET(fd, &fdset)
    int fd;
    fd_set fdset;

    FD_SET(fd, &fdset)
    int fd;
    fd_set fdset;

    FD_ZERO(&fdset)
    fd_set fdset;


 Description

    select examines the I/O descriptor sets whose addresses are passed in
    readfds, writefds, and exceptfds to see if some of their descriptors are
    ready for reading, are ready for writing, or have an exceptional condi-
    tion pending, respectively.  The first nfds descriptors are checked in
    each set; that is, the descriptors from 0 through nfds-1 in the descrip-
    tor sets are examined.  On return, select replaces the given descriptor
    sets with subsets consisting of those descriptors that are ready for the
    requested operation.  The total number of ready descriptors in all the
    sets is returned in nfound.

    The descriptor sets are stored as bit fields in arrays of integers.  The
    following macros are provided for manipulating such descriptor sets:
    FDZERO(&fdset) initializes a descriptor set fdset to the null set.
    FDSET(fd, &fdset) includes a particular descriptor fd in fdset.
    FDCLR(fd, &fdset) removes fd from fdset.  FDISSET(fd, &fdset) is
    nonzero if fd is a member of fdset, zero otherwise.  The behavior of
    these macros is undefined if a descriptor value is less than zero or
    greater than or equal to FDSETSIZE, which is normally at least equal to
    the maximum number of descriptors supported by the system.

    If timeout is a non-NULL pointer, it specifies a maximum interval to wait
    for the selection to complete.  If timeout is a NULL pointer, the select
    blocks indefinitely.  To effect a poll, the timeout argument should be
    non-NULL, pointing to a zero-valued timeval structure.

    Any of readfds, writefds, and exceptfds may be given as NULL pointers if
    no descriptors are of interest.

 Return value

    select returns the number of ready descriptors that are contained in the
    descriptor sets, or -1 if an error occurred.  If the time limit expires
    then select returns 0.  If select returns with an error, including one
    due to an interrupted call, the descriptor sets are unmodified.

 Diagnostics

    An error return from select indicates:


    [EBADF]     One of the descriptor sets specified an invalid descriptor.

    [EINTR]     A signal was delivered before the time limit expired and
                before any of the selected events occurred.

    [EINVAL]    The specified time limit is invalid.  One of its components
                is negative or too large. Or, the device driver being polled
                has not implemented select support.


 Notes

    select should probably return the time remaining from the original
    timeout, if any, by modifying the time value in place.  This may be
    implemented in future versions of the system.  Thus, it is unwise to
    assume that the timeout value is unmodified by the select call.

 See also

    read(S), write(S)

 Standards conformance

    select is not part of any currently supported standard; it was developed
    at the University of California at Berkeley and is used by permission.