SELECT(2)                            BSD                             SELECT(2)



NAME
     select - synchronous I/O multiplexing

SYNOPSIS
     #include <sys/types.h>
     #include <sys/time.h>

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

     FD_SET(fd, &fdset)
     FD_CLR(fd, &fdset)
     FD_ISSET(fd, &fdset)
     FD_ZERO(&fdset)
     int fd;
     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
     condition pending, respectively.  The first nfds descriptors are checked
     in each set; that is, the descriptors from 0 through nfds-1 in the
     descriptor 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:
     FD_ZERO(&fdset) initializes a descriptor set fdset to the null set.
     FD_SET(fd, &fdset) includes a particular descriptor fd in fdset.
     FD_CLR(fd, &fdset) removes fd from fdset.  FD_ISSET(fd, &fdset) is
     nonzero if fd is a member of fdset, 0 otherwise.  The behavior of these
     macros is undefined if a descriptor value is less than 0 or greater than
     or equal to FD_SETSIZE, which is normally at least equal to the maximum
     number of descriptors supported by the system.

     If time-out is a nonzero pointer, it specifies a maximum interval to wait
     for the selection to complete.  If time-out is a 0 pointer, the select
     blocks indefinitely.  To affect a poll, the time-out argument should be
     nonzero, pointing to a zero-valued timeval structure.

     Any of readfds, writefds, and exceptfds may be given as 0 pointers if no
     descriptors are of interest.

ERRORS
     select will fail if any of the following are true:

     [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.

SEE ALSO
     accept(2), connect(2), read(2), write(2), recv(2), send(2),

     getdtablesize(2)

BUGS
     Although the provision of getdtablesize(2) was intended to allow user
     programs to be written independent of the kernel limit on the number of
     open files, the dimension of a sufficiently large bit field for select
     remains a problem.  The default size FD_SETSIZE (currently 256) is
     somewhat larger than the current kernel limit to the number of open
     files.  However, in order to accommodate programs that might potentially
     use a larger number of open files with select, it is possible to increase
     this size within a program by providing a larger definition of FD_SETSIZE
     before the inclusion of <sys/types.h>.

     select should probably return the time remaining from the original time-
     out, 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 time-out value will be unmodified by the select call.

DIAGNOSTICS
     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 will be unmodified.