SELECT(2)                                                            SELECT(2)


NAME
     select - synchronous I/O multiplexing

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

     int select (int nfds, fd_set *readfds, fd_set *writefds,
               fd_set *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 file descriptors are
     checked in each set; i.e. the file descriptors from 0 through nfds - 1
     will be examined (see getdtablehi(3) for largest open descriptor).  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 the return value.

     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, zero otherwise.  The behavior of
     these macros is undefined if a descriptor value is less than zero 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 timeout is a non-zero pointer, it specifies a maximum interval to wait
     for the selection to complete.  If timeout is a zero pointer, the select
     blocks indefinitely.  To effect a poll, the timeout argument should be
     non-zero, pointing to a zero-valued timeval structure.

     Any of readfds, writefds, and exceptfds may be given as zero 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 will be unmodified.

ERRORS
     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.

SEE ALSO
     poll(2), accept(2), connect(2), read(2), write(2), recv(2), send(2),
     getdtablehi(3C).

NOTES
     Some devices do not support polling via the select(2) and poll(2) system
     calls.  Doing a select or poll on a file descriptor associated with an
     "un-pollable" device will cause the select or poll to return immediately
     with a success value of 0 and the with the corresponding file descriptor
     events of queried set true.  For instance, if a select or poll is
     performed on a read file descriptor associated with an un-pollable
     device, the call would return immediately, even though there may be
     nothing to read on the device.  A subsequent read(2) in this situation
     might return with a "bytes-read" count of 0 or might block if the device
     supports read blocking.  Devices which exhibit this behavior (especially
     those from third-party vendors) should be suspected as not supporting
     polling.

BUGS
     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 will be unmodified by the select call.


                                                                        Page 2