SELECT(2)	    UNIX Programmer's Manual		SELECT(2)

     pselect, select - synchronous I/O multiplexing

     #include <sys/types.h>
     #include <sys/time.h>
     #include <sys/select.h>
     #include <signal.h>

     nfound = pselect(nfds, readfds, writefds, exceptfds, timeout, sigmask);
     int nfound, nfds;
     fd_set *readfds, *writefds, *exceptfds;
     struct timespec *timeout;
     sigset_t *sigmask;

     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)
     int fd;
     fd_set fdset;

     Pselect and select examine 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 two functions are identical except for
     the type and format of the timeout value, and the additonal
     sigmask parameter supplied to the pselect() call.

     The first nfds descriptors are checked in each set; i.e. 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 manipulat-
     ing 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,

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SELECT(2)	    UNIX Programmer's Manual		SELECT(2)

     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, select blocks indefinitely.  To affect a
     poll, the timeout argument should be non-zero, pointing to a
     zero-valued timeval structure.

     If the sigmask parameter to pselect() is not NULL, it points
     to a signal mask that replaces the previous signal mask for
     the process for the duration of the call, and the previous
     mask is restored upon return; see sigprocmask(3).	This is
     normally used so that signals can be blocked while preparing
     for a call to pselect() and then atomically unblocking the
     signals while selecting.

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

     Select returns the number of ready descriptors that are con-
     tained 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.

     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

     [EINVAL]	    The specified time limit is invalid.  One of
		    its components is negative or too large.

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

     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 suffi-
     ciently 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 which might

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SELECT(2)	    UNIX Programmer's Manual		SELECT(2)

     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 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.

     In 2BSD the timeout is implemented in the kernel using the
     callout table.  Since a callout structure only has a signed
     short to store the number of ticks till expiration the max-
     imum value of a kernel timeout is 32767 ticks.  In the US
     (60hz power) this gives a maximum timeout of approximately 9
     minutes.  In countries using 50hz power the maximum timeout
     is about 13 minutes.

     struct timespec on a PDP-11 is silly since the hardware has
     nowhere near microsecond much less nanosecond clock resolu-

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