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SELECT(2)                DragonFly System Calls Manual               SELECT(2)


NAME

     select -- synchronous I/O multiplexing


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <sys/select.h>

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

     FD_SET(fd, &fdset);

     FD_CLR(fd, &fdset);

     FD_ISSET(fd, &fdset);

     FD_ZERO(&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 only exceptional condition
     detectable is out-of-band data received on a socket.  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.  Select() returns the total
     number of ready descriptors in all the sets.

     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 non-
     zero 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 non-NULL, it specifies the maximum interval to wait for the
     selection to complete.  System activity can lengthen the interval by an
     indeterminate amount.

     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.

     Even if no descriptors are of interest, the timeout works as described,
     effectively putting the process into an interruptible sleep for the
     specified timeout.  If timeout is NULL, the process will block until a
     signal is received.


RETURN VALUES

     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, 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.

     [EINVAL]           nfds was invalid.


SEE ALSO

     accept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2),
     recv(2), send(2), write(2), clocks(7)


NOTES

     The default size of FD_SETSIZE is currently 1024.  In order to
     accommodate programs which might potentially use a larger number of open
     files with select(), it is possible to increase this size by having the
     program define FD_SETSIZE before the inclusion of any header which
     includes <sys/types.h>.

     If nfds is greater than the number of open files, select() is not
     guaranteed to examine the unused file descriptors.   For historical
     reasons, select() will always examine the first 256 descriptors.


HISTORY

     The select() function call appeared in 4.2BSD.


BUGS

     Version 2 of the Single UNIX Specification (``SUSv2'') allows systems to
     modify the original timeout in place.  Thus, it is unwise to assume that
     the timeout value will be unmodified by the select() call.

DragonFly 5.1                  February 22, 2018                 DragonFly 5.1

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