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SELECT(2)                 FreeBSD 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 *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);

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 a non-nil pointer, 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 nil pointer, the select blocks indefinitely.

     To effect a poll, the timeout argument should be non-nil, pointing to a
     zero-valued timeval structure.

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

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.

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.

STANDARDS
     The select() function and FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO()
     macros conform with IEEE Std 1003.1-2001 (``POSIX.1'').

HISTORY
     The select() function call appeared in 4.2BSD.

FreeBSD 11.0-PRERELEASE        November 17, 2002       FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | NOTES | BUGS | STANDARDS | HISTORY

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