Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
SELECT(2)                 FreeBSD System Calls Manual                SELECT(2)

     select -- synchronous I/O multiplexing

     Standard C Library (libc, -lc)

     #include <sys/select.h>

     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);


     The select() system call 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 condi-
     tion 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.  The select() system call
     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 max-
     imum number of descriptors supported by the system.

     If timeout is not a null 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 null pointer, the select blocks indefinitely.

     To effect a poll, the timeout argument should not be a null pointer, but
     it should point to a zero-valued timeval structure.

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

     The select() system call 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 system call, the descriptor
     sets will be unmodified.

     An error return from select() indicates:

     [EBADF]            One of the descriptor sets specified an invalid

     [EFAULT]           One of the arguments readfds, writefds, exceptfds, or
                        timeout points to an invalid address.

     [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 com-
                        ponents is negative or too large.

     [EINVAL]           The nfds argument was invalid.

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

     The default size of FD_SETSIZE is currently 1024.  In order to accommo-
     date 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

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

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

     The select() system call appeared in 4.2BSD.

     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() system call.

FreeBSD 6.2                    November 17, 2002                   FreeBSD 6.2


Want to link to this manual page? Use this URL:

home | help