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NAME    [Toc]    [Back]

     select - synchronous I/O multiplexing

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <sys/time.h>
     #include <unistd.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);


DESCRIPTION    [Toc]    [Back]

     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 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 a maximum interval to wait
     for the selection to complete.  If timeout is a nil pointer, the select
     blocks indefinitely.  To affect 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    [Toc]    [Back]

     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    [Toc]    [Back]

     An error return from select() indicates:

     [EFAULT]           One or more of readfds, writefds, or exceptfds points
                        outside the process's allocated address space.

     [EBADF]            One of the descriptor sets specified an invalid

     [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    [Toc]    [Back]

     accept(2), connect(2), gettimeofday(2), poll(2), read(2), recv(2),
     send(2), write(2), getdtablesize(3)

HISTORY    [Toc]    [Back]

     The select() function call appeared in 4.2BSD.

BUGS    [Toc]    [Back]

     Although the provision of getdtablesize(3) was intended to allow user
     programs to be written independent of the kernel limit on the number of
     open files, the dimension of a sufficiently 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 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.

BSD                             March 25, 1994                             BSD
[ Back ]
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