select(3C)           COMPATIBILITY FUNCTIONS           select(3C)



NAME
     select - synchronous I/O multiplexing

SYNOPSIS
     #include <sys/time.h>
     #include <sys/types.h>

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

     FD_SET(fd, &fdset);
     FD_CLR(fd, &fdset);
     FD_ISSET(fd, &fdset);
     FD_ZERO(&fdset);
     int fd;
     fd_set fdset;

DESCRIPTION
     select examines the I/O descriptor sets whose addresses  are
     passed  in readfds, writefds, and execptfds to see if any of
     their descriptors are ready for reading, are ready for writ-
     ing, or have an exceptional condition pending, respectively.
     nfds is the number of bits to be checked in  each  bit  mask
     that represents a file descriptor; the descriptors from 0 to
     -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 opera-
     tion.   The  return  value  from the call to select() is the
     number of ready descriptors.

     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.
     FD_SETSIZE is a constant defined in sys/select.h and is nor-
     mally at least equal to the maximum  number  of  descriptors
     supported by the system.

     If timeout is not a NULL pointer,  it  specifies  a  maximum
     interval  to wait for the selection to complete.  If timeout
     is a NULL  pointer,  the  select  blocks  indefinitely.   To
     affect  a  poll,  the  timeout argument should be a non-NULL
     pointer, pointing to a zero-valued timeval structure.





                          Last change:                          1

select(3C)           COMPATIBILITY FUNCTIONS           select(3C)



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

RETURN VALUE
     select returns the number of ready descriptors contained  in
     the descriptor sets or -1 if an error occurred.  If the time
     limit expires, then select returns 0.

ERRORS
     An error return from select indicates:

     EBADF          One of the I/O descriptor sets  specified  an
                    invalid I/O descriptor.

     EINTR          A signal was  delivered  before  any  of  the
                    selected  events  occurred, or the time limit
                    expired.

     EINVAL         A component of the pointed-to time  limit  is
                    outside  the acceptable range:  t_sec must be
                    between 0 and 10^8, inclusive. t_usec must be
                    greater-than  or  equal  to  0, and less than
                    10^6.

SEE ALSO
     poll(2), read(2), write(2)

NOTES
     The default value for FD_SETSIZE (currently 1024) is  larger
     than  the  default  limit  on  the number of open files.  In
     order to accommodate programs that may 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>.

     In future versions of the system, select may return the time
     remaining  from  the  original timeout, if any, by modifying
     the time value in place.  It is thus unwise to  assume  that
     the timeout value will be unmodified by the select call.

     The descriptor sets are always modified on return,  even  if
     the call returns as the result of a timeout.













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