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PSELECT(P)		   POSIX Programmer's Manual		    PSELECT(P)

NAME
       pselect, select - synchronous I/O multiplexing

SYNOPSIS
       #include <sys/select.h>

       int pselect(int nfds, fd_set *restrict readfds,
	      fd_set *restrict writefds, fd_set *restrict errorfds,
	      const struct timespec *restrict timeout,
	      const sigset_t *restrict sigmask);
       int select(int nfds, fd_set *restrict readfds,
	      fd_set *restrict writefds, fd_set *restrict errorfds,
	      struct timeval *restrict timeout);
       void FD_CLR(int fd, fd_set *fdset);
       int FD_ISSET(int fd, fd_set *fdset);
       void FD_SET(int fd, fd_set *fdset);
       void FD_ZERO(fd_set *fdset);

DESCRIPTION
       The  pselect()  function	 shall	examine the file descriptor sets whose
       addresses are passed in the readfds, writefds, and errorfds  parameters
       to  see	whether	 some  of their descriptors are ready for reading, are
       ready for writing, or have an exceptional  condition  pending,  respec‐
       tively.

       The  select()  function	shall be equivalent to the pselect() function,
       except as follows:

	* For the select() function, the timeout period is  given  in  seconds
	  and  microseconds in an argument of type struct timeval, whereas for
	  the pselect() function the timeout period is given  in  seconds  and
	  nanoseconds in an argument of type struct timespec.

	* The  select()	 function  has no sigmask argument; it shall behave as
	  pselect() does when sigmask is a null pointer.

	* Upon successful completion, the select()  function  may  modify  the
	  object pointed to by the timeout argument.

       The  pselect() and select() functions shall support regular files, ter‐
       minal and pseudo-terminal devices,  STREAMS-based files,	 FIFOs, pipes,
       and sockets. The behavior of pselect() and select() on file descriptors
       that refer to other types of file is unspecified.

       The nfds argument specifies the range of descriptors to be tested.  The
       first  nfds  descriptors	 shall	be  checked  in each set; that is, the
       descriptors from zero through nfds-1 in the descriptor  sets  shall  be
       examined.

       If  the	readfds argument is not a null pointer, it points to an object
       of type fd_set that on input  specifies	the  file  descriptors	to  be
       checked	for  being  ready  to read, and on output indicates which file
       descriptors are ready to read.

       If the writefds argument is not a null pointer, it points to an	object
       of  type	 fd_set	 that  on  input  specifies the file descriptors to be
       checked for being ready to write, and on output	indicates  which  file
       descriptors are ready to write.

       If  the errorfds argument is not a null pointer, it points to an object
       of type fd_set that on input  specifies	the  file  descriptors	to  be
       checked	for  error  conditions	pending, and on output indicates which
       file descriptors have error conditions pending.

       Upon successful completion, the pselect() or  select()  function	 shall
       modify  the  objects  pointed to by the readfds, writefds, and errorfds
       arguments to indicate which file descriptors  are  ready	 for  reading,
       ready  for  writing,  or have an error condition pending, respectively,
       and shall return the total number of ready descriptors in all the  out‐
       put  sets.  For	each file descriptor less than nfds, the corresponding
       bit shall be set on successful completion if it was set	on  input  and
       the associated condition is true for that file descriptor.

       If  none of the selected descriptors are ready for the requested opera‐
       tion, the pselect() or select() function shall block until at least one
       of the requested operations becomes ready, until the timeout occurs, or
       until interrupted by a signal. The timeout parameter controls how  long
       the pselect() or select() function shall take before timing out. If the
       timeout parameter is not a null pointer, it specifies a maximum	inter‐
       val  to	wait  for  the	selection  to complete.	 If the specified time
       interval expires without any requested operation	 becoming  ready,  the
       function shall return. If the timeout parameter is a null pointer, then
       the call to pselect() or select() shall	block  indefinitely  until  at
       least  one  descriptor  meets the specified criteria. To effect a poll,
       the timeout parameter should not be a null pointer, and should point to
       a zero-valued timespec structure.

       The  use	 of  a	timeout	 does  not affect any pending timers set up by
       alarm(), ualarm(), or setitimer().

       Implementations may place limitations on the maximum  timeout  interval
       supported. All implementations shall support a maximum timeout interval
       of at least 31 days. If the timeout argument specifies a timeout inter‐
       val  greater than the implementation-defined maximum value, the maximum
       value shall be used as the actual timeout  value.  Implementations  may
       also  place limitations on the granularity of timeout intervals. If the
       requested timeout interval requires a finer granularity than the imple‐
       mentation  supports, the actual timeout interval shall be rounded up to
       the next supported value.

       If sigmask is not a null pointer, then  the  pselect()  function	 shall
       replace the signal mask of the process by the set of signals pointed to
       by sigmask before examining the descriptors, and shall restore the sig‐
       nal mask of the process before returning.

       A  descriptor  shall  be considered ready for reading when a call to an
       input function with O_NONBLOCK clear would not block,  whether  or  not
       the  function  would  transfer  data  successfully. (The function might
       return data, an end-of-file indication, or  an  error  other  than  one
       indicating  that it is blocked, and in each of these cases the descrip‐
       tor shall be considered ready for reading.)

       A descriptor shall be considered ready for writing when a  call	to  an
       output  function	 with O_NONBLOCK clear would not block, whether or not
       the function would transfer data successfully.

       If a socket has a pending error, it shall  be  considered  to  have  an
       exceptional  condition  pending.	 Otherwise, what constitutes an excep‐
       tional condition is file type-specific. For a file descriptor  for  use
       with a socket, it is protocol-specific except as noted below. For other
       file types it is implementation-defined. If the operation  is  meaning‐
       less  for  a particular file type, pselect() or select() shall indicate
       that the descriptor is ready for read or write  operations,  and	 shall
       indicate that the descriptor has no exceptional condition pending.

       If  a  descriptor refers to a socket, the implied input function is the
       recvmsg() function with	parameters  requesting	normal	and  ancillary
       data,  such  that the presence of either type shall cause the socket to
       be marked as readable.  The  presence  of  out-of-band  data  shall  be
       checked	if the socket option SO_OOBINLINE has been enabled, as out-of-
       band data is enqueued with normal data. If the socket is currently lis‐
       tening,	then  it shall be marked as readable if an incoming connection
       request has been received, and a call to the  accept()  function	 shall
       complete without blocking.

       If  a descriptor refers to a socket, the implied output function is the
       sendmsg() function supplying an amount of normal data equal to the cur‐
       rent  value of the SO_SNDLOWAT option for the socket. If a non-blocking
       call to the connect() function has been made for a socket, and the con‐
       nection attempt has either succeeded or failed leaving a pending error,
       the socket shall be marked as writable.

       A socket shall be considered to have an exceptional  condition  pending
       if a receive operation with O_NONBLOCK clear for the open file descrip‐
       tion and with the MSG_OOB flag set would return out-of-band data	 with‐
       out  blocking.  (It is protocol-specific whether the MSG_OOB flag would
       be used to read out-of-band data.) A socket shall also be considered to
       have  an	 exceptional  condition pending if an out-of-band data mark is
       present in the receive queue. Other circumstances under which a	socket
       may  be	considered to have an exceptional condition pending are proto‐
       col-specific and implementation-defined.

       If the readfds, writefds, and errorfds arguments are all null  pointers
       and  the	 timeout  argument  is	not  a	null pointer, the pselect() or
       select() function shall block for the time specified, or	 until	inter‐
       rupted  by  a  signal. If the readfds, writefds, and errorfds arguments
       are all null pointers and the timeout argument is a null	 pointer,  the
       pselect()  or select() function shall block until interrupted by a sig‐
       nal.

       File descriptors associated with regular files shall always select true
       for ready to read, ready to write, and error conditions.

       On  failure,  the  objects  pointed  to	by  the readfds, writefds, and
       errorfds arguments shall not be	modified.   If	the  timeout  interval
       expires without the specified condition being true for any of the spec‐
       ified  file  descriptors,  the  objects	pointed	 to  by	 the  readfds,
       writefds, and errorfds arguments shall have all bits set to 0.

       File descriptor masks of type fd_set can be initialized and tested with
       FD_CLR(),  FD_ISSET(),  FD_SET(),  and  FD_ZERO().  It  is  unspecified
       whether	each  of these is a macro or a function. If a macro definition
       is suppressed in order to access	 an  actual  function,	or  a  program
       defines an external identifier with any of these names, the behavior is
       undefined.

       FD_CLR(fd, fdsetp) shall remove the file descriptor  fd	from  the  set
       pointed to by fdsetp. If fd is not a member of this set, there shall be
       no effect on the set, nor will an error be returned.

       FD_ISSET(fd, fdsetp) shall evaluate to non-zero if the file  descriptor
       fd  is  a member of the set pointed to by fdsetp, and shall evaluate to
       zero otherwise.

       FD_SET(fd, fdsetp) shall add the file descriptor fd to the set  pointed
       to  by  fdsetp. If the file descriptor fd is already in this set, there
       shall be no effect on the set, nor will an error be returned.

       FD_ZERO(fdsetp) shall initialize	 the  descriptor  set  pointed	to  by
       fdsetp to the null set. No error is returned if the set is not empty at
       the time FD_ZERO() is invoked.

       The behavior of these macros is undefined if the fd  argument  is  less
       than  0 or greater than or equal to FD_SETSIZE, or if fd is not a valid
       file descriptor, or if any of the arguments are expressions  with  side
       effects.

RETURN VALUE
       Upon  successful completion, the pselect() and select() functions shall
       return the total number of bits set in the  bit	masks.	Otherwise,  -1
       shall be returned, and errno shall be set to indicate the error.

       FD_CLR(),  FD_SET(),  and  FD_ZERO()  do not return a value. FD_ISSET()
       shall return a non-zero value if the bit for the file descriptor fd  is
       set in the file descriptor set pointed to by fdset, and 0 otherwise.

ERRORS
       Under  the  following conditions, pselect() and select() shall fail and
       set errno to:

       EBADF  One or more  of  the  file  descriptor  sets  specified  a  file
	      descriptor that is not a valid open file descriptor.

       EINTR  The  function  was interrupted before any of the selected events
	      occurred and before the timeout interval expired.

       If SA_RESTART has been set for the interrupting signal, it is implemen‐
       tation-defined whether the function restarts or returns with [EINTR].

       EINVAL An invalid timeout interval was specified.

       EINVAL The nfds argument is less than 0 or greater than FD_SETSIZE.

       EINVAL One of the specified file descriptors refers to a STREAM or mul‐
	      tiplexer that is linked (directly or indirectly) downstream from
	      a multiplexer.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       In  previous  versions  of  the Single UNIX Specification, the select()
       function was defined in the <sys/time.h> header. This is now changed to
       <sys/select.h>.	The  rationale	for  this  change  was as follows: the
       introduction of the  pselect()  function	 included  the	<sys/select.h>
       header  and  the	 <sys/select.h> header defines all the related defini‐
       tions for the pselect() and select() functions. Backwards-compatibility
       to  existing XSI implementations is handled by allowing <sys/time.h> to
       include <sys/select.h>.

FUTURE DIRECTIONS
       None.

SEE ALSO
       accept() , alarm() , connect() , fcntl() , poll() , read() ,  recvmsg()
       ,  sendmsg()  , setitimer() , ualarm() , write() , the Base Definitions
       volume of IEEE Std 1003.1-2001, <sys/select.h>, <sys/time.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),	The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the	referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group		     2003			    PSELECT(P)
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