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PSELECT(3P)                POSIX Programmer's Manual               PSELECT(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
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(3P)