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SELECT(2)			    Linux Programmer's Manual				SELECT(2)

       select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing

       /* According to POSIX 1003.1-2001 */
       #include <sys/select.h>

       /* According to earlier standards */
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       int  select(int	n,  fd_set  *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval

       int pselect(int n, fd_set *readfds, fd_set  *writefds,  fd_set  *exceptfds,  const  struct
       timespec *timeout, const sigset_t *sigmask);

       FD_CLR(int fd, fd_set *set);
       FD_ISSET(int fd, fd_set *set);
       FD_SET(int fd, fd_set *set);
       FD_ZERO(fd_set *set);

       The functions select and pselect wait for a number of file descriptors to change status.

       Their function is identical, with three differences:

       (i)    The  select  function  uses  a  timeout  that is a struct timeval (with seconds and
	      microseconds), while pselect uses a struct timespec (with seconds and nanoseconds).

       (ii)   The select function may update the timeout parameter to indicate how much time  was
	      left. The pselect function does not change this parameter.

       (iii)  The  select  function  has no sigmask parameter, and behaves as pselect called with
	      NULL sigmask.

       Three independent sets of descriptors are  watched.   Those  listed  in	readfds  will  be
       watched	to  see  if  characters become available for reading (more precisely, to see if a
       read will not block - in particular, a file descriptor  is  also  ready	on  end-of-file),
       those in writefds will be watched to see if a write will not block, and those in exceptfds
       will be watched for exceptions.	On exit, the sets are modified in place to indicate which
       descriptors actually changed status.

       Four  macros  are  provided to manipulate the sets.  FD_ZERO will clear a set.  FD_SET and
       FD_CLR add or remove a given descriptor from a set.  FD_ISSET tests to see if a descriptor
       is part of the set; this is useful after select returns.

       n is the highest-numbered descriptor in any of the three sets, plus 1.

       timeout	is  an upper bound on the amount of time elapsed before select returns. It may be
       zero, causing select to return immediately. (This is useful for polling.)  If  timeout  is
       NULL (no timeout), select can block indefinitely.

       sigmask	is  a pointer to a signal mask (see sigprocmask(2)); if it is not NULL, then pse-
       lect first replaces the current signal mask by the one pointed to by  sigmask,  then  does
       the `select' function, and then restores the original signal mask again.

       The  idea  of  pselect is that if one wants to wait for an event, either a signal or some-
       thing on a file descriptor, an atomic test is needed to prevent race conditions.  (Suppose
       the  signal  handler  sets a global flag and returns. Then a test of this global flag fol-
       lowed by a call of select() could hang indefinitely if the signal arrived just  after  the
       test  but  just before the call. On the other hand, pselect allows one to first block sig-
       nals, handle the signals that have come in, then call pselect() with the desired  sigmask,
       avoiding  the race.)  Since Linux today does not have a pselect() system call, the current
       glibc2 routine still contains this race.

   The timeout
       The time structures involved are defined in <sys/time.h> and look like

	      struct timeval {
		  long	  tv_sec;	  /* seconds */
		  long	  tv_usec;	  /* microseconds */


	      struct timespec {
		  long	  tv_sec;	  /* seconds */
		  long	  tv_nsec;	  /* nanoseconds */

       (However, see below on the POSIX 1003.1-2001 versions.)

       Some code calls select with all three sets empty, n zero, and  a  non-null  timeout  as	a
       fairly portable way to sleep with subsecond precision.

       On  Linux,  the	function select modifies timeout to reflect the amount of time not slept;
       most other implementations do not do this.  This causes	problems  both	when  Linux  code
       which reads timeout is ported to other operating systems, and when code is ported to Linux
       that reuses a struct timeval for multiple selects in a  loop  without  reinitializing  it.
       Consider timeout to be undefined after select returns.

       On  success, select and pselect return the number of descriptors contained in the descrip-
       tor sets, which may be zero if the timeout expires before  anything  interesting  happens.
       On  error,  -1  is  returned,  and errno is set appropriately; the sets and timeout become
       undefined, so do not rely on their contents after an error.

       EBADF  An invalid file descriptor was given in one of the sets.

       EINTR  A non blocked signal was caught.

       EINVAL n is negative.

       ENOMEM select was unable to allocate memory for internal tables.

       #include <stdio.h>
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       main(void) {
	   fd_set rfds;
	   struct timeval tv;
	   int retval;

	   /* Watch stdin (fd 0) to see when it has input. */
	   FD_SET(0, &rfds);
	   /* Wait up to five seconds. */
	   tv.tv_sec = 5;
	   tv.tv_usec = 0;

	   retval = select(1, &rfds, NULL, NULL, &tv);
	   /* Don't rely on the value of tv now! */

	   if (retval)
	       printf("Data is available now.\n");
	       /* FD_ISSET(0, &rfds) will be true. */
	       printf("No data within five seconds.\n");

	   return 0;

       4.4BSD (the select function first appeared in 4.2BSD).  Generally portable to/from non-BSD
       systems supporting clones of the BSD socket layer (including System V variants).  However,
       note that the System V variant typically sets the timeout variable before  exit,  but  the
       BSD variant does not.

       The  pselect  function  is  defined in IEEE Std 1003.1g-2000 (POSIX.1g), and part of POSIX
       1003.1-2001.  It is found in glibc2.1 and later.  Glibc2.0 has a function with this  name,
       that however does not take a sigmask parameter.

       Concerning  the types involved, the classical situation is that the two fields of a struct
       timeval are longs (as shown above), and the struct is defined in <sys/time.h>.  The  POSIX
       1003.1-2001 situation is

	      struct timeval {
		  time_t	 tv_sec;     /* seconds */
		  suseconds_t	 tv_usec;    /* microseconds */

       where  the  struct  is defined in <sys/select.h> and the data types time_t and suseconds_t
       are defined in <sys/types.h>.

       Concerning prototypes, the classical situation is that one  should  include  <time.h>  for
       select.	 The  POSIX  1003.1-2001  situation is that one should include <sys/select.h> for
       select and pselect.  Libc4 and libc5 do not have a <sys/select.h> header; under glibc  2.0
       and  later  this header exists.	Under glibc 2.0 it unconditionally gives the wrong proto-
       type for pselect, under glibc 2.1-2.2.1 it gives  pselect  when	_GNU_SOURCE  is  defined,
       under  glibc  2.2.2-2.2.4 it gives it when _XOPEN_SOURCE is defined and has a value of 600
       or larger.  No doubt, since POSIX 1003.1-2001, it should give the prototype by default.

       For a tutorial with discussion and examples, see select_tut(2).

       For vaguely related stuff, see accept(2), connect(2), poll(2), read(2), recv(2),  send(2),
       sigprocmask(2), write(2)

Linux 2.4				    2001-02-09					SELECT(2)
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