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NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

       #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 *timeout);

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

DESCRIPTION    [Toc]    [Back]

       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

       sigmask	is  a  pointer to a signal mask (see sigprocmask(2)); if it is
       not NULL, then pselect 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  something 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 followed 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 signals, 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.

RETURN VALUE    [Toc]    [Back]

       On  success,  select  and pselect return the number of descriptors contained
 in the descriptor 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.

ERRORS    [Toc]    [Back]

       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.

NOTES    [Toc]    [Back]

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

       On  Linux, timeout is modified 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.

EXAMPLE    [Toc]    [Back]

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

       int 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;

CONFORMING TO    [Toc]    [Back]

       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).
       It  is  found in glibc2.1 and later.  Glibc2.0 has a function with this
       name, that however does not take a sigmask parameter.

SEE ALSO    [Toc]    [Back]

       accept(2), connect(2), poll(2),	read(2),  recv(2),  send(2),  sigproc-
       mask(2), write(2)

Linux 1.2			  1996-02-11			     SELECT(2)
[ Back ]
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