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

NAME

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

SYNOPSIS

       /* 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 *timeout);

       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);

DESCRIPTION

       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 read-
       ing (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 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.

   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 */
	      };

       and

	      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 preci-
       sion.

       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.

RETURN VALUE

       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

       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.

EXAMPLE

       #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_ZERO(&rfds);
	   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.
");
	       /* FD_ISSET(0, &rfds) will be true. */
	   else
	       printf("No data within five seconds.
");

	   return 0;
       }

CONFORMING TO

       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.

NOTES

       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 prototype 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.

SEE ALSO

       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)