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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.1-2001 */
       #include	<sys/select.h>

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

       int select(int nfds, fd_set *readfds, fd_set *writefds,
		  fd_set *exceptfds, struct timeval *timeout);

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

       #include	<sys/select.h>

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

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       pselect(): _POSIX_C_SOURCE >= 200112L ||	_XOPEN_SOURCE >= 600

DESCRIPTION
       select()	and pselect() allow a program to  monitor  multiple  file  de-
       scriptors,  waiting  until  one	or more	of the file descriptors	become
       "ready" for some	class of I/O operation (e.g., input possible).	A file
       descriptor  is  considered  ready if it is possible to perform a	corre-
       sponding	I/O operation (e.g., read(2) without  blocking,	 or  a	suffi-
       ciently small write(2)).

       The  operation of select() and pselect()	is identical, other than these
       three differences:

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

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

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

       Three independent sets of file 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  particu-
       lar, a file descriptor is also ready on end-of-file), those in writefds
       will be watched to see if space is available for	write (though a	 large
       write  may still	block),	and those in exceptfds will be watched for ex-
       ceptions.  On exit, the sets are	modified in place  to  indicate	 which
       file  descriptors  actually changed status.  Each of the	three file de-
       scriptor	sets may be specified as NULL if no file descriptors are to be
       watched for the corresponding class of events.

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

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

       The  timeout argument specifies the interval that select() should block
       waiting for a file descriptor to	become ready.  The call	will block un-
       til either:

       *  a file descriptor becomes ready;

       *  the call is interrupted by a signal handler; or

       *  the timeout expires.

       Note  that  the timeout interval	will be	rounded	up to the system clock
       granularity, and	kernel scheduling delays mean that the blocking	inter-
       val  may	 overrun  by  a	 small	amount.	 If both fields	of the timeval
       structure are zero, then	select() returns immediately.  (This is	useful
       for  polling.)  If timeout is NULL (no timeout),	select() can block in-
       definitely.

       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.

       Other than the difference in the	precision of the timeout argument, the
       following pselect() call:

	   ready = pselect(nfds, &readfds, &writefds, &exceptfds,
			   timeout, &sigmask);

       is equivalent to	atomically executing the following calls:

	   sigset_t origmask;

	   pthread_sigmask(SIG_SETMASK,	&sigmask, &origmask);
	   ready = select(nfds,	&readfds, &writefds, &exceptfds, timeout);
	   pthread_sigmask(SIG_SETMASK,	&origmask, NULL);

       The reason that pselect() is needed is that if one wants	 to  wait  for
       either  a  signal  or  for  a  file descriptor to become	ready, then 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 sig-
       nal arrived just	after the test but just	before the call.  By contrast,
       pselect() allows	one to first block signals, handle  the	 signals  that
       have  come  in,	then call pselect() with the desired sigmask, avoiding
       the 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.1-2001 versions.)

       Some code calls select()	with all three sets empty, nfds	 zero,	and  a
       non-NULL	 timeout as a fairly portable way to sleep with	subsecond pre-
       cision.

       On Linux, select() modifies timeout to reflect the amount of  time  not
       slept;  most  other implementations do not do this.  (POSIX.1-2001 per-
       mits either behavior.)  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 select()s  in
       a loop without reinitializing it.  Consider timeout to be undefined af-
       ter select() returns.

RETURN VALUE
       On success, select() and	pselect() return the number of	file  descrip-
       tors  contained in the three returned descriptor	sets (that is, the to-
       tal number of bits that are set in readfds, writefds, exceptfds)	 which
       may be zero if the timeout expires before anything interesting happens.
       On error, -1 is returned, and errno is set to indicate the  error;  the
       file descriptor sets are	unmodified, and	timeout	becomes	undefined.

ERRORS
       EBADF  An  invalid file descriptor was given in one of the sets.	 (Per-
	      haps a file descriptor that was already closed, or one on	 which
	      an error has occurred.)

       EINTR  A	signal was caught; see signal(7).

       EINVAL nfds  is	negative  or the value contained within	timeout	is in-
	      valid.

       ENOMEM unable to	allocate memory	for internal tables.

VERSIONS
       pselect() was added to Linux in kernel 2.6.16.	Prior  to  this,  pse-
       lect() was emulated in glibc (but see BUGS).

CONFORMING TO
       select()	 conforms  to POSIX.1-2001 and 4.4BSD (select()	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  be-
       fore exit, but the BSD variant does not.

       pselect() is defined in POSIX.1g, and in	POSIX.1-2001.

NOTES
       An  fd_set is a fixed size buffer.  Executing FD_CLR() or FD_SET() with
       a value of fd that is negative or is equal to or	larger than FD_SETSIZE
       will result in undefined	behavior.  Moreover, POSIX requires fd to be a
       valid file descriptor.

       Concerning the types involved, the classical situation is that the  two
       fields  of  a timeval structure are typed as long (as shown above), and
       the structure is	defined	in _sys/time.h_.  The  POSIX.1-2001  situation
       is

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

       where  the  structure  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  in-
       clude  _time.h_	for  select().	The POSIX.1-2001 situation is that one
       should include _sys/select.h_ for select() and pselect().

       Under glibc 2.0,	_sys/select.h_ gives  the  wrong  prototype  for  pse-
       lect().	 Under glibc 2.1 to 2.2.1, it gives pselect() when _GNU_SOURCE
       is defined.  Since glibc	2.2.2, the requirements	are as	shown  in  the
       SYNOPSIS.

   Multithreaded applications
       If  a  file descriptor being monitored by select() is closed in another
       thread, the result is unspecified.  On some UNIX	systems, select()  un-
       blocks  and  returns,  with  an	indication that	the file descriptor is
       ready (a	subsequent I/O operation will likely fail with an  error,  un-
       less another the	file descriptor	reopened between the time select() re-
       turned and the I/O operations was performed).  On Linux (and some other
       systems),  closing  the file descriptor in another thread has no	effect
       on select().  In	summary, any application that relies on	 a  particular
       behavior	in this	scenario must be considered buggy.

   C library/kernel ABI	differences
       The pselect() interface described in this page is implemented by	glibc.
       The underlying Linux system call	is named pselect6().  This system call
       has somewhat different behavior from the	glibc wrapper function.

       The  Linux  pselect6() system call modifies its timeout argument.  How-
       ever, the glibc wrapper function	hides this behavior by using  a	 local
       variable	 for  the  timeout argument that is passed to the system call.
       Thus, the glibc pselect() function does not modify  its	timeout	 argu-
       ment; this is the behavior required by POSIX.1-2001.

       The  final  argument  of	the pselect6() system call is not a sigset_t *
       pointer,	but is instead a structure of the form:

	   struct {
	       const sigset_t *ss;     /* Pointer to signal set	*/
	       size_t	       ss_len; /* Size (in bytes) of object pointed
					  to by	'ss' */
	   };

       This allows the system call to obtain both a pointer to the signal  set
       and  its	size, while allowing for the fact that most architectures sup-
       port a maximum of 6 arguments to	a system call.

BUGS
       Glibc 2.0 provided a version of pselect() that did not take  a  sigmask
       argument.

       Starting	 with  version	2.1,  glibc provided an	emulation of pselect()
       that was	implemented using sigprocmask(2) and select().	This implemen-
       tation  remained	 vulnerable  to	the very race condition	that pselect()
       was designed to prevent.	 Modern	versions of glibc use the  (race-free)
       pselect() system	call on	kernels	where it is provided.

       On  systems  that  lack	pselect(), reliable (and more portable)	signal
       trapping	can be achieved	using the self-pipe trick.  In this technique,
       a  signal  handler writes a byte	to a pipe whose	other end is monitored
       by select() in the main program.	  (To  avoid  possibly	blocking  when
       writing	to  a pipe that	may be full or reading from a pipe that	may be
       empty, nonblocking I/O is used when reading from	 and  writing  to  the
       pipe.)

       Under Linux, select() may report	a socket file descriptor as "ready for
       reading", while nevertheless a subsequent read blocks.  This could  for
       example	happen	when  data  has	arrived	but upon examination has wrong
       checksum	and is discarded.  There may be	other circumstances in which a
       file  descriptor	is spuriously reported as ready.  Thus it may be safer
       to use O_NONBLOCK on sockets that should	not block.

       On Linux, select() also modifies	timeout	if the call is interrupted  by
       a signal	handler	(i.e., the EINTR error return).	 This is not permitted
       by POSIX.1-2001.	 The Linux pselect() system call has the  same	behav-
       ior,  but  the  glibc wrapper hides this	behavior by internally copying
       the timeout to a	local variable and passing that	variable to the	system
       call.

EXAMPLE
       #include	<stdio.h>
       #include	<stdlib.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 == -1)
	       perror("select()");
	   else	if (retval)
	       printf("Data is available now.\n");
	       /* FD_ISSET(0, &rfds) will be true. */
	   else
	       printf("No data within five seconds.\n");

	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       accept(2),  connect(2),	poll(2),  read(2),  recv(2), send(2), sigproc-
       mask(2),	write(2), epoll(7), time(7)

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

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-08-19			     SELECT(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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