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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

       /* 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

       select() and  pselect()	allow  a  program  to  monitor	multiple  file
       descriptors,  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 the corre‐
       sponding I/O operation (e.g., read(2)) without blocking.

       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  a  write	 will  not block, and those in
       exceptfds will be watched for exceptions.  On exit, the sets are	 modi‐
       fied  in place to indicate which file descriptors actually changed sta‐
       tus.  Each of the three file descriptor sets may be specified  as  NULL
       if no file descriptors are to be watched for the corresponding class of

       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.	 This interval will be
       rounded up to the  system  clock	 granularity,  and  kernel  scheduling
       delays  mean  that the blocking interval 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 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.

       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 */


	   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‐

       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
       after select() returns.

       On success, select() and pselect() return the number of	file  descrip‐
       tors  contained	in  the	 three	returned descriptor sets (that is, the
       total 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 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.	 (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

       ENOMEM unable to allocate memory for internal tables.

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

       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
       before exit, but the BSD variant does not.

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

       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

	   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
       include <time.h> for select().  The POSIX.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 to 2.2.1 it gives
       pselect() when _GNU_SOURCE is defined.  Since glibc 2.2.2 the  require‐
       ments 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()
       unblocks	 and  returns,	with an indication that the file descriptor is
       ready (a subsequent I/O operation  will	likely	fail  with  an	error,
       unless  another	the file descriptor reopened between the time select()
       returned 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  par‐
       ticular behavior in this scenario must be considered buggy.

   Linux notes
       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.

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

       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

       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

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

	   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 == -1)
	   else if (retval)
	       printf("Data is available now.\n");
	       /* FD_ISSET(0, &rfds) will be true. */
	       printf("No data within five seconds.\n");


       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).

       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2013-09-04			     SELECT(2)

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