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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.  The call will block
       until 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

       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 to indicate the
       error; the file descriptor sets are  unmodified,	 and  timeout  becomes

       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 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2014-01-31			     SELECT(2)

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