select manpage

Search topic Section

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, POSIX.1-2008 */
       #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

       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 a corre-
       sponding I/O operation (e.g., read(2) without  blocking,	 or  a	suffi-
       ciently small write(2)).

       select()	 can  monitor only file descriptors numbers that are less than
       FD_SETSIZE; poll(2) does not have this limitation.  See BUGS.

       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
       exceptions.   On exit, the sets are modified in place to indicate which
       file descriptors actually changed  status.   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 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
       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 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  permits
       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  exceeds the RLIMIT_NOFILE resource limit
	      (see getrlimit(2)).

       EINVAL The value contained within timeout is invalid.

       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, POSIX.1-2008, 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 vari-
       ants).  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 and POSIX.1-2008.

       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.

       On some other UNIX systems, select() can fail with the error EAGAIN  if
       the  system  fails  to  allocate kernel-internal resources, rather than
       ENOMEM as Linux does.  POSIX specifies this error for poll(2), but  not
       for select().  Portable programs may wish to check for EAGAIN and loop,
       just as with EINTR.

       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 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
       include <time.h> for select().	The  POSIX.1  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

   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.

   C library/kernel differences
       The Linux kernel allows file descriptor sets of arbitrary size,	deter-
       mining  the  length  of	the sets to be checked from the value of nfds.
       However, in the glibc implementation, the fd_set type is fixed in size.
       See also BUGS.

       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 kernel_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.	See sigprocmask(2) for
       a  discussion  of  the difference between the kernel and libc notion of
       the signal set.

       POSIX allows an implementation to define an upper limit, advertised via
       the  constant  FD_SETSIZE, on the range of file descriptors that can be
       specified in a file descriptor set.  The Linux kernel imposes no	 fixed
       limit,  but  the	 glibc	implementation makes fd_set a fixed-size type,
       with FD_SETSIZE defined	as  1024,  and	the  FD_*()  macros  operating
       according  to  that  limit.   To	 monitor file descriptors greater than
       1023, use poll(2) instead.

       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.  The Linux pselect() system call has the same behavior, but
       the glibc wrapper hides this behavior by internally copying the timeout
       to a local variable and passing that variable to the system call.

       #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),  restart_syscall(2),
       send(2), sigprocmask(2), write(2), epoll(7), time(7)

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

       This  page  is  part of release 4.10 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

Linux				  2017-03-13			     SELECT(2)