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 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
       corresponding I/O operation (e.g., read(2)) without blocking.

       The operation of select() and pselect() is identical, with three differences:

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

       timeout	is  an	upper bound on the amount of time elapsed before select() returns.  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;

	   sigprocmask(SIG_SETMASK, &sigmask, &origmask);
	   ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
	   sigprocmask(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 signal arrived just after the test but just before the call.  By con-
       trast, 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  unde-
       fined after select() returns.

RETURN VALUE

       On  success,  select()  and  pselect()  return the number of file descriptors 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.

ERRORS

       EBADF  An invalid file descriptor was given in one of the sets.	(Perhaps 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 invalid.

       ENOMEM unable to allocate memory for internal tables.

VERSIONS

       pselect() was added to Linux in kernel 2.6.16.  Prior to this, pselect() 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
       before 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_SET-
       SIZE 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 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
       requirements are as shown in the SYNOPSIS.

   Linux Notes
       The Linux pselect() system call modifies its timeout argument.  However, 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.

BUGS

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

       Since version 2.1, glibc has provided an emulation of pselect() that is implemented using sigprocmask(2) and select().  This implementation
       remains	vulnerable  to the very race condition that pselect() was designed to prevent.	On systems that lack pselect(), reliable (and more
       portable) signal trapping can be achieved using the self-pipe trick (where a signal handler writes a byte to a pipe whose other end is mon-
       itored by select() in the main program.)

       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 per-
       mitted by POSIX.1-2001.	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.

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

	   exit(EXIT_SUCCESS);
       }

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), epoll(7), time(7)

COLOPHON

       This  page is part of release 3.25 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								    2008-12-05								 SELECT(2)