-.\" Hey Emacs! This file is -*- nroff -*- source.
-.\"
.\" This manpage is copyright (C) 1992 Drew Eckhardt,
.\" copyright (C) 1995 Michael Shields.
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
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+.\" %%%LICENSE_END
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.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1995-05-18 by Jim Van Zandt <jrv@vanzandt.mv.com>
.\" 2005-03-11, mtk, modified pselect() text (it is now a system
.\" call in 2.6.16.
.\"
-.TH SELECT 2 2012-05-02 "Linux" "Linux Programmer's Manual"
+.TH SELECT 2 2015-01-22 "Linux" "Linux Programmer's Manual"
.SH NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \-
synchronous I/O multiplexing
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.,
-.BR read (2))
-without blocking.
+perform a corresponding I/O operation (e.g.,
+.BR read (2)
+without blocking, or a sufficiently small
+.BR write (2)).
.PP
The operation of
.BR select ()
and
.BR pselect ()
-is identical, with three differences:
+is identical, other than these three differences:
.TP
(i)
.BR select ()
block; in particular, a file descriptor is also ready on end-of-file),
those in
.I writefds
-will be watched to see if a write will not block, and
-those in
+will be watched to see if space is available for write (though a large
+write may still block), and those in
.I exceptfds
will be watched for exceptions.
On exit, the sets are modified in place
.PP
The
.I timeout
-argument specifies the minimum interval that
+argument specifies the interval that
.BR select ()
should block waiting for a file descriptor to become ready.
-(This interval will be rounded up to the system clock granularity,
+The call will block until either:
+.IP * 3
+a file descriptor becomes ready;
+.IP *
+the call is interrupted by a signal handler; or
+.IP *
+the timeout expires.
+.PP
+Note that the
+.I timeout
+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.)
+may overrun by a small amount.
If both fields of the
.I timeval
structure are zero, then
sigset_t origmask;
- sigprocmask(SIG_SETMASK, &sigmask, &origmask);
+ pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
- sigprocmask(SIG_SETMASK, &origmask, NULL);
+ pthread_sigmask(SIG_SETMASK, &origmask, NULL);
.fi
.PP
The reason that
with the desired
.IR sigmask ,
avoiding the race.)
-.SS "The timeout"
+.SS The timeout
The time structures involved are defined in
.I <sys/time.h>
and look like
.\" On BSD, when a timeout occurs, the file descriptor bits are not changed.
.\" - it is certainly true that:
.\" Linux follows SUSv2 and sets the bit masks to zero upon a timeout.
-.SH "RETURN VALUE"
+.SH RETURN VALUE
On success,
.BR select ()
and
which may be zero if the timeout expires before anything interesting happens.
On error, \-1 is returned, and
.I errno
-is set appropriately; the sets and
+is set to indicate the error;
+the file descriptor sets are unmodified,
+and
.I timeout
-become undefined, so do not
-rely on their contents after an error.
+becomes undefined.
.SH ERRORS
.TP
.B EBADF
.TP
.B EINVAL
.I nfds
-is negative or the value contained within
+is negative or exceeds the
+.BR RLIMIT_NOFILE
+resource limit (see
+.BR getrlimit (2)).
+.TP
+.B EINVAL
+the value contained within
.I timeout
is invalid.
.TP
Prior to this,
.BR pselect ()
was emulated in glibc (but see BUGS).
-.SH "CONFORMING TO"
+.SH CONFORMING TO
.BR select ()
conforms to POSIX.1-2001 and
4.4BSD
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
+System\ V variants).
+However, note that the System\ V variant typically
sets the timeout variable before exit, but the BSD variant does not.
.PP
.BR pselect ()
and
.BR pselect ().
-Libc4 and libc5 do not have a
+Under glibc 2.0,
.I <sys/select.h>
-header; under glibc 2.0 and later this header exists.
-Under glibc 2.0 it unconditionally gives the wrong prototype for
+gives the wrong prototype for
.BR pselect ().
-Under glibc 2.1 to 2.2.1 it gives
+Under glibc 2.1 to 2.2.1, it gives
.BR pselect ()
when
.B _GNU_SOURCE
is defined.
-Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
-.SS "Linux Notes"
+Since glibc 2.2.2, the requirements are as shown in the SYNOPSIS.
+.SS Multithreaded applications
+If a file descriptor being monitored by
+.BR select ()
+is closed in another thread, the result is unspecified.
+On some UNIX systems,
+.BR 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
+.BR 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
+.BR select ().
+In summary, any application that relies on a particular behavior
+in this scenario must be considered buggy.
+.\"
+.SS C library/kernel ABI differences
The
.BR pselect ()
interface described in this page is implemented by glibc.
The underlying Linux system call is named
.BR pselect6 ().
-This system call has somewhat different behavior from the gibc
+This system call has somewhat different behavior from the glibc
wrapper function.
The Linux
this is the behavior required by POSIX.1-2001.
The final argument of the
-.BR pselect6()
+.BR pselect6 ()
system call is not a
.I "sigset_t\ *"
pointer, but is instead a structure of the form:
exit(EXIT_SUCCESS);
}
.fi
-.SH "SEE ALSO"
-For a tutorial with discussion and examples, see
-.BR select_tut (2).
-.LP
-For vaguely related stuff, see
+.SH SEE ALSO
.BR accept (2),
.BR connect (2),
.BR poll (2),
.BR read (2),
.BR recv (2),
+.BR restart_syscall (2),
.BR send (2),
.BR sigprocmask (2),
.BR write (2),
.BR epoll (7),
.BR time (7)
+
+For a tutorial with discussion and examples, see
+.BR select_tut (2).
+.SH COLOPHON
+This page is part of release 3.79 of the Linux
+.I 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/.
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