-.\" Hey Emacs! This file is -*- nroff -*- source.
-.\"
.\" This manpage is copyright (C) 1992 Drew Eckhardt,
.\" copyright (C) 1995 Michael Shields.
.\"
+.\" %%%LICENSE_START(VERBATIM)
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.\" manual provided the copyright notice and this permission notice are
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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 2008-12-05 "Linux" "Linux Programmer's Manual"
+.TH SELECT 2 2014-01-31 "Linux" "Linux Programmer's Manual"
.SH NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \-
synchronous I/O multiplexing
.BR select ()
and
.BR pselect ()
-is identical, with three differences:
+is identical, other than these three differences:
.TP
(i)
.BR select ()
.I nfds
is the highest-numbered file descriptor in any of the three sets, plus 1.
.PP
+The
.I timeout
-is an upper bound on the amount of time elapsed before
+argument specifies the interval that
.BR select ()
-returns.
+should block waiting for a file descriptor to become ready.
+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.
If both fields of the
.I timeval
-stucture are zero, then
+structure are zero, then
.BR select ()
returns immediately.
(This is useful for polling.)
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
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
.B _GNU_SOURCE
is defined.
Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
-.SS "Linux Notes"
-The Linux
+.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 Linux notes
+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 glibc
+wrapper function.
+
+The Linux
+.BR pselect6 ()
system call modifies its
.I timeout
argument.
is passed to the system call.
Thus, the glibc
.BR pselect ()
-function does not modify its timeout argument;
+function does not modify its
+.I timeout
+argument;
this is the behavior required by POSIX.1-2001.
+
+The final argument of the
+.BR pselect6 ()
+system call is not a
+.I "sigset_t\ *"
+pointer, but is instead a structure of the form:
+.in +4
+.nf
+
+struct {
+ const sigset_t *ss; /* Pointer to signal set */
+ size_t ss_len; /* Size (in bytes) of object pointed
+ to by 'ss' */
+};
+
+.fi
+.in
+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
+support a maximum of 6 arguments to a system call.
.SH BUGS
Glibc 2.0 provided a version of
.BR pselect ()
.I sigmask
argument.
-Since version 2.1, glibc has provided an emulation of
+Starting with version 2.1, glibc provided an emulation of
.BR pselect ()
-that is implemented using
+that was implemented using
.BR sigprocmask (2)
and
.BR select ().
-This implementation remains vulnerable to the very race condition that
+This implementation remained vulnerable to the very race condition that
.BR pselect ()
was designed to prevent.
+Modern versions of glibc use the (race-free)
+.BR pselect ()
+system call on kernels where it is provided.
+
On systems that lack
.BR 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
+using the self-pipe trick.
+In this technique,
+a signal handler writes a byte to a pipe whose other end
is monitored by
.BR select ()
-in the main program.)
+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 pipe.)
Under Linux,
.BR select ()
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 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 3.65 of the Linux
+.I man-pages
+project.
+A description of the project,
+and information about reporting bugs,
+can be found at
+\%http://www.kernel.org/doc/man\-pages/.
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