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Fixes for pselect() description.

This commit is contained in:
Michael Kerrisk 2006-03-12 03:09:50 +00:00
parent 19017a1626
commit d02aa9bc8c
1 changed files with 36 additions and 6 deletions
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42
man2/select.2
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@ -32,8 +32,10 @@
.\" Modified Thu Feb 24 01:41:09 CET 2000 by aeb
.\" Modified Thu Feb 9 22:32:09 CET 2001 by bert hubert <ahu@ds9a.nl>, aeb
.\" Modified Mon Nov 11 14:35:00 PST 2002 by Ben Woodard <ben@zork.net>
.\" 2005-03-11, mtk, modified pselect() text (it is now a system
.\" call in 2.6.16.
.\"
.TH SELECT 2 2001-02-09 "Linux 2.4" "Linux Programmer's Manual"
.TH SELECT 2 2006-03-11 "Linux 2.6.16" "Linux Programmer's Manual"
.SH NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \- synchronous I/O multiplexing
.SH SYNOPSIS
@ -149,15 +151,40 @@ first replaces the current signal mask by the one pointed to by
then does the `select' function, and then restores the original
signal mask.
.PP
The idea of
Other than the difference in the precision of the
.I timeout
argument, the following
.BR pselect ()
is that if one wants to wait for an event, either a signal
or something on a file descriptor, an atomic test is needed to prevent
race conditions. (Suppose the signal handler sets a global flag and
call:
.nf
ready = pselect(n, &readfds, &writefds, &exceptfds,
timeout, &sigmask);
.fi
is equivalent to
.I atomically
executing the following calls:
.nf
sigset_t origmask;
sigprocmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
sigprocmask(SIG_SETMASK, &origmask, NULL);
.fi
.PP
The reason that
.BR 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
.BR select ()
could hang indefinitely if the signal arrived just after the test
but just before the call. On the other hand,
but just before the call.
By contrast,
.BR pselect ()
allows one to first block signals, handle the signals that have come in,
then call
@ -405,6 +432,9 @@ checksum and is discarded. There may be other circumstances.
.\" returns successfully because of an intervening RST from the client.
Thus it may be safer to use O_NONBLOCK on sockets that should not block.
.\" Maybe the kernel should have returned EIO in such a situation?
.\"
.\" FIXME select() (and pselect()?) also modify the timeout
.\" on an EINTR error return; POSIX.1-2001 doesn' permit this.
.SH "SEE ALSO"
For a tutorial with discussion and examples, see
.BR select_tut (2).