From: Jamie Lokier <jamie@shareable.org>
To: mtk-manpages@gmx.net
Subject: Update to fcntl(2) man page
Hi Michael,
I have a correction to the fcntl(2) man page.
Under the section for F_SETOWN, which describes how to set the
recipient process or group for SIGIO signals, see this paragraph:
The process or process group to receive the signal can be
selected by using the F_SETOWN command to the fcntl function.
If the file descriptor is a socket, this also selects the recip-
ient of SIGURG signals that are delivered when out-of-band data
arrives on that socket. (SIGURG is sent in any situation where
select(2) would report the socket as having an "exceptional con-
dition".) If the file descriptor corresponds to a terminal
device, then SIGIO signals are sent to the foreground process
group of the terminal.
I would like to add an additional paragraph:
The value given to F_SETOWN has a slightly different meaning
when F_SETSIG is used in a multi-threaded process.
If a non-zero value is given to F_SETSIG, then a positive
value given to F_SETOWN identifies a specific thread within a
process, instead of a whole process. The value is a thread id
not a process id, so you may need to pass the result of
gettid() instead of getpid() to get sensible results when
F_SETSIG is used. (Thread ids are different from process ids,
although they have the same value for some threads depending
on details of the threading library used).
Also, this is the first paragraph of the F_SETSIG section:
Sets the signal sent when input or output becomes possible. A
value of zero means to send the default SIGIO signal. Any other
value (including SIGIO) is the signal to send instead, and in
this case additional info is available to the signal handler if
installed with SA_SIGINFO.
I'd like to append another paragraph right after that one:
Additionally, passing a non-zero value to F_SETSIG changes the
signal recipient from a whole process to a specific thread
within a process. The section on F_SETOWN gives more details.
Date: Thu, 7 Apr 2005 17:58:59 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Michael Kerrisk <mtk-manpages@gmx.net>
Subject: Re: Update to fcntl(2) man page
[[...]]
> I've been trying to follow the kernel source code to verify
> the details you describe above. The relevant place is the
> 'switch' in fs/fcntl.c::send_sigio_to_task() right?
Yes.
> Also, for NPTL, perhaps one needs to mention that for the main
> thread, gettid() == getpid(), which allows the traditional
> use of F_SETSIG / F_SETOWN in programs consisting of a single
> thread -- right?
Yes, that makes sense. It's also fine for the "main thread" with
NPTL, so programs which spawn threads can still use F_SETOWN/F_SETSIG
in the main thread using getpid().
Date: Tue, 12 Apr 2005 15:25:49 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Michael Kerrisk <mtk-manpages@gmx.net>
Subject: Re: Update to fcntl(2) man page
Michael Kerrisk wrote:
> F_SETOWN
> Set the process ID or process group ID that will
> receive SIGIO and SIGURG signals for events on
> file descriptor fd. A process ID is specified as
> a positive value; a process group ID is specified
> as a negative value.
>
> If you set the O_ASYNC status flag on a file
> descriptor (either by providing this flag with the
> open(2) call, or by using the F_SETFL command of
> fcntl), a SIGIO signal is sent whenever input or
> output becomes possible on that file descriptor.
> F_SETSIG can be used to obtain delivery of a sig‐
> nal other than SIGIO.
>
> If the file descriptor fd refers to a socket,
> F_SETOWN also selects the recipient of SIGURG sig‐
> nals that are delivered when out-of-band data
> arrives on that socket. (SIGURG is sent in any
> situation where select(2) would report the socket
> as having an "exceptional condition".)
>
> If a non-zero value is given to F_SETSIG in a
> multi-threaded process, then a positive value
> value given to F_SETOWN has a different meaning:
> instead of being a process ID identifying a whole
> process, it is a thread ID identifying a specific
> thread within a process. Consequently, it may be
> necessary to pass F_SETOWN the result of gettid()
> instead of getpid() to get sensible results when
> F_SETSIG is used. (In current Linux threading
> implementations, a main thread's thread ID is the
> same as its process ID. This means that a single-
> threaded program can equally use gettid() or get‐
> pid() in this scenario.) Note, however, that the
> statements in this paragraph do not apply to the
> SIGURG signal generated for out-of-band data on a
> socket: this signal is always sent to either a
> process or a process group, depending on the value
> given to F_SETOWN.
>
> And the first part of the description of F_SETSIG now reads:
>
> F_SETSIG
> Sets the signal sent when input or output becomes
> possible. A value of zero means to send the
> default SIGIO signal. Any other value (including
> SIGIO) is the signal to send instead, and in this
> case additional info is available to the signal
> handler if installed with SA_SIGINFO.
>
> Additionally, passing a non-zero value to F_SETSIG
> changes the signal recipient from a whole process
> to a specific thread within a process. See the
> desciption of F_SETOWN for more details.
>
> Does the above seem okay to you?
It looks good, but:
1. An omission: It mentions that SIGURG is always sent to the whole
process. SIGIO is also sent to the whole process, instead of
queueing a thread-specific signal, when the signal queue is full.
Programs that mustn't miss readiness events need to handle it.
2. The description could be confusing to LinuxThreads users, because
all the signals are thread-specific in LinuxThreads.
Date: Tue, 12 Apr 2005 16:53:19 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Michael Kerrisk <mtk-manpages@gmx.net>
Subject: Re: Update to fcntl(2) man page
Michael Kerrisk wrote:
> > 1. An omission: It mentions that SIGURG is always sent to the whole
> > process. SIGIO is also sent to the whole process, instead of
> > queueing a thread-specific signal, when the signal queue is full.
> > Programs that mustn't miss readiness events need to handle it.
>
> Sorry -- can you point me to the relevant code for the
> above point please.
In 2.6:
switch (fown->signum) {
siginfo_t si;
default:
/* Queue a rt signal with the appropriate fd as its
value. We use SI_SIGIO as the source, not
SI_KERNEL, since kernel signals always get
delivered even if we can't queue. Failure to
queue in this case _should_ be reported; we fall
back to SIGIO in that case. --sct */
si.si_signo = fown->signum;
si.si_errno = 0;
si.si_code = reason;
/* Make sure we are called with one of the POLL_*
reasons, otherwise we could leak kernel stack into
userspace. */
if ((reason & __SI_MASK) != __SI_POLL)
BUG();
if (reason - POLL_IN >= NSIGPOLL)
si.si_band = ~0L;
else
si.si_band = band_table[reason - POLL_IN];
si.si_fd = fd;
if (!send_sig_info(fown->signum, &si, p))
break;
/* fall-through: fall back on the old plain SIGIO signal */
case 0:
send_group_sig_info(SIGIO, SEND_SIG_PRIV, p);
2.4 is exactly the same, except:
/* fall-through: fall back on the old plain SIGIO signal */
case 0:
send_sig(SIGIO, p, 1);
The fall-through happens when send_sig_info() fails, which happens
when the real-time signal queue is full.
Programs using a queued signal to track file readiness efficiently (as
an alternative to select/poll), must listen for SIGIO in addition to
the real-time signal, as otherwise they will miss notifications when
the queue is full (which happens often on a busy server).
Multi-threaded programs using NPTL must be aware this SIGIO is
process-wide - so receiving it on one thread must cause all threads to
assume a queued signal may be lost. Programs using LinuxThreads do
not have to assume this (but it's safe if they do).
Date: Wed, 13 Apr 2005 17:25:44 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Michael Kerrisk <mtk-manpages@gmx.net>
Subject: Re: Update to fcntl(2) man page
Michael Kerrisk wrote:
> I added a few more sentences to the end of that paragraph
> on F_SETOWN:
>
> If a non-zero value is given to F_SETSIG in a multi-threaded
> process running with a threading library that supports thread
> groups (e.g., NPTL), then a positive value value given to
> F_SETOWN has a different meaning: instead of being a process ID
> identifying a whole process, it is a thread ID identifying a
> specific thread within a process. Consequently, it may be nec-
> essary to pass F_SETOWN the result of gettid() instead of get
> pid() to get sensible results when F_SETSIG is used. (In cur-
> rent Linux threading implementations, a main thread's thread ID
> is the same as its process ID. This means that a single-
> threaded program can equally use gettid() or getpid() in this
> scenario.) Note, however, that the statements in this paragraph
> do not apply to the SIGURG signal generated for out-of-band data
> on a socket: this signal is always sent to either a process or a
> process group, depending on the value given to F_SETOWN. Note
> also that Linux imposes a limit on the number of real-time sig-
> nals that may be queued to a process (see getrlimit(2) and sig-
> nal(7)) and if this limit is reached, then the kernel reverts to
> delivering SIGIO, and this signal is delivered to the entire
> process rather than to a specifc thread.
>
> Look oay now?
Looks ood.
It will take a minor genius to translate that to working
multi-threaded RT-SIGIO code without a tutorial -- and in fact I
haven't heard of any program or library which does it (though I'm
trying to write one) -- but technically it seems to include everything.
as per http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=242638
Noted use of _XOPEN_SOURCE to get macros <sytdlib.h>
as for wait(2).
mtk
Changed name of argument from 'string' to 'command' (like POSIX).
Noted that glibc does nowadays explicitly check for the existence
of the shell if 'command' is NULL, rather than the older behaviour
of assuming the shell exists and always returning 1 if
'command' is NULL.
Other wording and formatting clean-ups.
as per http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=276248
Changed range for "%S" from 0..61 to 0..60.
SUSv3 says 0..60. I think the manual page probably says
0..61, because that's what SUSv2 said.
(Some other implementations' man pages also say 0..61 --
e.g., Solaris 8 & 9, Tru64 5.1B; FreeBSD 5.1 says 0..60.)
The glibc manual currently says 0..60.
Given that SUSv3 says 0..60, I've changed this the
manual page to also say this:
-The second as a decimal number (range 00 to 61).
+The second as a decimal number (range 00 to 60).
+(The range is up to 60 to allow for occasional leap seconds.)
Michael Kerrisk