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fcntl-linux.h: add new definitions and manual updates for open file description locks

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Open file description locks have been merged into the Linux kernel for
v3.15.  Add the appropriate command-value definitions and an update to
the manual that describes their usage.
This commit is contained in:
Jeff Layton 2014-07-23 14:21:05 -04:00 committed by Carlos O'Donell
parent cc5fb300fc
commit 0961f7e1e3
5 changed files with 361 additions and 17 deletions
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9
ChangeLog
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@ -1,3 +1,12 @@
2014-07-29 Jeff Layton <jlayton@poochiereds.net>
[BZ #16839]
* manual/llio.texi: Add section about open file description locks.
* manual/examples/ofdlocks.c: Example of open file description
lock usage.
* sysdeps/unix/sysv/linux/bits/fcntl-linux.h: Define F_OFD_GETLK,
F_OFD_SETLK, and F_OFD_SETLKW.
2014-07-23 Allan McRae <allan@archlinux.org>
* po/es.po: Update Spanish translation from translation project.

34
NEWS
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@ -9,20 +9,26 @@ Version 2.20
* The following bugs are resolved with this release:
6804, 9894, 12994, 13347, 13651, 14308, 14770, 15119, 15132, 15347, 15514,
15698, 15804, 15894, 15946, 16002, 16064, 16095, 16198, 16284, 16287,
16315, 16348, 16349, 16354, 16357, 16362, 16447, 16516, 16532, 16539,
16545, 16561, 16562, 16564, 16574, 16599, 16600, 16609, 16610, 16611,
16613, 16619, 16623, 16629, 16632, 16634, 16639, 16642, 16648, 16649,
16670, 16674, 16677, 16680, 16681, 16683, 16689, 16695, 16701, 16706,
16707, 16712, 16713, 16714, 16724, 16731, 16739, 16740, 16743, 16754,
16758, 16759, 16760, 16770, 16786, 16789, 16791, 16796, 16799, 16800,
16815, 16823, 16824, 16831, 16838, 16849, 16854, 16876, 16877, 16878,
16882, 16885, 16888, 16890, 16912, 16915, 16916, 16917, 16918, 16922,
16927, 16928, 16932, 16943, 16958, 16965, 16966, 16967, 16977, 16978,
16984, 16990, 16996, 17009, 17022, 17031, 17042, 17048, 17050, 17058,
17061, 17062, 17069, 17075, 17078, 17079, 17084, 17086, 17088, 17092,
17097, 17125, 17135, 17137, 17150, 17153.
6804, 9894, 12994, 13347, 13651, 14308, 14770, 15119, 15132, 15347,
15514, 15698, 15804, 15894, 15946, 16002, 16064, 16095, 16198, 16284,
16287, 16315, 16348, 16349, 16354, 16357, 16362, 16447, 16516, 16532,
16539, 16545, 16561, 16562, 16564, 16574, 16599, 16600, 16609, 16610,
16611, 16613, 16619, 16623, 16629, 16632, 16634, 16639, 16642, 16648,
16649, 16670, 16674, 16677, 16680, 16681, 16683, 16689, 16695, 16701,
16706, 16707, 16712, 16713, 16714, 16724, 16731, 16739, 16740, 16743,
16754, 16758, 16759, 16760, 16770, 16786, 16789, 16791, 16796, 16799,
16800, 16815, 16823, 16824, 16831, 16838, 16839, 16849, 16854, 16876,
16877, 16878, 16882, 16885, 16888, 16890, 16912, 16915, 16916, 16917,
16918, 16922, 16927, 16928, 16932, 16943, 16958, 16965, 16966, 16967,
16977, 16978, 16984, 16990, 16996, 17009, 17022, 17031, 17042, 17048,
17050, 17058, 17061, 17062, 17069, 17075, 17078, 17079, 17084, 17086,
17088, 17092, 17097, 17125, 17135, 17137, 17150, 17153.
* Support for file description locks is added to systems running the
Linux kernel. The standard file locking interfaces are extended to
operate on file descriptions, not file descriptors, via the use of
F_OFD_GETLK, F_OFD_SETLK, and F_OFD_SETLKW. File description locks
are associated with an open file instead of a process.
* Optimized strchr implementation for AArch64. Contributed by ARM Ltd.

77
manual/examples/ofdlocks.c Normal file
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@ -0,0 +1,77 @@
/* Open File Description Locks Usage Example
Copyright (C) 1991-2014 Free Software Foundation, Inc.
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, see <http://www.gnu.org/licenses/>.
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <pthread.h>
#define FILENAME "/tmp/foo"
#define NUM_THREADS 3
#define ITERATIONS 5
void *
thread_start (void *arg)
{
int i, fd, len;
long tid = (long) arg;
char buf[256];
struct flock lck = {
.l_whence = SEEK_SET,
.l_start = 0,
.l_len = 1,
};
fd = open ("/tmp/foo", O_RDWR | O_CREAT, 0666);
for (i = 0; i < ITERATIONS; i++)
{
lck.l_type = F_WRLCK;
fcntl (fd, F_OFD_SETLKW, &lck);
len = sprintf (buf, "%d: tid=%ld fd=%d\n", i, tid, fd);
lseek (fd, 0, SEEK_END);
write (fd, buf, len);
fsync (fd);
lck.l_type = F_UNLCK;
fcntl (fd, F_OFD_SETLK, &lck);
/* sleep to ensure lock is yielded to another thread */
usleep (1);
}
pthread_exit (NULL);
}
int
main (int argc, char **argv)
{
long i;
pthread_t threads[NUM_THREADS];
truncate (FILENAME, 0);
for (i = 0; i < NUM_THREADS; i++)
pthread_create (&threads[i], NULL, thread_start, (void *) i);
pthread_exit (NULL);
return 0;
}

241
manual/llio.texi
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@ -57,6 +57,10 @@ directly.)
flags associated with open files.
* File Locks:: Fcntl commands for implementing
file locking.
* Open File Description Locks:: Fcntl commands for implementing
open file description locking.
* Open File Description Locks Example:: An example of open file description lock
usage
* Interrupt Input:: Getting an asynchronous signal when
input arrives.
* IOCTLs:: Generic I/O Control operations.
@ -2890,7 +2894,7 @@ Get flags associated with the open file. @xref{File Status Flags}.
Set flags associated with the open file. @xref{File Status Flags}.
@item F_GETLK
Get a file lock. @xref{File Locks}.
Test a file lock. @xref{File Locks}.
@item F_SETLK
Set or clear a file lock. @xref{File Locks}.
@ -2898,6 +2902,18 @@ Set or clear a file lock. @xref{File Locks}.
@item F_SETLKW
Like @code{F_SETLK}, but wait for completion. @xref{File Locks}.
@item F_OFD_GETLK
Test an open file description lock. @xref{Open File Description Locks}.
Specific to Linux.
@item F_OFD_SETLK
Set or clear an open file description lock. @xref{Open File Description Locks}.
Specific to Linux.
@item F_OFD_SETLKW
Like @code{F_OFD_SETLK}, but block until lock is acquired.
@xref{Open File Description Locks}. Specific to Linux.
@item F_GETOWN
Get process or process group ID to receive @code{SIGIO} signals.
@xref{Interrupt Input}.
@ -3576,6 +3592,10 @@ set_nonblock_flag (int desc, int value)
@cindex file locks
@cindex record locking
This section describes record locks that are associated with the process.
There is also a different type of record lock that is associated with the
open file description instead of the process. @xref{Open File Description Locks}.
The remaining @code{fcntl} commands are used to support @dfn{record
locking}, which permits multiple cooperating programs to prevent each
other from simultaneously accessing parts of a file in error-prone
@ -3641,7 +3661,10 @@ the file.
@item pid_t l_pid
This field is the process ID (@pxref{Process Creation Concepts}) of the
process holding the lock. It is filled in by calling @code{fcntl} with
the @code{F_GETLK} command, but is ignored when making a lock.
the @code{F_GETLK} command, but is ignored when making a lock. If the
conflicting lock is an open file description lock
(@pxref{Open File Description Locks}), then this field will be set to
@math{-1}.
@end table
@end deftp
@ -3813,10 +3836,222 @@ that part of the file for writing.
@c ??? This section could use an example program.
Remember that file locks are only a @emph{voluntary} protocol for
Remember that file locks are only an @emph{advisory} protocol for
controlling access to a file. There is still potential for access to
the file by programs that don't use the lock protocol.
@node Open File Description Locks
@section Open File Description Locks
In contrast to process-associated record locks (@pxref{File Locks}),
open file description record locks are associated with an open file
description rather than a process.
Using @code{fcntl} to apply an open file description lock on a region that
already has an existing open file description lock that was created via the
same file descriptor will never cause a lock conflict.
Open file description locks are also inherited by child processes across
@code{fork}, or @code{clone} with @code{CLONE_FILES} set
(@pxref{Creating a Process}), along with the file descriptor.
It is important to distinguish between the open file @emph{description} (an
instance of an open file, usually created by a call to @code{open}) and
an open file @emph{descriptor}, which is a numeric value that refers to the
open file description. The locks described here are associated with the
open file @emph{description} and not the open file @emph{descriptor}.
Using @code{dup} (@pxref{Duplicating Descriptors}) to copy a file
descriptor does not give you a new open file description, but rather copies a
reference to an existing open file description and assigns it to a new
file descriptor. Thus, open file description locks set on a file
descriptor cloned by @code{dup} will never conflict with open file
description locks set on the original descriptor since they refer to the
same open file description. Depending on the range and type of lock
involved, the original lock may be modified by a @code{F_OFD_SETLK} or
@code{F_OFD_SETLKW} command in this situation however.
Open file description locks always conflict with process-associated locks,
even if acquired by the same process or on the same open file
descriptor.
Open file description locks use the same @code{struct flock} as
process-associated locks as an argument (@pxref{File Locks}) and the
macros for the @code{command} values are also declared in the header file
@file{fcntl.h}. To use them, the macro @code{_GNU_SOURCE} must be
defined prior to including any header file.
In contrast to process-associated locks, any @code{struct flock} used as
an argument to open file description lock commands must have the @code{l_pid}
value set to @math{0}. Also, when returning information about an
open file description lock in a @code{F_GETLK} or @code{F_OFD_GETLK} request,
the @code{l_pid} field in @code{struct flock} will be set to @math{-1}
to indicate that the lock is not associated with a process.
When the same @code{struct flock} is reused as an argument to a
@code{F_OFD_SETLK} or @code{F_OFD_SETLKW} request after being used for an
@code{F_OFD_GETLK} request, it is necessary to inspect and reset the
@code{l_pid} field to @math{0}.
@pindex fcntl.h.
@deftypevr Macro int F_OFD_GETLK
This macro is used as the @var{command} argument to @code{fcntl}, to
specify that it should get information about a lock. This command
requires a third argument of type @w{@code{struct flock *}} to be passed
to @code{fcntl}, so that the form of the call is:
@smallexample
fcntl (@var{filedes}, F_OFD_GETLK, @var{lockp})
@end smallexample
If there is a lock already in place that would block the lock described
by the @var{lockp} argument, information about that lock is written to
@code{*@var{lockp}}. Existing locks are not reported if they are
compatible with making a new lock as specified. Thus, you should
specify a lock type of @code{F_WRLCK} if you want to find out about both
read and write locks, or @code{F_RDLCK} if you want to find out about
write locks only.
There might be more than one lock affecting the region specified by the
@var{lockp} argument, but @code{fcntl} only returns information about
one of them. Which lock is returned in this situation is undefined.
The @code{l_whence} member of the @var{lockp} structure are set to
@code{SEEK_SET} and the @code{l_start} and @code{l_len} fields are set
to identify the locked region.
If no conflicting lock exists, the only change to the @var{lockp} structure
is to update the @code{l_type} field to the value @code{F_UNLCK}.
The normal return value from @code{fcntl} with this command is either @math{0}
on success or @math{-1}, which indicates an error. The following @code{errno}
error conditions are defined for this command:
@table @code
@item EBADF
The @var{filedes} argument is invalid.
@item EINVAL
Either the @var{lockp} argument doesn't specify valid lock information,
the operating system kernel doesn't support open file description locks, or the file
associated with @var{filedes} doesn't support locks.
@end table
@end deftypevr
@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int F_OFD_SETLK
This macro is used as the @var{command} argument to @code{fcntl}, to
specify that it should set or clear a lock. This command requires a
third argument of type @w{@code{struct flock *}} to be passed to
@code{fcntl}, so that the form of the call is:
@smallexample
fcntl (@var{filedes}, F_OFD_SETLK, @var{lockp})
@end smallexample
If the open file already has a lock on any part of the
region, the old lock on that part is replaced with the new lock. You
can remove a lock by specifying a lock type of @code{F_UNLCK}.
If the lock cannot be set, @code{fcntl} returns immediately with a value
of @math{-1}. This command does not wait for other tasks
to release locks. If @code{fcntl} succeeds, it returns @math{0}.
The following @code{errno} error conditions are defined for this
command:
@table @code
@item EAGAIN
The lock cannot be set because it is blocked by an existing lock on the
file.
@item EBADF
Either: the @var{filedes} argument is invalid; you requested a read lock
but the @var{filedes} is not open for read access; or, you requested a
write lock but the @var{filedes} is not open for write access.
@item EINVAL
Either the @var{lockp} argument doesn't specify valid lock information,
the operating system kernel doesn't support open file description locks, or the
file associated with @var{filedes} doesn't support locks.
@item ENOLCK
The system has run out of file lock resources; there are already too
many file locks in place.
Well-designed file systems never report this error, because they have no
limitation on the number of locks. However, you must still take account
of the possibility of this error, as it could result from network access
to a file system on another machine.
@end table
@end deftypevr
@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int F_OFD_SETLKW
This macro is used as the @var{command} argument to @code{fcntl}, to
specify that it should set or clear a lock. It is just like the
@code{F_OFD_SETLK} command, but causes the process to wait until the request
can be completed.
This command requires a third argument of type @code{struct flock *}, as
for the @code{F_OFD_SETLK} command.
The @code{fcntl} return values and errors are the same as for the
@code{F_OFD_SETLK} command, but these additional @code{errno} error conditions
are defined for this command:
@table @code
@item EINTR
The function was interrupted by a signal while it was waiting.
@xref{Interrupted Primitives}.
@end table
@end deftypevr
Open file description locks are useful in the same sorts of situations as
process-associated locks. They can also be used to synchronize file
access between threads within the same process by having each thread perform
its own @code{open} of the file, to obtain its own open file description.
Because open file description locks are automatically freed only upon
closing the last file descriptor that refers to the open file
description, this locking mechanism avoids the possibility that locks
are inadvertently released due to a library routine opening and closing
a file without the application being aware.
As with process-associated locks, open file description locks are advisory.
@node Open File Description Locks Example
@section Open File Description Locks Example
Here is an example of using open file description locks in a threaded
program. If this program used process-associated locks, then it would be
subject to data corruption because process-associated locks are shared
by the threads inside a process, and thus cannot be used by one thread
to lock out another thread in the same process.
Proper error handling has been omitted in the following program for
brevity.
@smallexample
@include ofdlocks.c.texi
@end smallexample
This example creates three threads each of which loops five times,
appending to the file. Access to the file is serialized via open file
description locks. If we compile and run the above program, we'll end up
with /tmp/foo that has 15 lines in it.
If we, however, were to replace the @code{F_OFD_SETLK} and
@code{F_OFD_SETLKW} commands with their process-associated lock
equivalents, the locking essentially becomes a noop since it is all done
within the context of the same process. That leads to data corruption
(typically manifested as missing lines) as some threads race in and
overwrite the data written by others.
@node Interrupt Input
@section Interrupt-Driven Input

17
sysdeps/unix/sysv/linux/bits/fcntl-linux.h
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@ -117,6 +117,23 @@
# define F_SETLKW64 14 /* Set record locking info (blocking). */
#endif
/* open file description locks.
Usually record locks held by a process are released on *any* close and are
not inherited across a fork.
These cmd values will set locks that conflict with process-associated record
locks, but are "owned" by the opened file description, not the process.
This means that they are inherited across fork or clone with CLONE_FILES
like BSD (flock) locks, and they are only released automatically when the
last reference to the the file description against which they were acquired
is put. */
#ifdef __USE_GNU
# define F_OFD_GETLK 36
# define F_OFD_SETLK 37
# define F_OFD_SETLKW 38
#endif
#ifdef __USE_LARGEFILE64
# define O_LARGEFILE __O_LARGEFILE
#endif