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Date: Wed, 30 Apr 2014 08:15:01 -0400
From: Jeff Layton <jlayton@...chiereds.net>
To: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>
Subject: Re: [PATCH man-pages v1] fcntl.2: update manpage with verbiage
about open file description locks
On Wed, 30 Apr 2014 12:50:23 +0200
"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> wrote:
> [CC += linux-man]
>
> Jeff,
>
> Thanks very much for writing this patch!
>
> I've taken your patch into a branch and add a number of details. I have
> one or two questions below.
>
> On 04/29/2014 08:51 PM, Jeff Layton wrote:
> > Signed-off-by: Jeff Layton <jlayton@...chiereds.net>
> > ---
> > man2/fcntl.2 | 112 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 109 insertions(+), 3 deletions(-)
> >
> > diff --git a/man2/fcntl.2 b/man2/fcntl.2
> > index d0154a6d9f42..8d119dfec24c 100644
> > --- a/man2/fcntl.2
> > +++ b/man2/fcntl.2
> > @@ -191,6 +191,9 @@ and
> > .BR O_SYNC
> > flags; see BUGS, below.
> > .SS Advisory locking
> > +This section describes traditional POSIX record locks. Also see the section on
> > +open file description locks below.
> > +.PP
> > .BR F_SETLK ,
> > .BR F_SETLKW ,
> > and
> > @@ -213,7 +216,8 @@ struct flock {
> > off_t l_start; /* Starting offset for lock */
> > off_t l_len; /* Number of bytes to lock */
> > pid_t l_pid; /* PID of process blocking our lock
> > - (F_GETLK only) */
> > + (returned for F_GETLK and F_OFD_GETLK only. Set
> > + to 0 for open file description locks) */
> > ...
> > };
> > .fi
> > @@ -349,9 +353,13 @@ returns details about one of these locks in the
> > .IR l_type ", " l_whence ", " l_start ", and " l_len
> > fields of
> > .I lock
> > -and sets
> > +.
> > +If the conflicting lock is a traditional POSIX lock, then the
> > +.I l_pid
> > +to be the PID of the process holding that lock. If the
> > +conflicting lock is an open file description lock, then the
> > .I l_pid
> > -to be the PID of the process holding that lock.
> > +will be set to \-1.
> > Note that the information returned by
> > .BR F_GETLK
> > may already be out of date by the time the caller inspects it.
> > @@ -394,6 +402,104 @@ should be avoided; use
> > and
> > .BR write (2)
> > instead.
> > +.SS Open file description locks (non-POSIX)
> > +.BR F_OFD_GETLK ", " F_OFD_SETLK " and " F_OFD_SETLKW
> > +are used to acquire, release and test open file description record locks.
> > +These are byte-range locks that work identically to the traditional advisory
> > +record locks described above, but are associated with the open file description
> > +on which they were acquired rather than the process, much like locks acquired
> > +with
> > +.BR flock (2)
> > +.
> > +.PP
> > +Unlike traditional advisory record locks, these locks are inherited
> > +across
> > +.BR fork (2)
> > +and
> > +.BR clone (2)
> > +with
> > +.BR CLONE_FILES
> > +and are only released on the last close of the open file description instead
> > +of being released on any close of the file.
> > +.PP
> > +Open file description locks always conflict with traditional record locks,
> > +even when they are acquired by the same process on the same file descriptor.
> > +They only conflict with each other when they are acquired on different
> > +open file descriptions.
> > +.PP
> > +Note that in contrast to traditional record locks, the
> > +.I flock
> > +structure passed in as an argument to the open file description lock commands
> > +must have the
> > +.I l_pid
> > +value set to 0.
>
> In ERRORS, I added EINVAL for this case.
>
> > +.TP
> > +.BR F_OFD_SETLK " (\fIstruct flock *\fP)"
> > +Acquire an open file description lock (when
> > +.I l_type
> > +is
> > +.B F_RDLCK
> > +or
> > +.BR F_WRLCK )
> > +or release an open file description lock (when
> > +.I l_type
> > +is
> > +.BR F_UNLCK )
> > +on the bytes specified by the
> > +.IR l_whence ", " l_start ", and " l_len
> > +fields of
> > +.IR lock .
> > +If a conflicting lock is held by another process,
> > +this call returns \-1 and sets
> > +.I errno
> > +to
> > +.B EACCES
> > +or
> > +.BR EAGAIN .
>
> The "EACCES or EAGAIN" thing comes from POSIX, because different
> implementations of tradition record locks returned one of these errors.
> So, portable applications using traditional locks must handle either
> possibility. However, that argument doesn't apply for these new locks.
> Surely, we just want to say "set errno to EAGAIN" for this case?
>
> > +.TP
> > +.BR F_OFD_SETLKW " (\fIstruct flock *\fP)"
> > +As for
> > +.BR F_OFD_SETLK ,
> > +but if a conflicting lock is held on the file, then wait for that lock to be
> > +released. If a signal is caught while waiting, then the call is interrupted
> > +and (after the signal handler has returned) returns immediately (with return
> > +value \-1 and
> > +.I errno
> > +set to
> > +.BR EINTR ;
> > +see
> > +.BR signal (7)).
> > +.TP
> > +.BR F_OFD_GETLK " (\fIstruct flock *\fP)"
> > +On input to this call,
> > +.I lock
> > +describes an open file description lock we would like to place on the file.
> > +If the lock could be placed,
> > +.BR fcntl ()
> > +does not actually place it, but returns
> > +.B F_UNLCK
> > +in the
> > +.I l_type
> > +field of
> > +.I lock
> > +and leaves the other fields of the structure unchanged.
> > +If one or more incompatible locks would prevent
> > +this lock being placed, then
> > +.BR fcntl ()
> > +returns details about one of these locks in the
> > +.IR l_type ", " l_whence ", " l_start ", and " l_len
> > +fields of
> > +.I lock
> > +.
> > +If the conflicting lock is a process-associated record lock, then the
> > +.I l_pid
> > +will be set to the PID of the process holding that lock. If the
> > +conflicting lock is an open file description lock, then the
> > +.I l_pid
> > +will be set to -1 to indicate that it is not associated with a process.
> > +Note that the information returned by
> > +.BR F_OFD_GETLK
> > +may already be out of date by the time the caller inspects it.
> > .SS Mandatory locking
> > (Non-POSIX.)
> > The above record locks may be either advisory or mandatory,
>
> Based on some past conversations, I added a number of details
> to the page, and also reworked your text a little to eliminate some
> of the redundancy with the discussion of traditional locks. Below,
> I've reproduced all of the relevant pieces from the current draft
> (including the existing text on traditional locks). Could I ask
> you to take a look at the pieces marked with '#' in column 1
> (which are places where I either tweaked your text significantly,
> or added details) and let me know if it looks okay.
>
> DESCRIPTION
> Advisory record locking
> # Linux implements traditional ("process-associated") UNIX record
> # locks, as standardized by POSIX. For a Linux-specific alterna‐
> # tive with better semantics, see the discussion of open file
> # description locks below.
>
> F_SETLK, F_SETLKW, and F_GETLK are used to acquire, release, and
> test for the existence of record locks (also known as byte-range,
> file-segment, or file-region locks). The third argument, lock,
> is a pointer to a structure that has at least the following
> fields (in unspecified order).
>
> struct flock {
> ...
> short l_type; /* Type of lock: F_RDLCK,
> F_WRLCK, F_UNLCK */
> short l_whence; /* How to interpret l_start:
> SEEK_SET, SEEK_CUR, SEEK_END */
> off_t l_start; /* Starting offset for lock */
> off_t l_len; /* Number of bytes to lock */
> pid_t l_pid; /* PID of process blocking our lock
> (set by F_GETLK and F_OFD_GETLK) */
> ...
> };
>
> The l_whence, l_start, and l_len fields of this structure specify
> the range of bytes we wish to lock. Bytes past the end of the
> file may be locked, but not bytes before the start of the file.
>
> l_start is the starting offset for the lock, and is interpreted
> relative to either: the start of the file (if l_whence is
> SEEK_SET); the current file offset (if l_whence is SEEK_CUR); or
> the end of the file (if l_whence is SEEK_END). In the final two
> cases, l_start can be a negative number provided the offset does
> not lie before the start of the file.
>
> l_len specifies the number of bytes to be locked. If l_len is
> positive, then the range to be locked covers bytes l_start up to
> and including l_start+l_len-1. Specifying 0 for l_len has the
> special meaning: lock all bytes starting at the location speci‐
> fied by l_whence and l_start through to the end of file, no mat‐
> ter how large the file grows.
>
> POSIX.1-2001 allows (but does not require) an implementation to
> support a negative l_len value; if l_len is negative, the inter‐
> val described by lock covers bytes l_start+l_len up to and
> including l_start-1. This is supported by Linux since kernel
> versions 2.4.21 and 2.5.49.
>
> The l_type field can be used to place a read (F_RDLCK) or a write
> (F_WRLCK) lock on a file. Any number of processes may hold a
> read lock (shared lock) on a file region, but only one process
> may hold a write lock (exclusive lock). An exclusive lock
> excludes all other locks, both shared and exclusive. A single
> process can hold only one type of lock on a file region; if a new
> lock is applied to an already-locked region, then the existing
> lock is converted to the new lock type. (Such conversions may
> involve splitting, shrinking, or coalescing with an existing lock
> if the byte range specified by the new lock does not precisely
> coincide with the range of the existing lock.)
>
> F_SETLK (struct flock *)
> Acquire a lock (when l_type is F_RDLCK or F_WRLCK) or
> release a lock (when l_type is F_UNLCK) on the bytes spec‐
> ified by the l_whence, l_start, and l_len fields of lock.
> If a conflicting lock is held by another process, this
> call returns -1 and sets errno to EACCES or EAGAIN.
>
> F_SETLKW (struct flock *)
> As for F_SETLK, but if a conflicting lock is held on the
> file, then wait for that lock to be released. If a signal
> is caught while waiting, then the call is interrupted and
> (after the signal handler has returned) returns immedi‐
> ately (with return value -1 and errno set to EINTR; see
> signal(7)).
>
> F_GETLK (struct flock *)
> On input to this call, lock describes a lock we would like
> to place on the file. If the lock could be placed,
> fcntl() does not actually place it, but returns F_UNLCK in
> the l_type field of lock and leaves the other fields of
> the structure unchanged.
>
> If one or more incompatible locks would prevent this lock
> being placed, then fcntl() returns details about one of
> these locks in the l_type, l_whence, l_start, and l_len
> fields of lock. If the conflicting lock is a traditional
> (process-associated) record lock, then the l_pid field is
> set to the PID of the process holding that lock. If the
> conflicting lock is an open file description lock, then
> l_pid is set to -1. Note that the returned information
> may already be out of date by the time the caller inspects
> it.
>
> In order to place a read lock, fd must be open for reading. In
> order to place a write lock, fd must be open for writing. To
> place both types of lock, open a file read-write.
>
> As well as being removed by an explicit F_UNLCK, record locks are
> automatically released when the process terminates.
>
> Record locks are not inherited by a child created via fork(2),
> but are preserved across an execve(2).
>
> Because of the buffering performed by the stdio(3) library, the
> use of record locking with routines in that package should be
> avoided; use read(2) and write(2) instead.
>
> # The record locks described above are associated with the process
> # (unlike the open file description locks described below). This
> # has some unfortunate consequences:
>
> # * If a process holding a lock on a file closes any file descrip‐
> # tor referring to the file, then all of the process's locks on
> # the file are released, no matter which file descriptor they
> # were obtained via. This is bad: it means that a process can
> # lose its locks on a file such as /etc/passwd or /etc/mtab when
> # for some reason a library function decides to open, read, and
> # close the same file.
>
> # * The threads in a process share locks. In other words, a mul‐
> # tithreaded program can't use record locking to ensure that
> # threads don't simultaneously access the same region of a file.
>
> # Open file description locks solve both of these problems.
>
> Open file description locks (non-POSIX)
> # Open file description locks are advisory byte-range locks whose
> # operation is in most respects identical to the traditional record
> # locks described above. This lock type is Linux-specific, and
> # available since Linux 3.15.
>
> # The principal difference between the two lock types is that
> # whereas traditional record locks are associated with a process,
> # open file description locks are associated with the open file
> # description on which they are acquired, much like locks acquired
> # with flock(2). Consequently (and unlike traditional advisory
> # record locks), open file description locks are inherited across
> # fork(2) (and clone(2) with CLONE_FILES), and are only automati‐
> # cally released on the last close of the open file description,
> # instead of being released on any close of the file.
>
> Open file description locks always conflict with traditional
> record locks, even when they are acquired by the same process on
> the same file descriptor.
>
> # Open file description locks placed via the same open file
> # description (i.e., via the same file descriptor, or via a dupli‐
> # cate of the file descriptor created by fork(2), dup(2), fcntl(2)
> # F_DUPFD, and so on) are always compatible: if a new lock is
> # placed on an already locked region, then the existing lock is
> # converted to the new lock type. (Such conversions may result in
> # splitting, shrinking, or coalescing with an existing lock as dis‐
> # cussed above.)
>
> # On the other hand, open file description locks may conflict with
> # each other when they are acquired via different open file
> # descriptions. Thus, the threads in a multithreaded program can
> # use open file description locks to synchronize access to a file
> # region by having each thread perform its own open(2) on the file
> # and applying locks via the resulting file descriptor.
>
> As with traditional advisory locks, the third argument to
> fcntl(), lock, is a pointer to an flock structure. By contrast
> with traditional record locks, the l_pid field of that structure
> must be set to zero when using the commands described below.
>
> The commands for working with open file description locks are
> analogous to those used with traditional locks:
>
> F_OFD_SETLK (struct flock *)
> Acquire an open file description lock (when l_type is
> F_RDLCK or F_WRLCK) or release an open file description
> lock (when l_type is F_UNLCK) on the bytes specified by
> the l_whence, l_start, and l_len fields of lock. If a
> conflicting lock is held by another process, this call
> returns -1 and sets errno to EACCES or EAGAIN.
>
> F_OFD_SETLKW (struct flock *)
> As for F_OFD_SETLK, but if a conflicting lock is held on
> the file, then wait for that lock to be released. If a
> signal is caught while waiting, then the call is inter‐
> rupted and (after the signal handler has returned) returns
> immediately (with return value -1 and errno set to EINTR;
> see signal(7)).
>
> F_OFD_GETLK (struct flock *)
> On input to this call, lock describes an open file
> description lock we would like to place on the file. If
> the lock could be placed, fcntl() does not actually place
> it, but returns F_UNLCK in the l_type field of lock and
> leaves the other fields of the structure unchanged. If
> one or more incompatible locks would prevent this lock
> being placed, then details about one of these locks are
> returned via lock, as described above for F_GETLK.
>
> Mandatory locking
> Warning: the Linux implementation of mandatory locking is unreli‐
> able. See BUGS below.
>
> # By default, both traditional (process-associated) and open file
> # description record locks are advisory. Advisory locks are not
> # enforced and are useful only between cooperating processes.
>
> Both lock types can also be mandatory. Mandatory locks are
> enforced for all processes. If a process tries to perform an
> incompatible access (e.g., read(2) or write(2)) on a file region
> that has an incompatible mandatory lock, then the result depends
> upon whether the O_NONBLOCK flag is enabled for its open file
> description. If the O_NONBLOCK flag is not enabled, then the
> system call is blocked until the lock is removed or converted to
> a mode that is compatible with the access. If the O_NONBLOCK
> flag is enabled, then the system call fails with the error
> EAGAIN.
>
> To make use of mandatory locks, mandatory locking must be enabled
> both on the filesystem that contains the file to be locked, and
> on the file itself. Mandatory locking is enabled on a filesystem
> using the "-o mand" option to mount(8), or the MS_MANDLOCK flag
> for mount(2). Mandatory locking is enabled on a file by dis‐
> abling group execute permission on the file and enabling the set-
> group-ID permission bit (see chmod(1) and chmod(2)).
>
> Mandatory locking is not specified by POSIX. Some other systems
> also support mandatory locking, although the details of how to
> enable it vary across systems.
>
> RETURN VALUE
> For a successful call, the return value depends on the operation:
>
> F_DUPFD The new descriptor.
>
> F_GETFD Value of file descriptor flags.
>
> F_GETFL Value of file status flags.
>
> F_GETLEASE
> Type of lease held on file descriptor.
>
> F_GETOWN Value of descriptor owner.
>
> F_GETSIG Value of signal sent when read or write becomes possi‐
> ble, or zero for traditional SIGIO behavior.
>
> F_GETPIPE_SZ
> The pipe capacity.
>
> # All other commands
> # Zero.
>
> # On error, -1 is returned, and errno is set appropriately.
>
> ERRORS
> [...]
>
> # EINVAL cmd is F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and
> # l_pid was not specified as zero.
>
> [...]
>
> CONFORMING TO
> [...]
> F_OFD_SETLK, F_OFD_SETLKW, and F_OFD_GETLK are Linux-specific,
> but work is being done to have them included in the next version
> of POSIX.1.
> $ vi f
> f ==> /hdd/backup/home/mtk/man-pages/man-pages/man2/f/2014-04-30_12:44:55
>
> $ cat f
> DESCRIPTION
> [...]
>
> Advisory record locking
> # Linux implements traditional ("process-associated") UNIX record
> # locks, as standardized by POSIX. For a Linux-specific alterna‐
> # tive with better semantics, see the discussion of open file
> # description locks below.
>
> F_SETLK, F_SETLKW, and F_GETLK are used to acquire, release, and
> test for the existence of record locks (also known as byte-range,
> file-segment, or file-region locks). The third argument, lock,
> is a pointer to a structure that has at least the following
> fields (in unspecified order).
>
> struct flock {
> ...
> short l_type; /* Type of lock: F_RDLCK,
> F_WRLCK, F_UNLCK */
> short l_whence; /* How to interpret l_start:
> SEEK_SET, SEEK_CUR, SEEK_END */
> off_t l_start; /* Starting offset for lock */
> off_t l_len; /* Number of bytes to lock */
> pid_t l_pid; /* PID of process blocking our lock
> (set by F_GETLK and F_OFD_GETLK) */
> ...
> };
>
> The l_whence, l_start, and l_len fields of this structure specify
> the range of bytes we wish to lock. Bytes past the end of the
> file may be locked, but not bytes before the start of the file.
>
> l_start is the starting offset for the lock, and is interpreted
> relative to either: the start of the file (if l_whence is
> SEEK_SET); the current file offset (if l_whence is SEEK_CUR); or
> the end of the file (if l_whence is SEEK_END). In the final two
> cases, l_start can be a negative number provided the offset does
> not lie before the start of the file.
>
> l_len specifies the number of bytes to be locked. If l_len is
> positive, then the range to be locked covers bytes l_start up to
> and including l_start+l_len-1. Specifying 0 for l_len has the
> special meaning: lock all bytes starting at the location speci‐
> fied by l_whence and l_start through to the end of file, no mat‐
> ter how large the file grows.
>
> POSIX.1-2001 allows (but does not require) an implementation to
> support a negative l_len value; if l_len is negative, the inter‐
> val described by lock covers bytes l_start+l_len up to and
> including l_start-1. This is supported by Linux since kernel
> versions 2.4.21 and 2.5.49.
>
> The l_type field can be used to place a read (F_RDLCK) or a write
> (F_WRLCK) lock on a file. Any number of processes may hold a
> read lock (shared lock) on a file region, but only one process
> may hold a write lock (exclusive lock). An exclusive lock
> excludes all other locks, both shared and exclusive. A single
> process can hold only one type of lock on a file region; if a new
> lock is applied to an already-locked region, then the existing
> lock is converted to the new lock type. (Such conversions may
> involve splitting, shrinking, or coalescing with an existing lock
> if the byte range specified by the new lock does not precisely
> coincide with the range of the existing lock.)
>
> F_SETLK (struct flock *)
> Acquire a lock (when l_type is F_RDLCK or F_WRLCK) or
> release a lock (when l_type is F_UNLCK) on the bytes spec‐
> ified by the l_whence, l_start, and l_len fields of lock.
> If a conflicting lock is held by another process, this
> call returns -1 and sets errno to EACCES or EAGAIN.
>
> F_SETLKW (struct flock *)
> As for F_SETLK, but if a conflicting lock is held on the
> file, then wait for that lock to be released. If a signal
> is caught while waiting, then the call is interrupted and
> (after the signal handler has returned) returns immedi‐
> ately (with return value -1 and errno set to EINTR; see
> signal(7)).
>
> F_GETLK (struct flock *)
> On input to this call, lock describes a lock we would like
> to place on the file. If the lock could be placed,
> fcntl() does not actually place it, but returns F_UNLCK in
> the l_type field of lock and leaves the other fields of
> the structure unchanged.
>
> If one or more incompatible locks would prevent this lock
> being placed, then fcntl() returns details about one of
> these locks in the l_type, l_whence, l_start, and l_len
> fields of lock. If the conflicting lock is a traditional
> (process-associated) record lock, then the l_pid field is
> set to the PID of the process holding that lock. If the
> conflicting lock is an open file description lock, then
> l_pid is set to -1. Note that the returned information
> may already be out of date by the time the caller inspects
> it.
>
> In order to place a read lock, fd must be open for reading. In
> order to place a write lock, fd must be open for writing. To
> place both types of lock, open a file read-write.
>
> As well as being removed by an explicit F_UNLCK, record locks are
> automatically released when the process terminates.
>
> Record locks are not inherited by a child created via fork(2),
> but are preserved across an execve(2).
>
> Because of the buffering performed by the stdio(3) library, the
> use of record locking with routines in that package should be
> avoided; use read(2) and write(2) instead.
>
> # The record locks described above are associated with the process
> # (unlike the open file description locks described below). This
> # has some unfortunate consequences:
>
> # * If a process holding a lock on a file closes any file descrip‐
> # tor referring to the file, then all of the process's locks on
> # the file are released, no matter which file descriptor they
> # were obtained via. This is bad: it means that a process can
"were obtained via" is a little awkward. How about "regardless of which
file descriptor on which they were obtained".
> # lose its locks on a file such as /etc/passwd or /etc/mtab when
> # for some reason a library function decides to open, read, and
> # close the same file.
>
> # * The threads in a process share locks. In other words, a mul‐
> # tithreaded program can't use record locking to ensure that
> # threads don't simultaneously access the same region of a file.
>
> # Open file description locks solve both of these problems.
>
> Open file description locks (non-POSIX)
> # Open file description locks are advisory byte-range locks whose
> # operation is in most respects identical to the traditional record
> # locks described above. This lock type is Linux-specific, and
> # available since Linux 3.15.
>
> # The principal difference between the two lock types is that
> # whereas traditional record locks are associated with a process,
> # open file description locks are associated with the open file
> # description on which they are acquired, much like locks acquired
> # with flock(2). Consequently (and unlike traditional advisory
> # record locks), open file description locks are inherited across
> # fork(2) (and clone(2) with CLONE_FILES), and are only automati‐
> # cally released on the last close of the open file description,
> # instead of being released on any close of the file.
>
> Open file description locks always conflict with traditional
> record locks, even when they are acquired by the same process on
> the same file descriptor.
>
> # Open file description locks placed via the same open file
> # description (i.e., via the same file descriptor, or via a dupli‐
> # cate of the file descriptor created by fork(2), dup(2), fcntl(2)
> # F_DUPFD, and so on) are always compatible: if a new lock is
> # placed on an already locked region, then the existing lock is
> # converted to the new lock type. (Such conversions may result in
> # splitting, shrinking, or coalescing with an existing lock as dis‐
> # cussed above.)
>
> # On the other hand, open file description locks may conflict with
> # each other when they are acquired via different open file
> # descriptions. Thus, the threads in a multithreaded program can
> # use open file description locks to synchronize access to a file
> # region by having each thread perform its own open(2) on the file
> # and applying locks via the resulting file descriptor.
>
> As with traditional advisory locks, the third argument to
> fcntl(), lock, is a pointer to an flock structure. By contrast
> with traditional record locks, the l_pid field of that structure
> must be set to zero when using the commands described below.
>
> The commands for working with open file description locks are
> analogous to those used with traditional locks:
>
> F_OFD_SETLK (struct flock *)
> Acquire an open file description lock (when l_type is
> F_RDLCK or F_WRLCK) or release an open file description
> lock (when l_type is F_UNLCK) on the bytes specified by
> the l_whence, l_start, and l_len fields of lock. If a
> conflicting lock is held by another process, this call
> returns -1 and sets errno to EACCES or EAGAIN.
>
> F_OFD_SETLKW (struct flock *)
> As for F_OFD_SETLK, but if a conflicting lock is held on
> the file, then wait for that lock to be released. If a
> signal is caught while waiting, then the call is inter‐
> rupted and (after the signal handler has returned) returns
> immediately (with return value -1 and errno set to EINTR;
> see signal(7)).
>
> F_OFD_GETLK (struct flock *)
> On input to this call, lock describes an open file
> description lock we would like to place on the file. If
> the lock could be placed, fcntl() does not actually place
> it, but returns F_UNLCK in the l_type field of lock and
> leaves the other fields of the structure unchanged. If
> one or more incompatible locks would prevent this lock
> being placed, then details about one of those locks are
> returned via lock, as described above for F_GETLK.
>
> Mandatory locking
> Warning: the Linux implementation of mandatory locking is unreli‐
> able. See BUGS below.
>
> # By default, both traditional (process-associated) and open file
> # description record locks are advisory. Advisory locks are not
> # enforced and are useful only between cooperating processes.
>
> Both lock types can also be mandatory. Mandatory locks are
> enforced for all processes. If a process tries to perform an
> incompatible access (e.g., read(2) or write(2)) on a file region
> that has an incompatible mandatory lock, then the result depends
> upon whether the O_NONBLOCK flag is enabled for its open file
> description. If the O_NONBLOCK flag is not enabled, then the
> system call is blocked until the lock is removed or converted to
> a mode that is compatible with the access. If the O_NONBLOCK
> flag is enabled, then the system call fails with the error
> EAGAIN.
>
> To make use of mandatory locks, mandatory locking must be enabled
> both on the filesystem that contains the file to be locked, and
> on the file itself. Mandatory locking is enabled on a filesystem
> using the "-o mand" option to mount(8), or the MS_MANDLOCK flag
> for mount(2). Mandatory locking is enabled on a file by dis‐
> abling group execute permission on the file and enabling the set-
> group-ID permission bit (see chmod(1) and chmod(2)).
>
> Mandatory locking is not specified by POSIX. Some other systems
> also support mandatory locking, although the details of how to
> enable it vary across systems.
>
> [...]
>
> RETURN VALUE
> For a successful call, the return value depends on the operation:
>
> F_DUPFD The new descriptor.
>
> F_GETFD Value of file descriptor flags.
>
> F_GETFL Value of file status flags.
>
> F_GETLEASE
> Type of lease held on file descriptor.
>
> F_GETOWN Value of descriptor owner.
>
> F_GETSIG Value of signal sent when read or write becomes possi‐
> ble, or zero for traditional SIGIO behavior.
>
> F_GETPIPE_SZ
> The pipe capacity.
>
> # All other commands
> # Zero.
>
> # On error, -1 is returned, and errno is set appropriately.
>
> ERRORS
> [...]
>
> # EINVAL cmd is F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and
> # l_pid was not specified as zero.
>
The kernel will also return -EINVAL if it doesn't recognize the cmd
value being passed in. It may be worth mentioning that as well as
that's the best mechanism to tell whether the kernel actually supports
OFD locks.
> [...]
>
> CONFORMING TO
> [...]
> # F_OFD_SETLK, F_OFD_SETLKW, and F_OFD_GETLK are Linux-specific,
> # but work is being done to have them included in the next version
> # of POSIX.1.
>
>
> Cheers,
>
> Michael
>
>
Other than the two nits above, this looks great.
Thanks!
--
Jeff Layton <jlayton@...chiereds.net>
--
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