fcntl — manipulate file descriptor
#include <unistd.h> #include <fcntl.h>
int
fcntl( |
int fd, |
| int cmd, | |
... /* arg
*/); |
fcntl() performs one of the
operations described below on the open file descriptor
fd. The operation is
determined by cmd.
fcntl() can take an optional
third argument. Whether or not this argument is required is
determined by cmd.
The required argument type is indicated in parentheses after
each cmd name (in
most cases, the required type is int, and we identify the argument using the
name arg), or
void is specified if the argument
is not required.
Certain of the operations below are supported only since a
particular Linux kernel version. The preferred method of
checking whether the host kernel supports a particular
operation is to invoke fcntl()
with the desired cmd
value and then test whether the call failed with EINVAL, indicating that the kernel does
not recognize this value.
F_DUPFD (int)Find the lowest numbered available file descriptor
greater than or equal to arg and make it be a
copy of fd.
This is different from dup2(2), which uses
exactly the descriptor specified.
On success, the new descriptor is returned.
See dup(2) for further details.
F_DUPFD_CLOEXEC (int; since Linux 2.6.24)As for F_DUPFD, but
additionally set the close-on-exec flag for the
duplicate descriptor. Specifying this flag permits a
program to avoid an additional fcntl() F_SETFD operation to set the
FD_CLOEXEC flag. For an
explanation of why this flag is useful, see the
description of O_CLOEXEC in open(2).
The following commands manipulate the flags associated
with a file descriptor. Currently, only one such flag is
defined: FD_CLOEXEC, the
close-on-exec flag. If the FD_CLOEXEC bit is 0, the file descriptor
will remain open across an execve(2), otherwise it
will be closed.
F_GETFD (void)Read the file descriptor flags; arg is ignored.
F_SETFD (int)Set the file descriptor flags to the value
specified by arg.
In multithreaded programs, using fcntl() F_SETFD to set the close-on-exec flag at
the same time as another thread performs a fork(2) plus execve(2) is vulnerable
to a race condition that may unintentionally leak the file
descriptor to the program executed in the child process.
See the discussion of the O_CLOEXEC flag in open(2) for details and a
remedy to the problem.
Each open file description has certain associated status
flags, initialized by open(2) and possibly
modified by fcntl().
Duplicated file descriptors (made with dup(2), fcntl(F_DUPFD), fork(2), etc.) refer to
the same open file description, and thus share the same
file status flags.
The file status flags and their semantics are described in open(2).
F_GETFL (void)Get the file access mode and the file status
flags; arg
is ignored.
F_SETFL (int)Set the file status flags to the value specified
by arg.
File access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags
(i.e., O_CREAT,
O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored. On
Linux this command can change only the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags. It is not
possible to change the O_DSYNC and O_SYNC flags; see BUGS, below.
Linux implements traditional ("process-associated") UNIX record locks, as standardized by POSIX. For a Linux-specific alternative 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 specified
by l_whence and
l_start through
to the end of file, no matter 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
interval 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
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. (The error returned in
this case differs across implementations, so POSIX
requires a portable application to check for both
errors.)
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
immediately (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 those 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.
When placing locks with F_SETLKW, the kernel detects deadlocks, whereby two or
more processes have their lock requests mutually blocked by
locks held by the other processes. For example, suppose
process A holds a write lock on byte 100 of a file, and
process B holds a write lock on byte 200. If each process
then attempts to lock the byte already locked by the other
process using F_SETLKW, then,
without deadlock detection, both processes would remain
blocked indefinitely. When the kernel detects such
deadlocks, it causes one of the blocking lock requests to
immediately fail with the error EDEADLK; an application that encounters
such an error should release some of its locks to allow
other applications to proceed before attempting regain the
locks that it requires. Circular deadlocks involving more
than two processes are also detected. Note, however, that
there are limitations to the kernel's deadlock-detection
algorithm; see BUGS.
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 closes any file descriptor
referring to a file, then all of the process's locks
on that file are released, regardless of the file
descriptor(s) on which the locks were obtained. 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 multithreaded 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 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. For an explanation of open file descriptions, see open(2)
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
automatically 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 duplicate 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 discussed
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 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 interrupted
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.
In the current implementation, no deadlock detection is performed for open file description locks. (This contrasts with process-associated record locks, for which the kernel does perform deadlock detection.)
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Warning |
|---|---|
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the Linux implementation of mandatory locking is unreliable. 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 disabling 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.
F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG and F_SETSIG are used to manage I/O
availability signals:
F_GETOWN (void)Return (as the function result) the process ID or
process group currently receiving SIGIO and SIGURG signals for events on file
descriptor fd. Process IDs are
returned as positive values; process group IDs are
returned as negative values (but see BUGS below).
arg is
ignored.
F_SETOWN (int)Set the process ID or process group ID that will
receive SIGIO and
SIGURG signals for
events on file descriptor fd to the ID given in
arg. A
process ID is specified as a positive value; a
process group ID is specified as a negative value.
Most commonly, the calling process specifies itself
as the owner (that is, arg is specified as
getpid(2)).
If you set the O_ASYNC status flag on a file
descriptor 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 signal other than SIGIO. If this permission check
fails, then the signal is silently discarded.
Sending a signal to the owner process (group)
specified by F_SETOWN
is subject to the same permissions checks as are
described for kill(2), where the
sending process is the one that employs F_SETOWN (but see BUGS below).
If the file descriptor fd refers to a socket,
F_SETOWN also selects
the recipient 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
condition".)
The following was true in 2.6.x kernels up to and including kernel 2.6.11:
If a nonzero value is given to
F_SETSIGin a multithreaded process running with a threading library that supports thread groups (e.g., NPTL), then a positive value given toF_SETOWNhas 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 passF_SETOWNthe result of gettid(2) instead of getpid(2) to get sensible results whenF_SETSIGis 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(2) or getpid(2) in this scenario.) Note, however, that the statements in this paragraph do not apply to theSIGURGsignal 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 toF_SETOWN.
The above behavior was accidentally dropped in
Linux 2.6.12, and won't be restored. From Linux
2.6.32 onward, use F_SETOWN_EX to target SIGIO and SIGURG signals at a particular
thread.
F_GETOWN_EX (struct f_owner_ex *)
(since Linux 2.6.32)Return the current file descriptor owner settings
as defined by a previous F_SETOWN_EX operation. The
information is returned in the structure pointed to
by arg,
which has the following form:
struct f_owner_ex { int type;pid_t pid;};
The type
field will have one of the values F_OWNER_TID, F_OWNER_PID, or F_OWNER_PGRP. The pid field is a positive
integer representing a thread ID, process ID, or
process group ID. See F_SETOWN_EX for more details.
F_SETOWN_EX (struct f_owner_ex *)
(since Linux 2.6.32)This operation performs a similar task to
F_SETOWN. It allows the
caller to direct I/O availability signals to a
specific thread, process, or process group. The
caller specifies the target of signals via arg, which is a
pointer to a f_owner_ex structure.
The type
field has one of the following values, which define
how pid is
interpreted:
F_OWNER_TIDSend the signal to the thread whose thread ID (the value returned by a call to clone(2) or gettid(2)) is specified in
pid.F_OWNER_PIDSend the signal to the process whose ID is specified in
pid.F_OWNER_PGRPSend the signal to the process group whose ID is specified in
pid. (Note that, unlike withF_SETOWN, a process group ID is specified as a positive value here.)
F_GETSIG (void)Return (as the function result) the signal sent
when input or output becomes possible. A value of
zero means SIGIO is
sent. Any other value (including SIGIO) is the signal sent instead,
and in this case additional info is available to the
signal handler if installed with SA_SIGINFO. arg is ignored.
F_SETSIG (int)Set the signal sent when input or output becomes
possible to the value given in arg. 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.
By using F_SETSIG
with a nonzero value, and setting SA_SIGINFO for the signal handler
(see sigaction(2)),
extra information about I/O events is passed to the
handler in a siginfo_t
structure. If the si_code field
indicates the source is SI_SIGIO, the si_fd field gives the
file descriptor associated with the event. Otherwise,
there is no indication which file descriptors are
pending, and you should use the usual mechanisms
(select(2),
poll(2), read(2) with
O_NONBLOCK set etc.) to
determine which file descriptors are available for
I/O.
By selecting a real time signal (value >=
SIGRTMIN), multiple I/O
events may be queued using the same signal numbers.
(Queuing is dependent on available memory). Extra
information is available if SA_SIGINFO is set for the signal
handler, as above.
Note that Linux imposes a limit on the number of
real-time signals that may be queued to a process
(see getrlimit(2) and
signal(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 specific thread.
Using these mechanisms, a program can implement fully asynchronous I/O without using select(2) or poll(2) most of the time.
The use of O_ASYNC is
specific to BSD and Linux. The only use of F_GETOWN and F_SETOWN specified in POSIX.1 is in
conjunction with the use of the SIGURG signal on sockets. (POSIX does not
specify the SIGIO signal.)
F_GETOWN_EX, F_SETOWN_EX, F_GETSIG, and F_SETSIG are Linux-specific. POSIX has
asynchronous I/O and the aio_sigevent structure to
achieve similar things; these are also available in Linux
as part of the GNU C Library (Glibc).
F_SETLEASE and
F_GETLEASE (Linux 2.4 onward)
are used (respectively) to establish a new lease, and
retrieve the current lease, on the open file description
referred to by the file descriptor fd. A file lease provides a
mechanism whereby the process holding the lease (the "lease
holder") is notified (via delivery of a signal) when a
process (the "lease breaker") tries to open(2) or truncate(2) the file
referred to by that file descriptor.
F_SETLEASE (int)Set or remove a file lease according to which of
the following values is specified in the integer
arg:
F_RDLCKTake out a read lease. This will cause the calling process to be notified when the file is opened for writing or is truncated. A read lease can be placed only on a file descriptor that is opened read-only.
F_WRLCKTake out a write lease. This will cause the caller to be notified when the file is opened for reading or writing or is truncated. A write lease may be placed on a file only if there are no other open file descriptors for the file.
F_UNLCKRemove our lease from the file.
Leases are associated with an open file
description (see open(2)). This
means that duplicate file descriptors (created by,
for example, fork(2) or
dup(2)) refer to
the same lease, and this lease may be modified or
released using any of these descriptors. Furthermore,
the lease is released by either an explicit
F_UNLCK operation on
any of these duplicate descriptors, or when all such
descriptors have been closed.
Leases may be taken out only on regular files. An
unprivileged process may take out a lease only on a file
whose UID (owner) matches the filesystem UID of the
process. A process with the CAP_LEASE capability may take out leases
on arbitrary files.
F_GETLEASE (void)Indicates what type of lease is associated with
the file descriptor fd by returning either
F_RDLCK, F_WRLCK, or F_UNLCK, indicating, respectively,
a read lease , a write lease, or no lease. arg is ignored.
When a process (the "lease breaker") performs an
open(2) or truncate(2) that
conflicts with a lease established via F_SETLEASE, the system call is blocked by
the kernel and the kernel notifies the lease holder by
sending it a signal (SIGIO by
default). The lease holder should respond to receipt of
this signal by doing whatever cleanup is required in
preparation for the file to be accessed by another process
(e.g., flushing cached buffers) and then either remove or
downgrade its lease. A lease is removed by performing an
F_SETLEASE command specifying
arg as
F_UNLCK. If the lease holder
currently holds a write lease on the file, and the lease
breaker is opening the file for reading, then it is
sufficient for the lease holder to downgrade the lease to a
read lease. This is done by performing an F_SETLEASE command specifying arg as F_RDLCK.
If the lease holder fails to downgrade or remove the
lease within the number of seconds specified in
/proc/sys/fs/lease-break-time,
then the kernel forcibly removes or downgrades the lease
holder's lease.
Once a lease break has been initiated, F_GETLEASE returns the target lease type
(either F_RDLCK or
F_UNLCK, depending on what
would be compatible with the lease breaker) until the lease
holder voluntarily downgrades or removes the lease or the
kernel forcibly does so after the lease break timer
expires.
Once the lease has been voluntarily or forcibly removed or downgraded, and assuming the lease breaker has not unblocked its system call, the kernel permits the lease breaker's system call to proceed.
If the lease breaker's blocked open(2) or truncate(2) is
interrupted by a signal handler, then the system call fails
with the error EINTR, but
the other steps still occur as described above. If the
lease breaker is killed by a signal while blocked in
open(2) or truncate(2), then the
other steps still occur as described above. If the lease
breaker specifies the O_NONBLOCK flag when calling open(2), then the call
immediately fails with the error EWOULDBLOCK, but the other steps still
occur as described above.
The default signal used to notify the lease holder is
SIGIO, but this can be
changed using the F_SETSIG
command to fcntl(). If a
F_SETSIG command is performed
(even one specifying SIGIO),
and the signal handler is established using SA_SIGINFO, then the handler will receive
a siginfo_t structure as its
second argument, and the si_fd field of this
argument will hold the descriptor of the leased file that
has been accessed by another process. (This is useful if
the caller holds leases against multiple files).
F_NOTIFY (int)(Linux 2.4 onward) Provide notification when the
directory referred to by fd or any of the files
that it contains is changed. The events to be
notified are specified in arg, which is a bit
mask specified by ORing together zero or more of the
following bits:
DN_ACCESSA file was accessed (read(2), pread(2), readv(2), and similar)
DN_MODIFYA file was modified (write(2), pwrite(2), writev(2), truncate(2), ftruncate(2), and similar).
DN_CREATEA file was created (open(2), creat(2), mknod(2), mkdir(2), link(2), symlink(2), rename(2) into this directory).
DN_DELETEA file was unlinked (unlink(2), rename(2) to another directory, rmdir(2)).
DN_RENAMEA file was renamed within this directory (rename(2)).
DN_ATTRIBThe attributes of a file were changed (chown(2), chmod(2), utime(2), utimensat(2), and similar).
(In order to obtain these definitions, the
_GNU_SOURCE feature
test macro must be defined before including
any header
files.)
Directory notifications are normally "one-shot",
and the application must reregister to receive
further notifications. Alternatively, if DN_MULTISHOT is included in
arg, then
notification will remain in effect until explicitly
removed.
A series of F_NOTIFY
requests is cumulative, with the events in arg being added to
the set already monitored. To disable notification of
all events, make an F_NOTIFY call specifying arg as 0.
Notification occurs via delivery of a signal. The
default signal is SIGIO, but this can be changed
using the F_SETSIG
command to fcntl().
(Note that SIGIO is one
of the nonqueuing standard signals; switching to the
use of a real-time signal means that multiple
notifications can be queued to the process.) In the
latter case, the signal handler receives a
siginfo_t structure as its
second argument (if the handler was established using
SA_SIGINFO) and the
si_fd field
of this structure contains the file descriptor which
generated the notification (useful when establishing
notification on multiple directories).
Especially when using DN_MULTISHOT, a real time signal
should be used for notification, so that multiple
notifications can be queued.
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Note |
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New applications should use the |
F_SETPIPE_SZ (int; since Linux 2.6.35)Change the capacity of the pipe referred to by
fd to be at
least arg
bytes. An unprivileged process can adjust the pipe
capacity to any value between the system page size
and the limit defined in /proc/sys/fs/pipe-max-size (see
proc(5)). Attempts
to set the pipe capacity below the page size are
silently rounded up to the page size. Attempts by an
unprivileged process to set the pipe capacity above
the limit in /proc/sys/fs/pipe-max-size yield
the error EPERM; a
privileged process (CAP_SYS_RESOURCE) can override the
limit. When allocating the buffer for the pipe, the
kernel may use a capacity larger than arg, if that is
convenient for the implementation. The actual
capacity that is set is returned as the function
result. Attempting to set the pipe capacity smaller
than the amount of buffer space currently used to
store data produces the error EBUSY.
F_GETPIPE_SZ (void; since Linux 2.6.35)Return (as the function result) the capacity of
the pipe referred to by fd.
For a successful call, the return value depends on the operation:
F_DUPFDThe new descriptor.
F_GETFDValue of file descriptor flags.
F_GETFLValue of file status flags.
F_GETLEASEType of lease held on file descriptor.
F_GETOWNValue of descriptor owner.
F_GETSIGValue of signal sent when read or write becomes
possible, or zero for traditional SIGIO behavior.
F_GETPIPE_SZ, F_SETPIPE_SZThe pipe capacity.
Zero.
On error, −1 is returned, and errno is set appropriately.
Operation is prohibited by locks held by other processes.
The operation is prohibited because the file has been memory-mapped by another process.
fd is not an
open file descriptor, or the command was F_SETLK or F_SETLKW and the file descriptor open
mode doesn't match with the type of lock requested.
It was detected that the specified F_SETLKW command would cause a
deadlock.
lock is
outside your accessible address space.
For F_SETLKW, the
command was interrupted by a signal; see signal(7). For
F_GETLK and F_SETLK, the command was interrupted
by a signal before the lock was checked or acquired.
Most likely when locking a remote file (e.g., locking
over NFS), but can sometimes happen locally.
The value specified in cmd is not recognized by
this kernel.
For F_DUPFD,
arg is
negative or is greater than the maximum allowable
value. For F_SETSIG,
arg is not an
allowable signal number.
cmd is
F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and l_pid was not specified
as zero.
For F_DUPFD, the
process already has the maximum number of file
descriptors open.
Too many segment locks open, lock table is full, or a remote locking protocol failed (e.g., locking over NFS).
F_NOTIFY was specified
in cmd, but
fd does not
refer to a directory.
Attempted to clear the O_APPEND flag on a file that has the
append-only attribute set.
SVr4, 4.3BSD, POSIX.1-2001. Only the operations
F_DUPFD, F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK, and F_SETLKW are specified in POSIX.1-2001.
F_GETOWN and F_SETOWN are specified in POSIX.1-2001. (To
get their definitions, define either _BSD_SOURCE, or _XOPEN_SOURCE with the value 500 or
greater, or _POSIX_C_SOURCE
with the value 200809L or greater.)
F_DUPFD_CLOEXEC is specified
in POSIX.1-2008. (To get this definition, define _POSIX_C_SOURCE with the value 200809L or
greater, or _XOPEN_SOURCE with
the value 700 or greater.)
F_GETOWN_EX, F_SETOWN_EX, F_SETPIPE_SZ, F_GETPIPE_SZ, F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-specific. (Define the
_GNU_SOURCE macro to obtain
these definitions.)
F_OFD_SETLK, F_OFD_SETLKW, and F_OFD_GETLK are Linux-specific (and one
must define _GNU_SOURCE to
obtain their definitions), but work is being done to have
them included in the next version of POSIX.1.
The errors returned by dup2(2) are different from
those returned by F_DUPFD.
The original Linux fcntl()
system call was not designed to handle large file offsets
(in the flock
structure). Consequently, an fcntl64() system call was added in Linux
2.4. The newer system call employs a different structure
for file locking, flock64, and corresponding
commands, F_GETLK64,
F_SETLK64, and F_SETLKW64. However, these details can be
ignored by applications using glibc, whose fcntl() wrapper function transparently
employs the more recent system call where it is
available.
The errors returned by dup2(2) are different
from those returned by F_DUPFD.
Since kernel 2.0, there is no interaction between the
types of lock placed by flock(2) and fcntl().
Several systems have more fields in struct flock such as, for
example, l_sysid.
Clearly, l_pid
alone is not going to be very useful if the process holding
the lock may live on a different machine.
The original Linux fcntl()
system call was not designed to handle large file offsets
(in the flock
structure). Consequently, an fcntl64() system call was added in Linux
2.4. The newer system call employs a different structure
for file locking, flock64, and corresponding
commands, F_GETLK64,
F_SETLK64, and F_SETLKW64. However, these details can be
ignored by applications using glibc, whose fcntl() wrapper function transparently
employs the more recent system call where it is
available.
Before Linux 3.12, if an NFSv4 client loses contact with
the server for a period of time (defined as more than 90
seconds with no communication), it might lose and regain a
lock without ever being aware of the fact. (The period of
time after which contact is assumed lost is known as the
NFSv4 leasetime. On a Linux NFS server, this can be
determined by looking at /proc/fs/nfsd/nfsv4leasetime, which
expresses the period in seconds. The default value for this
file is 90.) This scenario potentially risks data
corruption, since another process might acquire a lock in
the intervening period and perform file I/O.
Since Linux 3.12, if an NFSv4 client loses contact with
the server, any I/O to the file by a process which "thinks"
it holds a lock will fail until that process closes and
reopens the file. A kernel parameter, nfs.recover_lost_locks, can
be set to 1 to obtain the pre-3.12 behavior, whereby the
client will attempt to recover lost locks when contact is
reestablished with the server. Because of the attendant
risk of data corruption, this parameter defaults to 0
(disabled).
It is not possible to use F_SETFL to change the state of the
O_DSYNC and O_SYNC flags. Attempts to change the
state of these flags are silently ignored.
A limitation of the Linux system call conventions on
some architectures (notably i386) means that if a
(negative) process group ID to be returned by F_GETOWN falls in the range −1 to
−4095, then the return value is wrongly interpreted
by glibc as an error in the system call; that is, the
return value of fcntl() will
be −1, and errno will
contain the (positive) process group ID. The Linux-specific
F_GETOWN_EX operation avoids
this problem. Since glibc version 2.11, glibc makes the
kernel F_GETOWN problem
invisible by implementing F_GETOWN using F_GETOWN_EX.
In Linux 2.4 and earlier, there is bug that can occur
when an unprivileged process uses F_SETOWN to specify the owner of a socket
file descriptor as a process (group) other than the caller.
In this case, fcntl() can
return −1 with errno set
to EPERM, even when the
owner process (group) is one that the caller has permission
to send signals to. Despite this error return, the file
descriptor owner is set, and signals will be sent to the
owner.
The deadlock-detection algorithm employed by the kernel
when dealing with F_SETLKW
requests can yield both false negatives (failures to detect
deadlocks, leaving a set of deadlocked processes blocked
indefinitely) and false positives (EDEADLK errors when there is no
deadlock). For example, the kernel limits the lock depth of
its dependency search to 10 steps, meaning that circular
deadlock chains that exceed that size will not be detected.
In addition, the kernel may falsely indicate a deadlock
when two or more processes created using the clone(2) CLONE_FILES flag place locks that appear
(to the kernel) to conflict.
The Linux implementation of mandatory locking is subject to race conditions which render it unreliable: a write(2) call that overlaps with a lock may modify data after the mandatory lock is acquired; a read(2) call that overlaps with a lock may detect changes to data that were made only after a write lock was acquired. Similar races exist between mandatory locks and mmap(2). It is therefore inadvisable to rely on mandatory locking.
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), feature_test_macros(7)
locks.txt,
mandatory-locking.txt, and
dnotify.txt in the
Linux kernel source directory Documentation/filesystems/ (on older
kernels, these files are directly under the Documentation/ directory, and mandatory-locking.txt is
called mandatory.txt)
This page is part of release 3.72 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
http://www.kernel.org/doc/man−pages/.
|
t This manpage is Copyright (C) 1992 Drew Eckhardt; and Copyright (C) 1993 Michael Haardt, Ian Jackson; and Copyright (C) 1998 Jamie Lokier; and Copyright (C) 2002-2010, 2014 Michael Kerrisk; and Copyright (C) 2014 Jeff Layton %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified 1993-07-24 by Rik Faith <faithcs.unc.edu> Modified 1995-09-26 by Andries Brouwer <aebcwi.nl> and again on 960413 and 980804 and 981223. Modified 1998-12-11 by Jamie Lokier <jamieimbolc.ucc.ie> Applied correction by Christian Ehrhardt - aeb, 990712 Modified 2002-04-23 by Michael Kerrisk <mtk.manpagesgmail.com> Added note on F_SETFL and O_DIRECT Complete rewrite + expansion of material on file locking Incorporated description of F_NOTIFY, drawing on Stephen Rothwell's notes in Documentation/dnotify.txt. Added description of F_SETLEASE and F_GETLEASE Corrected and polished, aeb, 020527. Modified 2004-03-03 by Michael Kerrisk <mtk.manpagesgmail.com> Modified description of file leases: fixed some errors of detail Replaced the term "lease contestant" by "lease breaker" Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirements Modified 2004-12-08, added O_NOATIME after note from Martin Pool 2004-12-10, mtk, noted F_GETOWN bug after suggestion from aeb. 2005-04-08 Jamie Lokier <jamieshareable.org>, mtk Described behavior of F_SETOWN/F_SETSIG in multithreaded processes, and generally cleaned up the discussion of F_SETOWN. 2005-05-20, Johannes Nicolai <johannes.nicolaihpi.uni-potsdam.de>, mtk: Noted F_SETOWN bug for socket file descriptor in Linux 2.4 and earlier. Added text on permissions required to send signal. 2009-09-30, Michael Kerrisk Note obsolete F_SETOWN behavior with threads. Document F_SETOWN_EX and F_GETOWN_EX 2010-06-17, Michael Kerrisk Document F_SETPIPE_SZ and F_GETPIPE_SZ. |