FCNTL
Section: Linux Programmer's Manual (2)
Updated: 2012-04-15
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NAME
fcntl - manipulate file descriptor
SYNOPSIS
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fd, int cmd, ... /* arg */ );
DESCRIPTION
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.
Duplicating a file descriptor
- 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).
File descriptor flags
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.
File status flags
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.
Advisory locking
F_GETLK, F_SETLK and F_SETLKW
are used to acquire, release, and test for the existence of record
locks (also known as 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
(F_GETLK only) */
...
};
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.
- 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 these locks in the
l_type, l_whence, l_start, and l_len
fields of
lock
and sets
l_pid
to be the PID of the process holding that lock.
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 or if it closes
any
file descriptor referring to a file on which locks are held.
This is bad: it means that a process can lose the locks on
a file like
/etc/passwd
or
/etc/mtab
when for some reason a library function decides to open, read
and close it.
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.
Mandatory locking
(Non-POSIX.)
The above record locks may be either advisory or mandatory,
and are advisory by default.
Advisory locks are not enforced and are useful only between
cooperating processes.
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
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)).
The Linux implementation of mandatory locking is unreliable.
See BUGS below.
Managing signals
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_SETSIG
in a multithreaded process running with a threading library
that supports thread groups (e.g., NPTL),
then a positive 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(2)
instead of
getpid(2)
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(2)
or
getpid(2)
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.
-
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_TID
-
Send 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_PID
-
Send the signal to the process whose ID
is specified in
pid.
- F_OWNER_PGRP
-
Send the signal to the process group whose ID
is specified in
pid.
(Note that, unlike with
F_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,
F_GETOWN,
F_SETOWN
is specific to BSD and Linux.
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).
Leases
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_RDLCK
-
Take 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_WRLCK
-
Take 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_UNLCK
-
Remove 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).
File and directory change notification (dnotify)
- 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_ACCESS
-
A file was accessed (read, pread, readv)
- DN_MODIFY
-
A file was modified (write, pwrite, writev, truncate, ftruncate).
- DN_CREATE
-
A file was created (open, creat, mknod, mkdir, link, symlink, rename).
- DN_DELETE
-
A file was unlinked (unlink, rename to another directory, rmdir).
- DN_RENAME
-
A file was renamed within this directory (rename).
- DN_ATTRIB
-
The attributes of a file were changed (chown, chmod, utime[s]).
-
(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().
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.
NOTE:
New applications should use the
inotify
interface (available since kernel 2.6.13),
which provides a much superior interface for obtaining notifications of
filesystem events.
See
inotify(7).
Changing the capacity of a pipe
- 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
F_GETPIPE_SZ
operation returns the actual size used.
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.
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 possible, 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
- EACCES or EAGAIN
-
Operation is prohibited by locks held by other processes.
- EAGAIN
-
The operation is prohibited because the file has been memory-mapped by
another process.
- EBADF
-
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.
- EDEADLK
-
It was detected that the specified
F_SETLKW
command would cause a deadlock.
- EFAULT
-
lock
is outside your accessible address space.
- EINTR
-
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.
- EINVAL
-
For
F_DUPFD,
arg
is negative or is greater than the maximum allowable value.
For
F_SETSIG,
arg
is not an allowable signal number.
- EMFILE
-
For
F_DUPFD,
the process already has the maximum number of file descriptors open.
- ENOLCK
-
Too many segment locks open, lock table is full, or a remote locking
protocol failed (e.g., locking over NFS).
- EPERM
-
Attempted to clear the
O_APPEND
flag on a file that has the append-only attribute set.
CONFORMING TO
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
BSD_SOURCE,
or
_XOPEN_SOURCE
with the value 500 or greater, or define
_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.)
NOTES
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.
BUGS
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 implementation of mandatory locking in all known versions of Linux
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.
SEE ALSO
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)
COLOPHON
This page is part of release 3.54 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Duplicating a file descriptor
-
- File descriptor flags
-
- File status flags
-
- Advisory locking
-
- Mandatory locking
-
- Managing signals
-
- Leases
-
- File and directory change notification (dnotify)
-
- Changing the capacity of a pipe
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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Time: 12:52:35 GMT, May 19, 2024