FCNTL(2)FCNTL(2)NAMEfcntl - file control
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fildes, int cmd, /* arg */ ...);
DESCRIPTION
The fcntl() function provides for control over open files. The fildes
argument is an open file descriptor.
The fcntl() function can take a third argument, arg, whose data type,
value, and use depend upon the value of cmd. The cmd argument specifies
the operation to be performed by fcntl().
The values for cmd are defined in <fcntl.h> and include:
F_DUPFD
Return a new file descriptor which is the lowest num‐
bered available (that is, not already open) file
descriptor greater than or equal to the third argument,
arg, taken as an integer of type int. The new file
descriptor refers to the same open file description as
the original file descriptor, and shares any locks. The
FD_CLOEXEC flag associated with the new file descriptor
is cleared to keep the file open across calls to one of
the exec(2) functions.
F_DUP2FD
Similar to F_DUPFD, but always returns arg. F_DUP2FD
closes arg if it is open and not equal to fildes.
F_DUP2FD is equivalent to dup2(fildes, arg).
F_DUPFD_CLOEXEC
Similar to F_DUPFD except that instead of clearing
FD_CLOEXEC it is explicitly set on the returned file
descriptor.
F_DUP2FD_CLOEXEC
Similar to F_DUP2FD with two exceptions. The FD_CLOEXEC
flag is explicitly set on the returned file descriptor.
If filedes equals arg, the call will fail setting errno
to EINVAL.
F_FREESP
Free storage space associated with a section of the
ordinary file fildes. The section is specified by a
variable of data type struct flock pointed to by arg.
The data type struct flock is defined in the <fcntl.h>
header (see fcntl.h(3HEAD)) and is described below. Note
that all file systems might not support all possible
variations of F_FREESP arguments. In particular, many
file systems allow space to be freed only at the end of
a file.
F_FREESP64
Equivalent to F_FREESP, but takes a struct flock64 argu‐
ment rather than a struct flock argument.
F_ALLOCSP
Allocate space for a section of the ordinary file
fildes. The section is specified by a variable of data
type struct flock pointed to by arg. The data type
struct flock is defined in the <fcntl.h> header (see
fcntl.h(3HEAD) and is described below.
F_ALLOCSP64
Equivalent to F_ALLOCSP, but takes a struct flock64
argument rather than a struct flock argument.
F_GETFD
Get the file descriptor flags defined in <fcntl.h> that
are associated with the file descriptor fildes. File
descriptor flags are associated with a single file
descriptor and do not affect other file descriptors that
refer to the same file.
F_GETFL
Get the file status flags and file access modes, defined
in <fcntl.h>, for the file descriptor specified by
fildes. The file access modes can be extracted from the
return value using the mask O_ACCMODE, which is defined
in <fcntl.h>. File status flags and file access modes do
not affect other file descriptors that refer to the same
file with different open file descriptions.
F_GETOWN
If fildes refers to a socket, get the process or process
group ID specified to receive SIGURG signals when out-
of-band data is available. Positive values indicate a
process ID; negative values, other than −1, indicate a
process group ID. If fildes does not refer to a socket,
the results are unspecified.
F_GETXFL
Get the file status flags, file access modes, and file
creation and assignment flags, defined in <fcntl.h>, for
the file descriptor specified by fildes. The file access
modes can be extracted from the return value using the
mask O_ACCMODE, which is defined in <fcntl.h>. File sta‐
tus flags, file access modes, and file creation and
assignment flags do not affect other file descriptors
that refer to the same file with different open file
descriptions.
F_SETFD
Set the file descriptor flags defined in <fcntl.h>, that
are associated with fildes, to the third argument, arg,
taken as type int. If the FD_CLOEXEC flag in the third
argument is 0, the file will remain open across the
exec() functions; otherwise the file will be closed upon
successful execution of one of the exec() functions.
F_SETFL
Set the file status flags, defined in <fcntl.h>, for the
file descriptor specified by fildes from the correspond‐
ing bits in the arg argument, taken as type int. Bits
corresponding to the file access mode and file creation
and assignment flags that are set in arg are ignored.
If any bits in arg other than those mentioned here are
changed by the application, the result is unspecified.
F_SETOWN
If fildes refers to a socket, set the process or process
group ID specified to receive SIGURG signals when out-
of-band data is available, using the value of the third
argument, arg, taken as type int. Positive values indi‐
cate a process ID; negative values, other than −1, indi‐
cate a process group ID. If fildes does not refer to a
socket, the results are unspecified.
The following commands are available for advisory record locking.
Record locking is supported for regular files, and may be supported for
other files.
F_GETLK
Get the first lock which blocks the lock description
pointed to by the third argument, arg, taken as a pointer
to type struct flock, defined in <fcntl.h>. The informa‐
tion retrieved overwrites the information passed to
fcntl() in the structure flock. If no lock is found that
would prevent this lock from being created, then the
structure will be left unchanged except for the lock type
which will be set to F_UNLCK.
F_GETLK64
Equivalent to F_GETLK, but takes a struct flock64 argu‐
ment rather than a struct flock argument.
F_SETLK
Set or clear a file segment lock according to the lock
description pointed to by the third argument, arg, taken
as a pointer to type struct flock, defined in <fcntl.h>.
F_SETLK is used to establish shared (or read) locks
(F_RDLCK) or exclusive (or write) locks (F_WRLCK), as
well as to remove either type of lock (F_UNLCK). F_RDLCK,
F_WRLCK and F_UNLCK are defined in <fcntl.h>. If a shared
or exclusive lock cannot be set, fcntl() will return
immediately with a return value of −1.
F_SETLK64
Equivalent to F_SETLK, but takes a struct flock64 argu‐
ment rather than a struct flock argument.
F_SETLKW
This command is the same as F_SETLK except that if a
shared or exclusive lock is blocked by other locks, the
process will wait until the request can be satisfied. If
a signal that is to be caught is received while fcntl()
is waiting for a region, fcntl() will be interrupted.
Upon return from the process' signal handler, fcntl()
will return −1 with errno set to EINTR, and the lock
operation will not be done.
F_SETLKW64
Equivalent to F_SETLKW, but takes a struct flock64 argu‐
ment rather than a struct flock argument.
When a shared lock is set on a segment of a file, other processes will
be able to set shared locks on that segment or a portion of it. A
shared lock prevents any other process from setting an exclusive lock
on any portion of the protected area. A request for a shared lock will
fail if the file descriptor was not opened with read access.
An exclusive lock will prevent any other process from setting a shared
lock or an exclusive lock on any portion of the protected area. A
request for an exclusive lock will fail if the file descriptor was not
opened with write access.
The flock structure contains at least the following elements:
short l_type; /* lock operation type */
short l_whence; /* lock base indicator */
off_t l_start; /* starting offset from base */
off_t l_len; /* lock length; l_len == 0 means
until end of file */
int l_sysid; /* system ID running process holding lock */
pid_t l_pid; /* process ID of process holding lock */
The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END, to indicate
that the relative offset l_start bytes will be measured from the start
of the file, current position or end of the file, respectively. The
value of l_len is the number of consecutive bytes to be locked. The
value of l_len may be negative (where the definition of off_t permits
negative values of l_len). After a successful F_GETLK or F_GETLK64
request, that is, one in which a lock was found, the value of l_whence
will be SEEK_SET.
The l_pid and l_sysid fields are used only with F_GETLK or F_GETLK64 to
return the process ID of the process holding a blocking lock and to
indicate which system is running that process.
If l_len is positive, the area affected starts at l_start and ends at
l_start + l_len − 1. If l_len is negative, the area affected starts at
l_start + l_len and ends at l_start − 1. Locks may start and extend
beyond the current end of a file, but must not be negative relative to
the beginning of the file. A lock will be set to extend to the largest
possible value of the file offset for that file by setting l_len to 0.
If such a lock also has l_start set to 0 and l_whence is set to
SEEK_SET, the whole file will be locked.
If a process has an existing lock in which l_len is 0 and which
includes the last byte of the requested segment, and an unlock
(F_UNLCK) request is made in which l_len is non-zero and the offset of
the last byte of the requested segment is the maximum value for an
object of type off_t, then the F_UNLCK request will be treated as a
request to unlock from the start of the requested segment with an l_len
equal to 0. Otherwise, the request will attempt to unlock only the
requested segment.
There will be at most one type of lock set for each byte in the file.
Before a successful return from an F_SETLK, F_SETLK64, F_SETLKW, or
F_SETLKW64 request when the calling process has previously existing
locks on bytes in the region specified by the request, the previous
lock type for each byte in the specified region will be replaced by the
new lock type. As specified above under the descriptions of shared
locks and exclusive locks, an F_SETLK, F_SETLK64, F_SETLKW, or
F_SETLKW64 request will (respectively) fail or block when another
process has existing locks on bytes in the specified region and the
type of any of those locks conflicts with the type specified in the
request.
All locks associated with a file for a given process are removed when a
file descriptor for that file is closed by that process or the process
holding that file descriptor terminates. Locks are not inherited by a
child process created using fork(2).
A potential for deadlock occurs if a process controlling a locked
region is put to sleep by attempting to lock another process' locked
region. If the system detects that sleeping until a locked region is
unlocked would cause a deadlock, fcntl() will fail with an EDEADLK
error.
The following values for cmd are used for file share reservations. A
share reservation is placed on an entire file to allow cooperating pro‐
cesses to control access to the file.
F_SHARE
Sets a share reservation on a file with the specified
access mode and designates which types of access to deny.
F_UNSHARE
Remove an existing share reservation.
File share reservations are an advisory form of access control among
cooperating processes, on both local and remote machines. They are most
often used by DOS or Windows emulators and DOS based NFS clients. How‐
ever, native UNIX versions of DOS or Windows applications may also
choose to use this form of access control.
A share reservation is described by an fshare structure defined in
<sys/fcntl.h>, which is included in <fcntl.h> as follows:
typedef struct fshare {
short f_access;
short f_deny;
int f_id;
} fshare_t;
A share reservation specifies the type of access, f_access, to be
requested on the open file descriptor. If access is granted, it further
specifies what type of access to deny other processes, f_deny. A single
process on the same file may hold multiple non-conflicting reserva‐
tions by specifying an identifier, f_id, unique to the process, with
each request.
An F_UNSHARE request releases the reservation with the specified f_id.
The f_access and f_deny fields are ignored.
Valid f_access values are:
F_RDACC
Set a file share reservation for read-only access.
F_WRACC
Set a file share reservation for write-only access.
F_RWACC
Set a file share reservation for read and write access.
Valid f_deny values are:
F_COMPAT
Set a file share reservation to compatibility mode.
F_RDDNY
Set a file share reservation to deny read access to other
processes.
F_WRDNY
Set a file share reservation to deny write access to other
processes.
F_RWDNY
Set a file share reservation to deny read and write access
to other processes.
F_NODNY
Do not deny read or write access to any other process.
RETURN VALUES
Upon successful completion, the value returned depends on cmd as fol‐
lows:
F_DUPFD
A new file descriptor.
F_FREESP
Value of 0.
F_GETFD
Value of flags defined in <fcntl.h>. The return value
will not be negative.
F_GETFL
Value of file status flags and access modes. The return
value will not be negative.
F_GETLK
Value other than −1.
F_GETLK64
Value other than −1.
F_GETOWN
Value of the socket owner process or process group; this
will not be −1.
F_GETXFL
Value of file status flags, access modes, and creation
and assignment flags. The return value will not be nega‐
tive.
F_SETFD
Value other than −1.
F_SETFL
Value other than −1.
F_SETLK
Value other than −1.
F_SETLK64
Value other than −1.
F_SETLKW
Value other than −1.
F_SETLKW64
Value other than −1.
F_SETOWN
Value other than −1.
F_SHARE
Value other than −1.
F_UNSHARE
Value other than −1.
Otherwise, −1 is returned and errno is set to indicate the error.
ERRORS
The fcntl() function will fail if:
EAGAIN
The cmd argument is F_SETLK or F_SETLK64, the type of lock
(l_type) is a shared (F_RDLCK) or exclusive (F_WRLCK)
lock, and the segment of a file to be locked is already
exclusive-locked by another process; or the type is an
exclusive lock and some portion of the segment of a file
to be locked is already shared-locked or exclusive-locked
by another process.
The cmd argument is F_FREESP, the file exists, mandatory
file/record locking is set, and there are outstanding
record locks on the file; or the cmd argument is F_SETLK,
F_SETLK64, F_SETLKW, or F_SETLKW64, mandatory file/record
locking is set, and the file is currently being mapped to
virtual memory using mmap(2).
The cmd argument is F_SHARE and f_access conflicts with an
existing f_deny share reservation.
EBADF
The fildes argument is not a valid open file descriptor;
or the cmd argument is F_SETLK, F_SETLK64, F_SETLKW, or
F_SETLKW64, the type of lock, l_type, is a shared lock
(F_RDLCK), and fildes is not a valid file descriptor open
for reading; or the type of lock l_type is an exclusive
lock (F_WRLCK) and fildes is not a valid file descriptor
open for writing.
The cmd argument is F_FREESP and fildes is not a valid
file descriptor open for writing.
The cmd argument is F_DUP2FD, and arg is negative or is
not less than the current resource limit for
RLIMIT_NOFILE.
The cmd argument is F_SHARE, the f_access share reserva‐
tion is for write access, and fildes is not a valid file
descriptor open for writing.
The cmd argument is F_SHARE, the f_access share reserva‐
tion is for read access, and fildes is not a valid file
descriptor open for reading.
EFAULT
The cmd argument is F_GETLK, F_GETLK64, F_SETLK,
F_SETLK64, F_SETLKW, F_SETLKW64, or F_FREESP and the arg
argument points to an illegal address.
The cmd argument is F_SHARE or F_UNSHARE and arg points to
an illegal address.
EINTR
The cmd argument is F_SETLKW or F_SETLKW64 and the func‐
tion was interrupted by a signal.
EINVAL
The cmd argument is invalid or not supported by the file
system; or the cmd argument is F_DUPFD and arg is negative
or greater than or equal to OPEN_MAX; or the cmd argument
is F_GETLK, F_GETLK64, F_SETLK, F_SETLK64, F_SETLKW, or
F_SETLKW64 and the data pointed to by arg is not valid; or
fildes refers to a file that does not support locking.
The cmd argument is F_UNSHARE and a reservation with this
f_id for this process does not exist.
The cmd argument is F_DUP2FD_CLOEXEC and fildes is equal
to arg.
EIO
An I/O error occurred while reading from or writing to the
file system.
EMFILE
The cmd argument is F_DUPFD and either OPEN_MAX file
descriptors are currently open in the calling process, or
no file descriptors greater than or equal to arg are
available.
ENOLCK
The cmd argument is F_SETLK, F_SETLK64, F_SETLKW, or
F_SETLKW64 and satisfying the lock or unlock request would
result in the number of locked regions in the system
exceeding a system-imposed limit.
ENOLINK
Either the fildes argument is on a remote machine and the
link to that machine is no longer active; or the cmd argu‐
ment is F_FREESP, the file is on a remote machine, and the
link to that machine is no longer active.
EOVERFLOW
One of the values to be returned cannot be represented
correctly.
The cmd argument is F_GETLK, F_SETLK, or F_SETLKW and the
smallest or, if l_len is non-zero, the largest, offset of
any byte in the requested segment cannot be represented
correctly in an object of type off_t.
The cmd argument is F_GETLK64, F_SETLK64, or F_SETLKW64
and the smallest or, if l_len is non-zero, the largest,
offset of any byte in the requested segment cannot be rep‐
resented correctly in an object of type off64_t.
The fcntl() function may fail if:
EAGAIN
The cmd argument is F_SETLK, F_SETLK64, F_SETLKW, or
F_SETLKW64, and the file is currently being mapped to vir‐
tual memory using mmap(2).
EDEADLK
The cmd argument is F_SETLKW or F_SETLKW64, the lock is
blocked by some lock from another process and putting the
calling process to sleep, waiting for that lock to become
free would cause a deadlock.
The cmd argument is F_FREESP, mandatory record locking is
enabled, O_NDELAY and O_NONBLOCK are clear and a deadlock
condition was detected.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬───────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼───────────────────┤
│Interface Stability │ Standard │
├────────────────────┼───────────────────┤
│MT-Level │ Async-Signal Safe │
└────────────────────┴───────────────────┘
SEE ALSOlockd(1M), chmod(2), close(2), creat(2), dup(2), exec(2), fork(2),
mmap(2), open(2), pipe(2), read(2), sigaction(2), write(2), dup2(3C),
fcntl.h(3HEAD), attributes(5), standards(5)
Programming Interfaces Guide
NOTES
In the past, the variable errno was set to EACCES rather than EAGAIN
when a section of a file is already locked by another process. There‐
fore, portable application programs should expect and test for either
value.
Advisory locks allow cooperating processes to perform consistent opera‐
tions on files, but do not guarantee exclusive access. Files can be
accessed without advisory locks, but inconsistencies may result. The
network share locking protocol does not support the f_deny value of
F_COMPAT. For network file systems, if f_access is F_RDACC, f_deny is
mapped to F_RDDNY. Otherwise, it is mapped to F_RWDNY.
To prevent possible file corruption, the system may reject mmap()
requests for advisory locked files, or it may reject advisory locking
requests for mapped files. Applications that require a file be both
locked and mapped should lock the entire file (l_start and l_len both
set to 0). If a file is mapped, the system may reject an unlock
request, resulting in a lock that does not cover the entire file.
The process ID returned for locked files on network file systems might
not be meaningful.
If the file server crashes and has to be rebooted, the lock manager
(see lockd(1M)) attempts to recover all locks that were associated with
that server. If a lock cannot be reclaimed, the process that held the
lock is issued a SIGLOST signal.
Apr 19, 2013 FCNTL(2)
