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.\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94
.\"
.Dd February 17, 2011
.Dt FCNTL 2
.Os BSD 4.2
.Sh NAME
.Nm fcntl
.Nd file control
.Sh SYNOPSIS
.Fd #include <fcntl.h>
.Ft int
.Fo fcntl
.Fa "int fildes"
.Fa "int cmd"
.Fa "..."
.Fc
.Sh DESCRIPTION
.Fn Fcntl
provides for control over descriptors.
The argument
.Fa fildes
is a descriptor to be operated on by
.Fa cmd
as follows:
.Bl -tag -width F_WRITEBOOTSTRAPX
.It Dv F_DUPFD
Return a new descriptor as follows:
.Pp
.Bl -bullet -compact -offset 4n
.It
Lowest numbered available descriptor greater than or equal to
.Fa arg .
.It
Same object references as the original descriptor.
.It
New descriptor shares the same file offset if the object
was a file.
.It
Same access mode (read, write or read/write).
.It
Same file status flags (i.e., both file descriptors
share the same file status flags).
.It
The close-on-exec flag associated with the new file descriptor
is cleared so that the descriptor remains open across an
.Xr execv 2
system call.
.El
.It Dv F_DUPFD_CLOEXEC
Like
.Dv F_DUPFD ,
except that the close-on-exec flag associated with the new file descriptor
is set.
.It Dv F_GETFD
Get the flags associated with the file descriptor
.Fa fildes ,
as described below
.Fa ( arg
is ignored).
.It Dv F_SETFD
Set the file descriptor flags to
.Fa arg .
.It Dv F_GETFL
Get descriptor status flags, as described below
.Fa ( arg
is ignored).
.It Dv F_SETFL
Set descriptor status flags to
.Fa arg .
.It Dv F_GETOWN
Get the process ID or process group
currently receiving
.Dv SIGIO
and
.Dv SIGURG
signals; process groups are returned
as negative values
.Fa ( arg
is ignored).
.It Dv F_SETOWN
Set the process or process group
to receive
.Dv SIGIO
and
.Dv SIGURG
signals;
process groups are specified by supplying
.Fa arg
as negative, otherwise
.Fa arg
is interpreted as a process ID.
.It Dv F_GETPATH
Get the path of the file descriptor
.Fa Fildes .
The argument must be a buffer of size
.Sy MAXPATHLEN
or greater.
.It Dv F_PREALLOCATE
Preallocate file storage space. Note: upon success,
the space that is allocated can be the same size or
larger than the space requested.
.It Dv F_SETSIZE
Truncate a file without zeroing space.
The calling process must have root privileges.
.It Dv F_RDADVISE
Issue an advisory read async with no copy to user.
.It Dv F_RDAHEAD
Turn read ahead off/on.
A zero value in
.Fa arg
disables read ahead.
A non-zero value in
.Fa arg
turns read ahead on.
.It Dv F_READBOOTSTRAP
Read bootstrap from disk.
.It Dv F_WRITEBOOTSTRAP
Write bootstrap on disk.
The calling process must have root privileges.
.It Dv F_NOCACHE
Turns data caching off/on. A non-zero value in
.Fa arg
turns data caching off.
A value of zero in
.Fa arg
turns data caching on.
.It Dv F_LOG2PHYS
Get disk device information.
Currently this only includes the
disk device address that corresponds
to the current file offset. Note that if the
file offset is not backed by physical blocks
we can return -1 as the offset. This is subject
to change.
.It Dv F_LOG2PHYS_EXT
Variant of F_LOG2PHYS that uses the passed in
file offset and length.
.It Dv F_FULLFSYNC
Does the same thing as
.Xr fsync 2
then asks the drive to
flush all buffered data to
the permanent storage device
.Fa ( arg
is ignored).
This is currently implemented on HFS, MS-DOS (FAT),
and Universal Disk Format (UDF) file systems.
The operation may take quite a while to complete.
Certain FireWire drives have also been known
to ignore the request to flush their buffered data.
.It Dv F_SETNOSIGPIPE
Determines whether a
.Dv SIGPIPE
signal will be generated when a write fails on a pipe or socket for
which there is no reader. If
.Fa arg
is non-zero,
.Dv SIGPIPE
generation is disabled for descriptor
.Fa fildes ,
while an
.Fa arg
of zero enables it (the default).
.It Dv F_GETNOSIGPIPE
Returns whether a
.Dv SIGPIPE
signal will be generated when a write fails on a pipe or socket
for which there is no reader. The semantics of the return value
match those of the
.Fa arg
of
.Dv F_SETNOSIGPIPE .
.El
.Pp
The flags for the
.Dv F_GETFD
and
.Dv F_SETFD
commands are as follows:
.Bl -tag -width FD_CLOEXECX -offset indent
.It Dv FD_CLOEXEC
Close-on-exec; the given file descriptor will be automatically
closed in the successor process image when one of the
.Xr execv 2
or
.Xr posix_spawn 2
family of system calls is invoked.
.El
.Pp
The flags for the
.Dv F_GETFL
and
.Dv F_SETFL
commands are as follows:
.Bl -tag -width O_NONBLOCKX -offset indent
.It Dv O_NONBLOCK
Non-blocking I/O; if no data is available to a
.Xr read
call, or if a
.Xr write
operation would block,
the read or write call returns -1 with the error
.Er EAGAIN .
.It Dv O_APPEND
Force each write to append at the end of file;
corresponds to the
.Dv O_APPEND
flag of
.Xr open 2 .
.It Dv O_ASYNC
Enable the
.Dv SIGIO
signal to be sent to the process group
when I/O is possible, e.g.,
upon availability of data to be read.
.El
.Pp
Several commands are available for doing advisory file locking;
they all operate on the following structure:
.ne 7v
.Bd -literal
struct flock {
off_t l_start; /* starting offset */
off_t l_len; /* len = 0 means until end of file */
pid_t l_pid; /* lock owner */
short l_type; /* lock type: read/write, etc. */
short l_whence; /* type of l_start */
};
.Ed
.Pp
The commands available for advisory record locking are as follows:
.Bl -tag -width F_SETLKWX
.It Dv F_GETLK
Get the first lock that blocks the lock description pointed to by the
third argument,
.Fa arg ,
taken as a pointer to a
.Fa "struct flock"
(see above).
The information retrieved overwrites the information passed to
.Nm fcntl
in the
.Fa flock
structure.
If no lock is found that would prevent this lock from being created,
the structure is left unchanged by this function call except for the
lock type which is set to
.Dv F_UNLCK .
.It Dv F_SETLK
Set or clear a file segment lock according to the lock description
pointed to by the third argument,
.Fa arg ,
taken as a pointer to a
.Fa "struct flock"
(see above).
.Dv F_SETLK
is used to establish shared (or read) locks
.Dv (F_RDLCK)
or exclusive (or write) locks,
.Dv (F_WRLCK) ,
as well as remove either type of lock
.Dv (F_UNLCK) .
If a shared or exclusive lock cannot be set,
.Nm fcntl
returns immediately with
.Er EAGAIN .
.It Dv F_SETLKW
This command is the same as
.Dv F_SETLK
except that if a shared or exclusive lock is blocked by other locks,
the process waits until the request can be satisfied.
If a signal that is to be caught is received while
.Nm fcntl
is waiting for a region, the
.Nm fcntl
will be interrupted if the signal handler has not specified the
.Dv SA_RESTART
(see
.Xr sigaction 2 ) .
.El
.Pp
When a shared lock has been set on a segment of a file,
other processes can 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 fails if the file descriptor was not
opened with read access.
.Pp
An exclusive lock prevents 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 fails if the file was not
opened with write access.
.Pp
The value of
.Fa l_whence
is
.Dv SEEK_SET ,
.Dv SEEK_CUR ,
or
.Dv SEEK_END
to indicate that the relative offset,
.Fa l_start
bytes, will be measured from the start of the file,
current position, or end of the file, respectively.
The value of
.Fa l_len
is the number of consecutive bytes to be locked.
If
.Fa l_len
is negative, the result is undefined.
The
.Fa l_pid
field is only used with
.Dv F_GETLK
to return the process ID of the process holding a blocking lock.
After a successful
.Dv F_GETLK
request, the value of
.Fa l_whence
is
.Dv SEEK_SET .
.Pp
Locks may start and extend beyond the current end of a file,
but may not start or extend before the beginning of the file.
A lock is set to extend to the largest possible value of the
file offset for that file if
.Fa l_len
is set to zero. If
.Fa l_whence
and
.Fa l_start
point to the beginning of the file, and
.Fa l_len
is zero, the entire file is locked.
If an application wishes only to do entire file locking, the
.Xr flock 2
system call is much more efficient.
.Pp
There is at most one type of lock set for each byte in the file.
Before a successful return from an
.Dv F_SETLK
or an
.Dv F_SETLKW
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 is replaced by the new lock type.
As specified above under the descriptions
of shared locks and exclusive locks, an
.Dv F_SETLK
or an
.Dv F_SETLKW
request fails or blocks respectively 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.
.Pp
This interface follows the completely stupid semantics of System V and
.St -p1003.1-88
that require that all locks associated with a file for a given process are
removed when \fIany\fP file descriptor for that file is closed by that process.
This semantic means that applications must be aware of any files that
a subroutine library may access.
For example if an application for updating the password file locks the
password file database while making the update, and then calls
.Xr getpwname 3
to retrieve a record,
the lock will be lost because
.Xr getpwname 3
opens, reads, and closes the password database.
The database close will release all locks that the process has
associated with the database, even if the library routine never
requested a lock on the database.
Another minor semantic problem with this interface is that
locks are not inherited by a child process created using the
.Xr fork 2
function.
The
.Xr flock 2
interface has much more rational last close semantics and
allows locks to be inherited by child processes.
.Xr Flock 2
is recommended for applications that want to ensure the integrity
of their locks when using library routines or wish to pass locks
to their children.
Note that
.Xr flock 2
and
.Xr fcntl 2
locks may be safely used concurrently.
.Pp
All locks associated with a file for a given process are
removed when the process terminates.
.Pp
A potential for deadlock occurs if a process controlling a locked region
is put to sleep by attempting to lock the locked region of another process.
This implementation detects that sleeping until a locked region is unlocked
would cause a deadlock and fails with an
.Er EDEADLK
error.
.Pp
The
.Dv F_PREALLOCATE
command operates on the following structure:
.ne 7v
.Bd -literal
typedef struct fstore {
u_int32_t fst_flags; /* IN: flags word */
int fst_posmode; /* IN: indicates offset field */
off_t fst_offset; /* IN: start of the region */
off_t fst_length; /* IN: size of the region */
off_t fst_bytesalloc; /* OUT: number of bytes allocated */
} fstore_t;
.Ed
.Pp
The flags (fst_flags) for the
.Dv F_PREALLOCATE
command are as follows:
.Bl -tag -width F_ALLOCATECONTIGX -offset indent
.It Dv F_ALLOCATECONTIG
Allocate contiguous space.
.It Dv F_ALLOCATEALL
Allocate all requested space or no space at all.
.El
.Pp
The position modes (fst_posmode) for the
.Dv F_PREALLOCATE
command indicate how to use the offset field.
The modes are as follows:
.Bl -tag -width F_PEOFPOSMODEX -offset indent
.It Dv F_PEOFPOSMODE
Allocate from the physical end of file.
.It Dv F_VOLPOSMODE
Allocate from the volume offset.
.El
.Pp
The
.Dv F_RDADVISE
command operates on the following structure
which holds information passed from the
user to the system:
.ne 7v
.Bd -literal
struct radvisory {
off_t ra_offset; /* offset into the file */
int ra_count; /* size of the read */
};
.Ed
.Pp
The
.Dv F_READBOOTSTRAP and F_WRITEBOOTSTRAP
commands operate on the following structure.
.ne 7v
.Bd -literal
typedef struct fbootstraptransfer {
off_t fbt_offset; /* IN: offset to start read/write */
size_t fbt_length; /* IN: number of bytes to transfer */
void *fbt_buffer; /* IN: buffer to be read/written */
} fbootstraptransfer_t;
.Ed
.Pp
The
.Dv F_LOG2PHYS
command operates on the following structure:
.ne 7v
.Bd -literal
struct log2phys {
u_int32_t l2p_flags; /* unused so far */
off_t l2p_contigbytes; /* unused so far */
off_t l2p_devoffset; /* bytes into device */
};
.Ed
.Pp
The
.Dv F_LOG2PHYS_EXT
command operates on the same structure as F_LOG2PHYS but treats it as an in/out:
.ne 7v
.Bd -literal
struct log2phys {
u_int32_t l2p_flags; /* unused so far */
off_t l2p_contigbytes; /* IN: number of bytes to be queried;
OUT: number of contiguous bytes allocated at this position */
off_t l2p_devoffset; /* IN: bytes into file;
OUT: bytes into device */
};
.Ed
.Pp
If
.Fa fildes
is a socket, then the
.Dv F_SETNOSIGPIPE
and
.Dv F_GETNOSIGPIPE
commands are directly analogous, and fully interoperate with the
.Dv SO_NOSIGPIPE
option of
.Xr setsockopt 2
and
.Xr getsockopt 2
respectively.
.Sh RETURN VALUES
Upon successful completion, the value returned depends on
.Fa cmd
as follows:
.Bl -tag -width F_GETOWNX -offset indent
.It Dv F_DUPFD
A new file descriptor.
.It Dv F_GETFD
Value of flag (only the low-order bit is defined).
.It Dv F_GETFL
Value of flags.
.It Dv F_GETOWN
Value of file descriptor owner.
.It other
Value other than -1.
.El
.Pp
Otherwise, a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn fcntl
system call will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EAGAIN
The argument
.Fa cmd
is
.Dv F_SETLK ,
the type of lock
.Fa (l_type)
is a shared lock
.Dv (F_RDLCK)
or exclusive lock
.Dv (F_WRLCK) ,
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.
.It Bq Er EACCESS
The argument
.Fa cmd
is either
.Dv F_SETSIZE
or
.Dv F_WRITEBOOTSTRAP
and the calling process does not have root privileges.
.\" ==========
.It Bq Er EBADF
.Fa Fildes
is not a valid open file descriptor.
.Pp
The argument
.Fa cmd
is
.Dv F_SETLK
or
.Dv F_SETLKW ,
the type of lock
.Fa (l_type)
is a shared lock
.Dv (F_RDLCK) ,
and
.Fa fildes
is not a valid file descriptor open for reading.
.Pp
The argument
.Fa cmd
is
.Dv F_SETLK
or
.Dv F_SETLKW ,
the type of lock
.Fa (l_type)
is an exclusive lock
.Dv (F_WRLCK) ,
and
.Fa fildes
is not a valid file descriptor open for writing.
.Pp
The argument
.Fa cmd
is
.Dv F_PREALLOCATE
and the calling process does not have
file write permission.
.Pp
The argument
.Fa cmd
is
.Dv F_LOG2PHYS
or
.Dv F_LOG2PHYS_EXT
and
.Fa fildes
is not a valid file descriptor open for reading.
.\" ==========
.It Bq Er EDEADLK
The argument
.Fa cmd
is
.Dv F_SETLKW ,
and a deadlock condition was detected.
.\" ==========
.It Bq Er EINTR
The argument
.Fa cmd
is
.Dv F_SETLKW ,
and the function was interrupted by a signal.
.\" ==========
.It Bq Er EINVAL
.Fa Cmd
is
.Dv F_DUPFD
and
.Fa arg
is negative or greater than the maximum allowable number
(see
.Xr getdtablesize 2 ) .
.Pp
The argument
.Fa cmd
is
.Dv F_GETLK ,
.Dv F_SETLK ,
or
.Dv F_SETLKW
and the data to which
.Fa arg
points is not valid, or
.Fa fildes
refers to a file that does not support locking.
.Pp
The argument
.Fa cmd
is
.Dv F_PREALLOCATE
and the
.Fa fst_posmode
is not a valid mode,
or when
.Dv F_PEOFPOSMODE
is set and
.Fa fst_offset
is a non-zero value,
or when
.Dv F_VOLPOSMODE
is set and
.Fa fst_offset
is a negative or zero value.
.Pp
The argument
.Fa cmd
is either
.Dv F_READBOOTSTRAP
or
.Dv F_WRITEBOOTSTRAP
and the operation was attempted on a non-HFS disk type.
.\" ==========
.It Bq Er EMFILE
.Fa Cmd
is
.Dv F_DUPFD
and the maximum allowed number of file descriptors are currently
open.
.\" ==========
.It Bq Er EMFILE
The argument
.Fa cmd
is
.Dv F_DUPED
and the maximum number of file descriptors permitted for the
process are already in use,
or no file descriptors greater than or equal to
.Fa arg
are available.
.\" ==========
.It Bq Er ENOLCK
The argument
.Fa cmd
is
.Dv F_SETLK
or
.Dv F_SETLKW ,
and satisfying the lock or unlock request would result in the
number of locked regions in the system exceeding a system-imposed limit.
.\" ==========
.It Bq Er EOVERFLOW
A return value would overflow its representation.
For example,
.Fa cmd
is F_GETLK, F_SETLK, or F_SETLKW
and the smallest (or, if l_len is non-zero, the largest) offset
of a byte in the requested segment
will not fit in an object of type off_t.
.\" ==========
.It Bq Er ESRCH
.Fa Cmd
is
.Dv F_SETOWN
and
the process ID given as argument is not in use.
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr execve 2 ,
.Xr flock 2 ,
.Xr getdtablesize 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr setsockopt 2 ,
.Xr sigaction 3
.Sh HISTORY
The
.Fn fcntl
function call appeared in
.Bx 4.2 .