FCNTL(2) BSD System Calls Manual FCNTL(2)
NAME
fcntl -- file control
SYNOPSIS
##include <>
int
fcntl(int fd, int cmd, int arg);
DESCRIPTION
Fcntl() provides for control over descriptors. The argument fd is a
descriptor to be operated on by cmd as follows:
FDUPFD Return a new descriptor as follows:
]o Lowest numbered available descriptor greater
than or equal to arg.
]o Same object references as the original
descriptor.
]o New descriptor shares the same file offset if
the object was a file.
]o Same access mode (read, write or read/write).
]o Same file status flags (i.e., both file
descriptors share the same file status flags).
]o The close-on-exec flag associated with the new
file descriptor is set to remain open across
execv(2) system calls.
FGETFD Get the close-on-exec flag associated with the file
descriptor fd. If the low-order bit of the returned
value is 0, the file will remain open across exec(),
otherwise the file will be closed upon execution of
exec() (arg is ignored).
FSETFD Set the close-on-exec flag associated with fd to the
low order bit of arg (0 or 1 as above).
FGETFL Get descriptor status flags, as described below (arg
is ignored).
FSETFL Set descriptor status flags to arg.
FGETOWN Get the process ID or process group currently receiv-
ing SIGIO and SIGURG signals; process groups are
returned as negative values (arg is ignored).
FSETOWN Set the process or process group to receive SIGIO and
SIGURG signals; process groups are specified by sup-
plying arg as negative, otherwise arg is interpreted
as a process ID.
FPREALOCATE Preallocate file storage space.
FSETSIZE Truncate a file without zeroing space. The calling
process must have root privileges.
FRDADVISE Issue an advisory read async with no copy to user.
FRDAHEAD Turn read ahead off/on. A zero value in arg disables
read ahead. A non-zero value in arg turns read ahead
on.
FREADBOTSTRAP Read bootstrap from disk.
FWRITEBOTSTRAP Write bootstrap on disk. The calling process must
have root privileges.
FNOCACHE Turns data caching off/on. A non-zero value in arg
turns data caching off. A value of zero in arg turns
data caching on.
FLOG2PHYS Get disk device information. Currently this only
includes the disk device address that corresponds to
the current file offset.
FULFSYNC Does the same thing as fsync(2) then asks the drive to
flush all buffered data to the permanent storage
device (arg is ignored). This is currently only
implemented on HFS filesystems and the operation may
take quite a while to complete. Certain FireWire
drives have also been known to ignore this request.
The flags for the FGETFL and FSETFL commands are as follows:
ONONBLOCK Non-blocking I/O; if no data is available to a read
call, or if a write operation would block, the read or
write call returns -1 with the error EAGAIN.
OAPEND Force each write to append at the end of file; corre-
sponds to the OAPEND flag of open(2).
OASYNC Enable the SIGIO signal to be sent to the process
group when I/O is possible, e.g., upon availability of
data to be read.
Several commands are available for doing advisory file locking; they all
operate on the following structure:
struct flock {
offt lstart; /* starting offset */
offt llen; /* len = 0 means until end of file */
pidt lpid; /* lock owner */
short ltype; /* lock type: read/write, etc. */
short lwhence; /* type of lstart */
};
The commands available for advisory record locking are as follows:
FGETLK Get the first lock that blocks the lock description pointed to
by the third argument, arg, taken as a pointer to a struct
flock (see above). The information retrieved overwrites the
information passed to fcntl in the 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 FUNLCK.
FSETLK Set or clear a file segment lock according to the lock
description pointed to by the third argument, arg, taken as a
pointer to a struct flock (see above). FSETLK is used to
establish shared (or read) locks (FRDLCK) or exclusive (or
write) locks, (FWRLCK), as well as remove either type of lock
(FUNLCK). If a shared or exclusive lock cannot be set, fcntl
returns immediately with EACES.
FSETLKW This command is the same as FSETLK 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 fcntl is waiting for a region, the
fcntl will be interrupted if the signal handler has not speci-
fied the SARESTART (see sigaction(2)).
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.
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.
The value of lwhence is SEKSET, SEKCUR, or SEKEND to indicate that
the relative offset, lstart bytes, will be measured from the start of
the file, current position, or end of the file, respectively. The value
of llen is the number of consecutive bytes to be locked. If llen is
negative, the result is undefined. The lpid field is only used with
FGETLK to return the process ID of the process holding a blocking lock.
After a successful FGETLK request, the value of lwhence is SEKSET.
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
llen is set to zero. If lwhence and lstart point to the beginning of
the file, and llen is zero, the entire file is locked. If an applica-
tion wishes only to do entire file locking, the flock(2) system call is
much more efficient.
There is at most one type of lock set for each byte in the file. Before
a successful return from an FSETLK or an FSETLKW request when the call-
ing process has previously existing locks on bytes in the region speci-
fied by the request, the previous lock type for each byte in the speci-
fied region is replaced by the new lock type. As specified above under
the descriptions of shared locks and exclusive locks, an FSETLK or an
FSETLKW 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.
This interface follows the completely stupid semantics of System V and
IE Std 1003.1-1988 (``POSIX.1'') that require that all locks associated
with a file for a given process are removed when any file descriptor for
that file is closed by that process. This semantic means that applica-
tions 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
getpwname(3) to retrieve a record, the lock will be lost because
getpwname(3) opens, reads, and closes the password database. The data-
base 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 fork(2)
function. The flock(2) interface has much more rational last close
semantics and allows locks to be inherited by child processes. 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 flock(2) and fcntl(2) locks may be safely used con-
currently.
All locks associated with a file for a given process are removed when the
process terminates.
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 EDEADLK error.
The FPREALOCATE command operates on the following structure:
typedef struct fstore {
uint32t fstflags; /* IN: flags word */
int fstposmode; /* IN: indicates offset field */
offt fstoffset; /* IN: start of the region */
offt fstlength; /* IN: size of the region */
offt fstbytesalloc; /* OUT: number of bytes allocated */
} fstoret;
The flags (fstflags) for the FPREALOCATE command are as follows:
FALOCATECONTIG Allocate contiguous space.
FALOCATEAL Allocate all requested space or no space at all.
The position modes (fstposmode) for the FPREALOCATE command indicate
how to use the offset field. The modes are as follows:
FPEOFPOSMODE Allocate from the physical end of file.
FVOLPOSMODE Allocate from the volume offset.
The FRDADVISE command operates on the following structure which holds
information passed from the user to the system:
struct radvisory {
offt raoffset; /* offset into the file */
int racount; /* size of the read */
};
The FREADBOTSTRAP and FWRITEBOTSTRAP commands operate on the follow-
ing structure.
typedef struct fbootstraptransfer {
offt fbtoffset; /* IN: offset to start read/write */
sizet fbtlength; /* IN: number of bytes to transfer */
void *fbtbuffer; /* IN: buffer to be read/written */
} fbootstraptransfert;
The FLOG2PHYS command operates on the following structure.
struct log2phys {
uint32t l2pflags; /* unused so far */
offt l2pcontigbytes; /* unused so far */
offt l2pdevoffset; /* bytes into device */
};
RETURN VALUES
Upon successful completion, the value returned depends on cmd as follows:
FDUPFD A new file descriptor.
FGETFD Value of flag (only the low-order bit is defined).
FGETFL Value of flags.
FGETOWN Value of file descriptor owner.
other Value other than -1.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
ERORS
Fcntl() will fail if:
[EACES] The argument cmd is FSETLK, the type of lock (ltype)
is a shared lock (FRDLCK) or exclusive lock
(FWRLCK), 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 seg-
ment of a file to be locked is already shared-locked
or exclusive-locked by another process.
The argument cmd is either FSETSIZE or
FWRITEBOTSTRAP and the calling process does not have
root privileges.
[EBADF] Fildes is not a valid open file descriptor.
The argument cmd is FSETLK or FSETLKW, the type of
lock (ltype) is a shared lock (FRDLCK), and fildes
is not a valid file descriptor open for reading.
The argument cmd is FSETLK or FSETLKW, the type of
lock (ltype) is an exclusive lock (FWRLCK), and
fildes is not a valid file descriptor open for writ-
ing.
The argument cmd is FPREALOCATE and the calling
process does not have file write permission.
The argument cmd is FLOG2PHYS and fildes is not a
valid file descriptor open for reading.
[EMFILE] Cmd is FDUPFD and the maximum allowed number of file
descriptors are currently open.
[EDEADLK] The argument cmd is FSETLKW, and a deadlock condition
was detected.
[EINTR] The argument cmd is FSETLKW, and the function was
interrupted by a signal.
[EINVAL] Cmd is FDUPFD and arg is negative or greater than the
maximum allowable number (see getdtablesize(2)).
The argument cmd is FGETLK, FSETLK, or FSETLKW and
the data to which arg points is not valid, or fildes
refers to a file that does not support locking.
The argument cmd is FPREALOCATE and the fstposmode
is not a valid mode, or when FPEOFPOSMODE is set and
fstoffset is a non-zero value, or when FVOLPOSMODE
is set and fstoffset is a negative or zero value.
The argument cmd is either FREADBOTSTRAP or
FWRITEBOTSTRAP and the operation was attempted on a
non-HFS disk type.
[EMFILE] The argument cmd is FDUPED and the maximum number of
file descriptors permitted for the process are already
in use, or no file descriptors greater than or equal
to arg are available.
[ENOLCK] The argument cmd is FSETLK or FSETLKW, and satisfy-
ing the lock or unlock request would result in the
number of locked regions in the system exceeding a
system-imposed limit.
[ESRCH] Cmd is FSETOWN and the process ID given as argument
is not in use.
SEE ALSO
close(2), execve(2), flock(2), getdtablesize(2), open(2), sigaction(3)
HISTORY
The fcntl() function call appeared in 4.2BSD.
4.2 Berkeley Distribution January 12, 1994 4.2 Berkeley Distribution
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