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FCNTL(2) BSD System Calls Manual FCNTL(2)
NAME
fcntl -- file control
SYNOPSIS
#include <fcntl.h>
int
fcntl(int fildes, int cmd, ...);
DESCRIPTION
Fcntl() provides for control over descriptors. The argument fildes is a
descriptor to be operated on by cmd as follows:
F_DUPFD Return a new descriptor as follows:
oo Lowest numbered available descriptor greater
than or equal to arg.
oo Same object references as the original
descriptor.
oo New descriptor shares the same file offset if
the object was a file.
oo Same access mode (read, write or read/write).
oo Same file status flags (i.e., both file
descriptors share the same file status flags).
oo The close-on-exec flag associated with the new
file descriptor is set to remain open across
execv(2) system calls.
F_GETFD Get the close-on-exec flag associated with the file
descriptor fildes. 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 execu-tion execution
tion of exec() (arg is ignored).
F_SETFD Set the close-on-exec flag associated with fildes to
the low order bit of arg (0 or 1 as above).
F_GETFL Get descriptor status flags, as described below (arg
is ignored).
F_SETFL Set descriptor status flags to arg.
F_GETOWN Get the process ID or process group currently receiv-ing receiving
ing SIGIO and SIGURG signals; process groups are
returned as negative values (arg is ignored).
F_SETOWN Set the process or process group to receive SIGIO and
SIGURG signals; process groups are specified by sup-plying supplying
plying arg as negative, otherwise arg is interpreted
as a process ID.
F_GETPATH Get the path of the file descriptor Fildes. The argu-ment argument
ment must be a buffer of size MAXPATHLEN or greater.
F_PREALLOCATE Preallocate file storage space.
F_SETSIZE Truncate a file without zeroing space. The calling
process must have root privileges.
F_RDADVISE Issue an advisory read async with no copy to user.
F_RDAHEAD Turn read ahead off/on. A zero value in arg disables
read ahead. A non-zero value in arg turns read ahead
on.
F_READBOOTSTRAP Read bootstrap from disk.
F_WRITEBOOTSTRAP Write bootstrap on disk. The calling process must
have root privileges.
F_NOCACHE 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.
F_LOG2PHYS Get disk device information. Currently this only
includes the disk device address that corresponds to
the current file offset.
F_FULLFSYNC 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 imple-mented implemented
mented 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.
The flags for the F_GETFL and F_SETFL commands are as follows:
O_NONBLOCK 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.
O_APPEND Force each write to append at the end of file; corre-sponds corresponds
sponds to the O_APPEND flag of open(2).
O_ASYNC 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 {
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 */
};
The commands available for advisory record locking are as follows:
F_GETLK 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 F_UNLCK.
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 a struct flock (see above). F_SETLK is used to
establish shared (or read) locks (F_RDLCK) or exclusive (or
write) locks, (F_WRLCK), as well as remove either type of lock
(F_UNLCK). If a shared or exclusive lock cannot be set, fcntl
returns immediately with EACCES.
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 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 specified
fied the SA_RESTART (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 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. If l_len is
negative, the result is undefined. The l_pid field is only used with
F_GETLK to return the process ID of the process holding a blocking lock.
After a successful F_GETLK request, the value of l_whence is SEEK_SET.
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
l_len is set to zero. If l_whence and l_start point to the beginning of
the file, and l_len is zero, the entire file is locked. If an applica-tion application
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 F_SETLK or an F_SETLKW request when the call-ing calling
ing process has previously existing locks on bytes in the region speci-fied specified
fied by the request, the previous lock type for each byte in the speci-fied specified
fied region is replaced by the new lock type. As specified above under
the descriptions of shared locks and exclusive locks, an F_SETLK or an
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.
This interface follows the completely stupid semantics of System V and
IEEE 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 applications
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 database
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. concurrently.
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 F_PREALLOCATE command operates on the following structure:
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;
The flags (fst_flags) for the F_PREALLOCATE command are as follows:
F_ALLOCATECONTIG Allocate contiguous space.
F_ALLOCATEALL Allocate all requested space or no space at all.
The position modes (fst_posmode) for the F_PREALLOCATE command indicate
how to use the offset field. The modes are as follows:
F_PEOFPOSMODE Allocate from the physical end of file.
F_VOLPOSMODE Allocate from the volume offset.
The F_RDADVISE command operates on the following structure which holds
information passed from the user to the system:
struct radvisory {
off_t ra_offset; /* offset into the file */
int ra_count; /* size of the read */
};
The F_READBOOTSTRAP and F_WRITEBOOTSTRAP commands operate on the follow-ing following
ing structure.
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;
The F_LOG2PHYS command operates on the following structure.
struct log2phys {
u_int32_t l2p_flags; /* unused so far */
off_t l2p_contigbytes; /* unused so far */
off_t l2p_devoffset; /* bytes into device */
};
RETURN VALUES
Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD A new file descriptor.
F_GETFD Value of flag (only the low-order bit is defined).
F_GETFL Value of flags.
F_GETOWN 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.
ERRORS
The fcntl() system call will fail if:
[EACCES] The argument cmd is F_SETLK, the type of lock (l_type)
is a shared lock (F_RDLCK) or exclusive lock
(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 seg-ment segment
ment of a file to be locked is already shared-locked
or exclusive-locked by another process.
The argument cmd is either F_SETSIZE or
F_WRITEBOOTSTRAP and the calling process does not have
root privileges.
[EBADF] Fildes is not a valid open file descriptor.
The argument cmd is F_SETLK or F_SETLKW, the type of
lock (l_type) is a shared lock (F_RDLCK), and fildes
is not a valid file descriptor open for reading.
The argument cmd is F_SETLK or F_SETLKW, the type of
lock (l_type) is an exclusive lock (F_WRLCK), and
fildes is not a valid file descriptor open for writ-ing. writing.
ing.
The argument cmd is F_PREALLOCATE and the calling
process does not have file write permission.
The argument cmd is F_LOG2PHYS and fildes is not a
valid file descriptor open for reading.
[EDEADLK] The argument cmd is F_SETLKW, and a deadlock condition
was detected.
[EINTR] The argument cmd is F_SETLKW, and the function was
interrupted by a signal.
[EINVAL] Cmd is F_DUPFD and arg is negative or greater than the
maximum allowable number (see getdtablesize(2)).
The argument cmd is F_GETLK, F_SETLK, or F_SETLKW 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 F_PREALLOCATE and the fst_posmode
is not a valid mode, or when F_PEOFPOSMODE is set and
fst_offset is a non-zero value, or when F_VOLPOSMODE
is set and fst_offset is a negative or zero value.
The argument cmd is either F_READBOOTSTRAP or
F_WRITEBOOTSTRAP and the operation was attempted on a
non-HFS disk type.
[EMFILE] Cmd is F_DUPFD and the maximum allowed number of file
descriptors are currently open.
[EMFILE] The argument cmd is 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 arg are available.
[ENOLCK] The argument cmd is F_SETLK or F_SETLKW, and satisfy-ing satisfying
ing the lock or unlock request would result in the
number of locked regions in the system exceeding a
system-imposed limit.
[EOVERFLOW] A return value would overflow its representation. For
example, cmd is F_GETLK, F_SETLK, or F_SETLKW and the
smallest (or, if l_len is non-zero, the largest) off-set offset
set of a byte in the requested segment will not fit in
an object of type off_t.
[ESRCH] Cmd is F_SETOWN 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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