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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:
• Lowest numbered available descriptor greater than or equal to arg.
• Same object references as the original descriptor.
• New descriptor shares the same file offset if the object was a file.
• Same access mode (read, write or read/write).
• Same file status flags (i.e., both file descriptors share the same file sta-tus status
tus flags).
• The close-on-exec flag associated with the new file descriptor is cleared so
that the descriptor remains open across an execv(2) system call.
F_DUPFD_CLOEXEC Like F_DUPFD, except that the close-on-exec flag associated with the new file
descriptor is set.
F_GETFD Get the flags associated with the file descriptor fildes, as described below (arg is
ignored).
F_SETFD Set the file descriptor flags to arg.
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 receiving 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 supplying arg as negative, otherwise arg is interpreted as a
process ID.
F_GETPATH Get the path of the file descriptor Fildes. The argument must be a buffer of size
MAXPATHLEN or greater.
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.
F_SETSIZE Truncate a file without zeroing space. The calling process must have root privi-leges. privileges.
leges.
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. Note that if the file offset is not
backed by physical blocks we can return -1 as the offset. This is subject to change.
F_LOG2PHYS_EXT Variant of F_LOG2PHYS that uses the passed in file offset and length.
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 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.
F_SETNOSIGPIPE Determines whether a SIGPIPE signal will be generated when a write fails on a pipe
or socket for which there is no reader. If arg is non-zero, SIGPIPE generation is
disabled for descriptor fildes, while an arg of zero enables it (the default).
F_GETNOSIGPIPE Returns whether a 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 arg of F_SETNOSIGPIPE.
The flags for the F_GETFD and F_SETFD commands are as follows:
FD_CLOEXEC Close-on-exec; the given file descriptor will be automatically closed in the succes-sor successor
sor process image when one of the execv(2) or posix_spawn(2) family of system calls
is invoked.
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; corresponds 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 estab-lish establish
lish 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 EAGAIN.
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 specified 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 hold-ing holding
ing 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 application 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 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 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 spec-ified specified
ified 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 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
getpwname(3) to retrieve a record, the lock will be lost because 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 seman-tic semantic
tic 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 concurrently.
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 following 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 */
};
The F_LOG2PHYS_EXT command operates on the same structure as F_LOG2PHYS but treats it as an in/out:
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 */
};
If fildes is a socket, then the F_SETNOSIGPIPE and F_GETNOSIGPIPE commands are directly analogous, and
fully interoperate with the SO_NOSIGPIPE option of setsockopt(2) and getsockopt(2) respectively.
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:
[EAGAIN] 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 exclu-sive-locked exclusive-locked
sive-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.
[EACCESS] 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 writing.
The argument cmd is F_PREALLOCATE and the calling process does not have file write
permission.
The argument cmd is F_LOG2PHYS or F_LOG2PHYS_EXT 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 satisfying the lock or unlock request
would result in the number of locked regions in the system exceeding a system-imposed systemimposed
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) offset
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), pipe(2), socket(2), setsockopt(2),
sigaction(3)
HISTORY
The fcntl() function call appeared in 4.2BSD.
4.2 Berkeley Distribution February 17, 2011 4.2 Berkeley Distribution
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