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FCNTL(2) | System Calls Manual | FCNTL(2) |
NAME¶
fcntl — file controlLIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <fcntl.h> intfcntl(int fd, int cmd, ...);
DESCRIPTION¶
The fcntl() system call provides for control over descriptors. The argument fd is a descriptor to be operated on by cmd as described below. Depending on the value of cmd, fcntl() can take an additional third argument int arg.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 status flags).
- The close-on-exec flag associated with the new file descriptor is set to remain open across execve(2) system calls.
F_DUP2FD
- It is functionally equivalent to
dup2(fd, arg)
F_DUP2FD
constant is not portable, so it should not be used if portability is needed. Use dup2() instead. F_GETFD
- Get the close-on-exec flag associated with the file
descriptor fd as
FD_CLOEXEC
. If the returned value ANDed withFD_CLOEXEC
is 0, the file will remain open across exec(), otherwise the file will be closed upon execution of exec() (arg is ignored). F_SETFD
- Set the close-on-exec flag associated with
fd to arg, where
arg is either 0 or
FD_CLOEXEC
, as described 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 receiving
SIGIO
andSIGURG
signals; process groups are returned as negative values (arg is ignored). F_SETOWN
- Set the process or process group to receive
SIGIO
andSIGURG
signals; process groups are specified by supplying arg as negative, otherwise arg is interpreted as a process ID.
F_GETFL
and
F_SETFL
flags are as follows:
O_NONBLOCK
- Non-blocking I/O; if no data is available to a
read(2) system call, or if a write(2)
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_DIRECT
- Minimize or eliminate the cache effects of reading and writing. The system will attempt to avoid caching the data you read or write. If it cannot avoid caching the data, it will minimize the impact the data has on the cache. Use of this flag can drastically reduce performance if not used with care.
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.
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 */ int l_sysid; /* remote system id or zero for local */ };
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 system 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 withEAGAIN
. 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 theSA_RESTART
(see sigaction(2)). F_READAHEAD
- Set or clear the read ahead amount for sequential access to the third argument, arg, which is rounded up to the nearest block size. A zero value in arg turns off read ahead.
F_RDAHEAD
- Equivalent to Darwin counterpart which sets read ahead amount of 128KB when the third argument, arg is non-zero. A zero value in arg turns off read ahead.
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,
l_start means end edge of the region. The
l_pid and l_sysid fields are only
used with F_GETLK
to return the process ID of the
process holding a blocking lock and the system ID of the system that owns that
process. Locks created by the local system will have a system ID of zero.
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 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 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 getpwnam(3) to retrieve a record, the lock
will be lost because getpwnam(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
fork(2) system call. The flock(2)
interface has much more rational last close semantics and allows locks to be
inherited by child processes. The flock(2) system call 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.
The fcntl(), flock(2), and
lockf(3) locks are compatible. Processes using different
locking interfaces can cooperate over the same file safely. However, only one
of such interfaces should be used within the same process. If a file is locked
by a process through flock(2), any record within the file
will be seen as locked from the viewpoint of another process using
fcntl() or lockf(3), and vice versa. Note
that fcntl(F_GETLK) returns -1 in
l_pid if the process holding a blocking lock previously
locked the file descriptor by flock(2).
All locks associated with a file for a given process are removed when the
process terminates.
All locks obtained before a call to execve(2) remain in effect
until the new program releases them. If the new program does not know about
the locks, they will not be released until the program exits.
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.
RETURN VALUES¶
Upon successful completion, the value returned depends on cmd as follows:F_DUPFD
- A new file descriptor.
F_DUP2FD
- A file descriptor equal to arg.
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.
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 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. - [
EBADF
] - The fd argument is not a valid open
file descriptor.
The argument cmd is
F_DUP2FD
, and arg is not a valid file descriptor. The argument cmd isF_SETLK
orF_SETLKW
, the type of lock (l_type) is a shared lock (F_RDLCK
), and fd is not a valid file descriptor open for reading. The argument cmd isF_SETLK
orF_SETLKW
, the type of lock (l_type) is an exclusive lock (F_WRLCK
), and fd is not a valid file descriptor open for writing. - [
EDEADLK
] - The argument cmd is
F_SETLKW
, and a deadlock condition was detected. - [
EINTR
] - The argument cmd is
F_SETLKW
, and the system call was interrupted by a signal. - [
EINVAL
] - The cmd argument is
F_DUPFD
and arg is negative or greater than the maximum allowable number (see getdtablesize(2)). The argument cmd isF_GETLK
,F_SETLK
orF_SETLKW
and the data to which arg points is not valid. - [
EMFILE
] - The argument cmd is
F_DUPFD
orF_DUP2FD
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
orF_SETLKW
, and satisfying the lock or unlock request would result in the number of locked regions in the system exceeding a system-imposed limit. - [
EOPNOTSUPP
] - The argument cmd is
F_GETLK
,F_SETLK
orF_SETLKW
and fd refers to a file for which locking is not supported. - [
EOVERFLOW
] - The argument cmd is
F_GETLK
,F_SETLK
orF_SETLKW
and an off_t calculation overflowed. - [
EPERM
] - The cmd argument is
F_SETOWN
and the process ID or process group given as an argument is in a different session than the caller. - [
ESRCH
] - The cmd argument is
F_SETOWN
and the process ID given as argument is not in use.
F_SETOWN
can fail for the
same reasons as in tcsetpgrp(3), and a
cmd of F_GETOWN
for the reasons
as stated in tcgetpgrp(3).
SEE ALSO¶
close(2), dup2(2), execve(2), flock(2), getdtablesize(2), open(2), sigvec(2), lockf(3), tcgetpgrp(3), tcsetpgrp(3)STANDARDS¶
TheF_DUP2FD
constant is non portable. It is provided
for compatibility with AIX and Solaris.
HISTORY¶
The fcntl() system call appeared in 4.2BSD. TheF_DUP2FD
constant first appeared in
FreeBSD 7.1.September 28, 2009 | Debian |