NAME
fcntl
—
file control
SYNOPSIS
#include
<fcntl.h>
int
fcntl
(int
fd, int cmd,
...);
DESCRIPTION
The
fcntl
()
provides control over the properties of a file that is already open. The
argument fd is a descriptor to be operated on by
cmd as described below. The third parameter is called
arg and is technically a pointer to
void, but is interpreted as an
int
by some commands, a pointer to a
struct flock
by others (see below), and ignored by
the rest.
The commands are:
F_DUPFD
- Return a new descriptor as follows:
- Lowest numbered available descriptor greater than or equal to
arg (interpreted as an
int
). - 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) calls.
- Lowest numbered available descriptor greater than or equal to
arg (interpreted as an
F_DUPFD_CLOEXEC
- Like
F_DUPFD
, but theFD_CLOEXEC
flag associated with the new file descriptor is set, so the file descriptor is closed when execve(2) is called. 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 acrossexec
(), otherwise the file will be closed upon execution ofexec
() (arg is ignored). F_SETFD
- Set the close-on-exec flag associated with fd to
arg, where arg (interpreted as
an
int
) is either 0 orFD_CLOEXEC
, as described above. F_GETFL
- Get file status flags associated with the file descriptor fd, as described below (arg is ignored).
F_SETFL
- Set file status flags associated with the file descriptor
fd to arg (interpreted as an
int
). 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 (interpreted as anint
) as negative, otherwise arg is taken as a process ID.
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(2) 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_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. O_SYNC
- Cause writes to be synchronous. Data will be written to the physical
device instead of just being stored in the buffer cache; corresponds to
the
O_SYNC
flag of open(2).
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 toF_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 whilefcntl
() is waiting for a region, thefcntl
() will be interrupted if the signal handler has not specified theSA_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 area starting at
l_start+l_len and ending at
l_start-1 is locked. 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 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) 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
()
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.
RETURN VALUES
Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD
- A new file descriptor.
F_DUPFD_CLOEXEC
- 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
fcntl
() 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
] - fd is not a valid open file descriptor.
The argument cmd is
F_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 is
F_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 invalid.
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(3)).The argument cmd is
F_GETLK
,F_SETLK
, orF_SETLKW
and the data to which arg points is not valid, or fd refers to a file that does not support locking. - [
EMFILE
] - The argument cmd is
F_DUPFD
and the maximum number of open 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. - [
EOVERFLOW
] - The argument cmd is
F_GETLK
,F_SETLK
orF_SETLKW
and the segment length of a file to be locked is too large to be represented by an off_t. - [
ESRCH
] - cmd is
F_SETOWN
and the process ID given in arg is not in use.
SEE ALSO
close(2), execve(2), flock(2), open(2), sigaction(2), getdtablesize(3)
STANDARDS
The fcntl
() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
The fcntl
() function call appeared in
4.2BSD.