FCNTL(2) BSD Programmer's Manual FCNTL(2)NAMEfcntl - file control
SYNOPSIS
#include <sys/types.h>
#include <fcntl.h>
int
fcntl(int fd, int cmd, ...);
DESCRIPTIONFcntl() provides control over file descriptors. The argument fd is a
file descriptor to be operated on by a command cmd. Some operations use a
third argument, which is either an integer int iarg or a pointer void
*parg as appropriate. The commands are:
F_DUPFD Return a new file descriptor as follows:
o It uses the lowest numbered available descriptor
greater than or equal to iarg.
o It references the same object as the original descrip-
tor.
o It shares the same file offset if the object was a
file.
o It shares file status flags with the old descriptor,
including the access mode (read, write or read/write).
o It does not share file descriptor flags (FD_CLOEXEC,
etc.) with the old descriptor.
F_GETFD Get the file descriptor flags, as specified below. File de-
scriptor flags apply to the given file descriptor and they are
not copied by F_DUPFD or any other descriptor duplication op-
eration. When a process calls fork(2) to create a new pro-
cess, the new process receives copies of the parent process's
file descriptor flags along with copies of the parent pro-
cess's file descriptors; the processes do not share file de-
scriptor flags.
F_SETFD Set the file descriptor flags to iarg.
F_GETFL Get the file status flags, as specified below. File status
flags are shared among all file descriptors that are descended
from an original file descriptor that was created by open(2)
or some other descriptor-creating call. Specifically, file
status flags are shared between old and new file descriptors
after an F_DUPFD operation, and they are shared with (not
copied to) the corresponding file descriptors in a child pro-
cess after a fork(2) call.
F_SETFL Set the file status flags to iarg.
F_GETOWN Get the process ID or process group currently receiving SIGIO
and SIGURG signals; process groups are returned as negative
values.
F_SETOWN Set the process or process group to receive SIGIO and SIGURG
signals; process groups are specified by supplying iarg as
negative, otherwise iarg is interpreted as a process ID.
The file descriptor flags are:
FD_CLOEXEC Automatically close this descriptor immediately before an
execve(2) call succeeds.
The file status flags are:
O_NONBLOCK Enable non-blocking I/O. If no data is available to a
read(2) operation, or if a write(2) operation would block
because of flow control or other reasons, and the file de-
scriptor for the given operation is non-blocking, then the
operation returns -1 with the global variable errno set to
EAGAIN.
O_APPEND Force each write(2) operation to append data at the end of
the given file. This flag 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, for example, 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:
F_GETLK Get the first lock that blocks the lock description pointed to
by parg, 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 de-
scription pointed to by parg, 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 -1
with the global variable errno set to EAGAIN.
F_SETLKW Set or clear a file segment lock like F_SETLK except that if a
shared or exclusive lock is blocked by other locks, the pro-
cess 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 de-
scriptor 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 ex-
tend 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 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 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 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.
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 pro-
cess. 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_GETFD The file descriptor flags.
F_GETFL The file status flags.
F_GETOWN The file descriptor owner.
other Value other than -1.
Otherwise, a value of -1 is returned and errno is set to indicate the er-
ror.
ERRORSFcntl() 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 al-
ready shared-locked or exclusive-locked by another process.
[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.
[EMFILE] The argument cmd is F_DUPFD and the maximum allowed number
of file descriptors are currently open.
[EDEADLK] The argument cmd is F_SETLKW, and a deadlock condition was
detected.
[EINTR] The argument cmd is F_SETLKW, and the function was inter-
rupted by a signal.
[EINVAL] The argument cmd is F_DUPFD and iarg 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 parg points is not valid, or fildes refers to
a file that does not support locking.
[EMFILE] The argument cmd is F_DUPFD and the maximum number of file
descriptors permitted for the process are already in use,
or no file descriptors greater than or equal to iarg 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 limit.
[ENOTTY] The filedes is not the controlling tty for the process.
[EPERM] The argument cmd is F_SETOWN and the user is not permitted
to signal the given process or group ID.
The argument cmd is F_SETOWN and filedes is a tty, and the
specified process group (or the process group of the speci-
fied process) is not in the same session as the caller.
[ESRCH] The argument cmd is F_SETOWN and the process or group ID
given as argument is not in use.
SEE ALSOclose(2), execve(2), flock(2), getdtablesize(2), open(2), sigvec(2)HISTORY
The fcntl function call appeared in 4.2BSD.
BUGS
This interface follows the completely stupid semantics of System V and
IEEE Std1003.1-1988 (``POSIX'') 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 getpw-
name(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 semantic problem with this interface is that locks are not inherit-
ed 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 applica-
tions 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.
The differences between file descriptor flags, file status flags and de-
vice parameters (from ioctl(2)) are subtle and confusing. Device param-
eters apply to an object; file status flags apply to an open instance of
an object; file descriptor flags apply to a reference to an open instance
of an object.
4.2 Berkeley Distribution April 10, 2000 5
