fcntl - file descriptor control
Standard C Library (libc, -lc)
#include <fcntl.h>
int
fcntl(int fd, int cmd, ...);
fcntl() provides for control over descriptors. 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 it is interpreted
as an int by some commands and ignored by others.
Commands are:
F_DUPFD Return a new descriptor as follows:
+o Lowest numbered available descriptor greater than or
equal to arg, which is interpreted as an int.
+o Same object references as the original descriptor.
+o New descriptor shares the same file offset if the
object was a file.
+o Same access mode (read, write or read/write).
+o Same file status flags (i.e., both file descriptors
share the same file status flags).
+o The close-on-exec flag associated with the new file
descriptor is set to remain open across execve(2) system
calls.
F_GETFD Get the close-on-exec flag associated with the file descriptor
fd. 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 execution of exec() (arg is ignored).
F_SETFD Set the close-on-exec flag associated with fd 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, which is interpreted as an
int.
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. The
argument arg is interpreted as an int.
The flags for the F_GETFL and F_SETFL flags 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.
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 l_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). As specified by the
value of l_type, 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
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 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 AT&T System V
UNIX 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. Calling
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.
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.
fcntl() will fail if:
[EAGAIN] The argument arg 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] 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] 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
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, or F_SETLKW and
the data to which arg 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 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 limit.
[ESRCH] cmd is F_SETOWN and the process ID given as argument
is not in use.
close(2), execve(2), flock(2), open(2), sigaction(2), getdtablesize(3)
The fcntl() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').
The fcntl() function call appeared in 4.2BSD.
BSD May 5, 2001 BSD
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