FCNTL(2) BSD Programmer's Manual FCNTL(2)NAMEfcntl - 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 al-
ready 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 execv(3) calls.
F_GETFD Get the close-on-exec flag associated with the file descriptor
fd as FD_CLOEXEC. If the returned value ANDed with FD_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 (interpreted as an int) is either 0 or FD_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
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 (inter-
preted as an int) 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 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 es-
tablish 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 descrip-
tions of shared locks and exclusive locks, an F_SETLK or an F_SETLKW re-
quest 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") 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
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 da-
tabase, even if the library routine never requested a lock on the data-
base. 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 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 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 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] fd 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 fd 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 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 function was inter-
rupted by a signal.
[EINVAL] cmd is F_DUPFD and arg is negative or greater than the max-
imum 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 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 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 in arg is not in
use.
SEE ALSOclose(2), execve(2), flock(2), open(2), sigaction(2), getdtablesize(3)HISTORY
The fcntl() function call appeared in 4.2BSD.
MirOS BSD #10-current January 12, 1994 3
