The fcntl(2) function provides for control over
descriptors. The argument fd is a descriptor to be operated
on by cmd. Cmd is one of F_DUPFD, F_GETFD, F_SETFD,
F_GETFL, F_SETFL, or one of the commands for advisory file locking,
F_GETLK, F_SETLK, or F_SETLKW.
Values for cmd are:
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 execv(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(2), otherwise the file will be closed
upon execution of exec(2). Arg is ignored.
F_SETFD
Set the file descriptor flags of fd to the values set in
arg (an int). If the FD_CLOEXEC flag is set to 1, the
file descriptor is closed upon execution of an exec(2)-family function. If
the flag is 0, the descriptor is not closed when an exec(2) function
is executed.
F_GETFL
Get descriptor status flags, as described below. Arg is
ignored. To extract the file access modes from the return value,
use the mask O_ACCMODE (defined in <fcntl.h>). The
access modes are the ones defined for open(2) (O_RDONLY, O_RDWR, and
O_WRONLY).
F_SETFL
Set descriptor status flags to arg, which is an
int. Bits in arg that correspond to access mode and
the oflag values of open(2) are ignored.
F_GETOWN
Get the process ID or process group currently receiving SIGIO
and SIGURG signals; process groups are returnerd as negative
values, and arg is ignored.
F_SETOWN
Set the process or process group to receive SIGIO and SIGURG
signals. To specify a process group, supply arg as a
negative ; if arg is positive, it's taken as a process
ID.
The flags for the F_GETFL and F_SETFL flags are as follows:
FAPPEND
Available for backward compatibility; equivalent to
O_APPEND.
FASYNC
Available for backward compatibility; equivalent to
O_ASYNC.
FNDELAY
Available for backward compatibility; equivalent to
O_NONBLOCK.
FNONBLOCK
Available for backward compatibility; equivalent to
O_NONBLOCK.
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 (for example, upon availablility of data to be
read).
O_NONBLOCK
Non-blocking I/O; if no data is available to a read(1)
call, or if a write(1) operation would block, the read or
write call returns -1 with the error [EAGAIN].
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(2) 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
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(2) returns
immediately with [EACCES].
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(2) is waiting for a region, the fcntl(2) will be
interrupted.
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.
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.
All locks associated with a file for a given process are removed
when the process terminates.
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]
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.
[EMFILE]
Cmd is F_DUPFD and the maximum allowed number of file
descriptors are currently open.
[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 {OPEN_MAX}.
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_DUPED 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. This error
is not possible under INTERIX because there is no system-imposed
limit.
[ESRCH]
For a cmd of F_SETOWN, the process ID given as
arg is not in use.