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:
•
Lowest numbered available descriptor greater than or equal to arg, which is 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 cleared to remain open across
execve(2) system 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 is either 0 or FD_CLOEXEC, as described 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.
F_CLOSEM
Close all file descriptors greater than or equal to fd.
F_MAXFD
Return the maximum file descriptor number currently open by the process.
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.