/**
* filp_open - open file and return file pointer
*
* @filename: path to open
* @flags: open flags as per the open(2) second argument
* @mode: mode for the new file if O_CREAT is set, else ignored
*
* This is the helper to open a file from kernelspace if you really
* have to. But in generally you should not do this, so please move
* along, nothing to see here..
*/
struct file *filp_open(const char *filename, int flags, umode_t mode)
在Linux内核中，可以使用flip_open来打开文件，跟应用层打开文件的方式基本一样，简要说一下两个参数flags和mode
和应用层的API
int open(const char *pathname, int flags, mode_t mode);
类似，
访问http://man7.org/linux/man-pages/man2/open.2.html
获取详细信息。
O_APPEND
The file is opened in append mode. Before each write(2), the
file offset is positioned at the end of the file, as if with
lseek(2). The modification of the file offset and the write
operation are performed as a single atomic step.
O_APPEND may lead to corrupted files on NFS filesystems if
more than one process appends data to a file at once. This is
because NFS does not support appending to a file, so the
client kernel has to simulate it, which can't be done without
a race condition.
O_ASYNC
Enable signal-driven I/O: generate a signal (SIGIO by default,
but this can be changed via fcntl(2)) when input or output
becomes possible on this file descriptor. This feature is
available only for terminals, pseudoterminals, sockets, and
(since Linux 2.6) pipes and FIFOs. See fcntl(2) for further
details. See also BUGS, below.
O_CLOEXEC (since Linux 2.6.23)
Enable the close-on-exec flag for the new file descriptor.
Specifying this flag permits a program to avoid additional
fcntl(2) F_SETFD operations to set the FD_CLOEXEC flag.
Note that the use of this flag is essential in some
multithreaded programs, because using a separate fcntl(2)
F_SETFD operation to set the FD_CLOEXEC flag does not suffice
to avoid race conditions where one thread opens a file
descriptor and attempts to set its close-on-exec flag using
fcntl(2) at the same time as another thread does a fork(2)
plus execve(2). Depending on the order of execution, the race
may lead to the file descriptor returned by open() being
unintentionally leaked to the program executed by the child
process created by fork(2). (This kind of race is in
principle possible for any system call that creates a file
descriptor whose close-on-exec flag should be set, and various
other Linux system calls provide an equivalent of the
O_CLOEXEC flag to deal with this problem.)
O_CREAT
If pathname does not exist, create it as a regular file.
The owner (user ID) of the new file is set to the effective
user ID of the process.
The group ownership (group ID) of the new file is set either
to the effective group ID of the process (System V semantics)
or to the group ID of the parent directory (BSD semantics).
On Linux, the behavior depends on whether the set-group-ID
mode bit is set on the parent directory: if that bit is set,
then BSD semantics apply; otherwise, System V semantics apply.
For some filesystems, the behavior also depends on the
bsdgroups and sysvgroups mount options described in mount(8)).
The mode argument specifies the file mode bits be applied when
a new file is created. This argument must be supplied when
O_CREAT or O_TMPFILE is specified in flags; if neither O_CREAT
nor O_TMPFILE is specified, then mode is ignored. The
effective mode is modified by the process's umask in the usual
way: in the absence of a default ACL, the mode of the created
file is (mode & ~umask). Note that this mode applies only to
future accesses of the newly created file; the open() call
that creates a read-only file may well return a read/write
file descriptor.
The following symbolic constants are provided for mode:
S_IRWXU 00700 user (file owner) has read, write, and execute
permission
S_IRUSR 00400 user has read permission
S_IWUSR 00200 user has write permission
S_IXUSR 00100 user has execute permission
S_IRWXG 00070 group has read, write, and execute permission
S_IRGRP 00040 group has read permission
S_IWGRP 00020 group has write permission
S_IXGRP 00010 group has execute permission
S_IRWXO 00007 others have read, write, and execute permission
S_IROTH 00004 others have read permission
S_IWOTH 00002 others have write permission
S_IXOTH 00001 others have execute permission
According to POSIX, the effect when other bits are set in mode
is unspecified. On Linux, the following bits are also honored
in mode:
S_ISUID 0004000 set-user-ID bit
S_ISGID 0002000 set-group-ID bit (see inode(7)).
S_ISVTX 0001000 sticky bit (see inode(7)).
O_DIRECT (since Linux 2.4.10)
Try to minimize cache effects of the I/O to and from this
file. In general this will degrade performance, but it is
useful in special situations, such as when applications do
their own caching. File I/O is done directly to/from user-
space buffers. The O_DIRECT flag on its own makes an effort
to transfer data synchronously, but does not give the
guarantees of the O_SYNC flag that data and necessary metadata
are transferred. To guarantee synchronous I/O, O_SYNC must be
used in addition to O_DIRECT. See NOTES below for further
discussion.
A semantically similar (but deprecated) interface for block
devices is described in raw(8).
O_DIRECTORY
If pathname is not a directory, cause the open to fail. This
flag was added in kernel version 2.1.126, to avoid denial-of-
service problems if opendir(3) is called on a FIFO or tape
device.
O_DSYNC
Write operations on the file will complete according to the
requirements of synchronized I/O data integrity completion.
By the time write(2) (and similar) return, the output data has
been transferred to the underlying hardware, along with any
file metadata that would be required to retrieve that data
(i.e., as though each write(2) was followed by a call to
fdatasync(2)). See NOTES below.
O_EXCL Ensure that this call creates the file: if this flag is
specified in conjunction with O_CREAT, and pathname already
exists, then open() fails with the error EEXIST.
When these two flags are specified, symbolic links are not
followed: if pathname is a symbolic link, then open() fails
regardless of where the symbolic link points.
In general, the behavior of O_EXCL is undefined if it is used
without O_CREAT. There is one exception: on Linux 2.6 and
later, O_EXCL can be used without O_CREAT if pathname refers
to a block device. If the block device is in use by the
system (e.g., mounted), open() fails with the error EBUSY.
On NFS, O_EXCL is supported only when using NFSv3 or later on
kernel 2.6 or later. In NFS environments where O_EXCL support
is not provided, programs that rely on it for performing
locking tasks will contain a race condition. Portable
programs that want to perform atomic file locking using a
lockfile, and need to avoid reliance on NFS support for
O_EXCL, can create a unique file on the same filesystem (e.g.,
incorporating hostname and PID), and use link(2) to make a
link to the lockfile. If link(2) returns 0, the lock is
successful. Otherwise, use stat(2) on the unique file to
check if its link count has increased to 2, in which case the
lock is also successful.
O_LARGEFILE
(LFS) Allow files whose sizes cannot be represented in an
off_t (but can be represented in an off64_t) to be opened.
The _LARGEFILE64_SOURCE macro must be defined (before
including any header files) in order to obtain this
definition. Setting the _FILE_OFFSET_BITS feature test macro
to 64 (rather than using O_LARGEFILE) is the preferred method
of accessing large files on 32-bit systems (see
feature_test_macros(7)).
O_NOATIME (since Linux 2.6.8)
Do not update the file last access time (st_atime in the
inode) when the file is read(2).
This flag can be employed only if one of the following
conditions is true:
* The effective UID of the process matches the owner UID of
the file.
* The calling process has the CAP_FOWNER capability in its
user namespace and the owner UID of the file has a mapping
in the namespace.
This flag is intended for use by indexing or backup programs,
where its use can significantly reduce the amount of disk
activity. This flag may not be effective on all filesystems.
One example is NFS, where the server maintains the access
time.
O_NOCTTY
If pathname refers to a terminal device—see tty(4)—it will not
become the process's controlling terminal even if the process
does not have one.
O_NOFOLLOW
If pathname is a symbolic link, then the open fails, with the
error ELOOP. Symbolic links in earlier components of the
pathname will still be followed. (Note that the ELOOP error
that can occur in this case is indistinguishable from the case
where an open fails because there are too many symbolic links
found while resolving components in the prefix part of the
pathname.)
This flag is a FreeBSD extension, which was added to Linux in
version 2.1.126, and has subsequently been standardized in
POSIX.1-2008.
See also O_PATH below.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in nonblocking mode.
Neither the open() nor any subsequent operations on the file
descriptor which is returned will cause the calling process to
wait.
Note that this flag has no effect for regular files and block
devices; that is, I/O operations will (briefly) block when
device activity is required, regardless of whether O_NONBLOCK
is set. Since O_NONBLOCK semantics might eventually be
implemented, applications should not depend upon blocking
behavior when specifying this flag for regular files and block
devices.
For the handling of FIFOs (named pipes), see also fifo(7).
For a discussion of the effect of O_NONBLOCK in conjunction
with mandatory file locks and with file leases, see fcntl(2).
O_PATH (since Linux 2.6.39)
Obtain a file descriptor that can be used for two purposes: to
indicate a location in the filesystem tree and to perform
operations that act purely at the file descriptor level. The
file itself is not opened, and other file operations (e.g.,
read(2), write(2), fchmod(2), fchown(2), fgetxattr(2),
ioctl(2), mmap(2)) fail with the error EBADF.
The following operations can be performed on the resulting
file descriptor:
* close(2).
* fchdir(2), if the file descriptor refers to a directory
(since Linux 3.5).
* fstat(2) (since Linux 3.6).
* fstatfs(2) (since Linux 3.12).
* Duplicating the file descriptor (dup(2), fcntl(2) F_DUPFD,
etc.).
* Getting and setting file descriptor flags (fcntl(2) F_GETFD
and F_SETFD).
* Retrieving open file status flags using the fcntl(2)
F_GETFL operation: the returned flags will include the bit
O_PATH.
* Passing the file descriptor as the dirfd argument of
openat() and the other "*at()" system calls. This includes
linkat(2) with AT_EMPTY_PATH (or via procfs using
AT_SYMLINK_FOLLOW) even if the file is not a directory.
* Passing the file descriptor to another process via a UNIX
domain socket (see SCM_RIGHTS in unix(7)).
When O_PATH is specified in flags, flag bits other than
O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.
Opening a file or directory with the O_PATH flag requires no
permissions on the object itself (but does require execute
permission on the directories in the path prefix). Depending
on the subsequent operation, a check for suitable file
permissions may be performed (e.g., fchdir(2) requires execute
permission on the directory referred to by its file descriptor
argument). By contrast, obtaining a reference to a filesystem
object by opening it with the O_RDONLY flag requires that the
caller have read permission on the object, even when the
subsequent operation (e.g., fchdir(2), fstat(2)) does not
require read permission on the object.
If pathname is a symbolic link and the O_NOFOLLOW flag is also
specified, then the call returns a file descriptor referring
to the symbolic link. This file descriptor can be used as the
dirfd argument in calls to fchownat(2), fstatat(2), linkat(2),
and readlinkat(2) with an empty pathname to have the calls
operate on the symbolic link.
If pathname refers to an automount point that has not yet been
triggered, so no other filesystem is mounted on it, then the
call returns a file descriptor referring to the automount
directory without triggering a mount. fstatfs(2) can then be
used to determine if it is, in fact, an untriggered automount
point (.f_type == AUTOFS_SUPER_MAGIC).
One use of O_PATH for regular files is to provide the
equivalent of POSIX.1's O_EXEC functionality. This permits us
to open a file for which we have execute permission but not
read permission, and then execute that file, with steps
something like the following:
char buf[PATH_MAX];
fd = open("some_prog", O_PATH);
snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd);
execl(buf, "some_prog", (char *) NULL);
An O_PATH file descriptor can also be passed as the argument
of fexecve(3).
O_SYNC Write operations on the file will complete according to the
requirements of synchronized I/O file integrity completion (by
contrast with the synchronized I/O data integrity completion
provided by O_DSYNC.)
By the time write(2) (or similar) returns, the output data and
associated file metadata have been transferred to the underly‐
ing hardware (i.e., as though each write(2) was followed by a
call to fsync(2)). See NOTES below.
O_TMPFILE (since Linux 3.11)
Create an unnamed temporary regular file. The pathname argu‐
ment specifies a directory; an unnamed inode will be created
in that directory's filesystem. Anything written to the
resulting file will be lost when the last file descriptor is
closed, unless the file is given a name.
O_TMPFILE must be specified with one of O_RDWR or O_WRONLY
and, optionally, O_EXCL. If O_EXCL is not specified, then
linkat(2) can be used to link the temporary file into the
filesystem, making it permanent, using code like the follow‐
ing:
char path[PATH_MAX];
fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
S_IRUSR | S_IWUSR);
/* File I/O on 'fd'... */
snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd);
linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
AT_SYMLINK_FOLLOW);
In this case, the open() mode argument determines the file
permission mode, as with O_CREAT.
Specifying O_EXCL in conjunction with O_TMPFILE prevents a
temporary file from being linked into the filesystem in the
above manner. (Note that the meaning of O_EXCL in this case
is different from the meaning of O_EXCL otherwise.)
There are two main use cases for O_TMPFILE:
* Improved tmpfile(3) functionality: race-free creation of
temporary files that (1) are automatically deleted when
closed; (2) can never be reached via any pathname; (3) are
not subject to symlink attacks; and (4) do not require the
caller to devise unique names.
* Creating a file that is initially invisible, which is then
populated with data and adjusted to have appropriate
filesystem attributes (fchown(2), fchmod(2), fsetxattr(2),
etc.) before being atomically linked into the filesystem
in a fully formed state (using linkat(2) as described
above).
O_TMPFILE requires support by the underlying filesystem; only
a subset of Linux filesystems provide that support. In the
initial implementation, support was provided in the ext2,
ext3, ext4, UDF, Minix, and shmem filesystems. Support for
other filesystems has subsequently been added as follows: XFS
(Linux 3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and ubifs
(Linux 4.9)
O_TRUNC
If the file already exists and is a regular file and the
access mode allows writing (i.e., is O_RDWR or O_WRONLY) it
will be truncated to length 0. If the file is a FIFO or ter‐
minal device file, the O_TRUNC flag is ignored. Otherwise,
the effect of O_TRUNC is unspecified.