http://blog.csdn.net/cywosp/article/details/8767327
SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
int creat(const char *pathname, mode_t mode);
DESCRIPTION
Given a pathname for a file, open() returns a file descriptor, a small,
non-negative integer for use in subsequent system calls (read(),
write(), lseek(), fcntl(), etc.). The file descriptor returned by a
successful call will be the lowest-numbered file descriptor not cur-
rently open for the process.
By default, the new file descriptor is set to remain open across an
execve() (i.e., the FD_CLOEXEC file descriptor flag described in
fcntl() is initially disabled; the Linux-specific O_CLOEXEC flag,
described below, can be used to change this default). The file offset
is set to the beginning of the file (see lseek()).
A call to open() creates a new open file description, an entry in the
system-wide table of open files. This entry records the file offset and
the file status flags (modifiable via the fcntl() F_SETFL operation).
A file descriptor is a reference to one of these entries; this reference
is unaffected if pathname is subsequently removed or modified to refer
to a different file. The new open file description is initially not
shared with any other process, but sharing may arise via fork().
The argument flags must include one of the following access modes:
O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read-
only, write-only, or read/write, respectively.
In addition, zero or more file creation flags and file status flags can
be bitwise-or’d in flags. The file creation flags are O_CREAT, O_EXCL,
O_NOCTTY, and O_TRUNC. The file status flags are all of the remaining
flags listed below. The distinction between these two groups of flags
is that the file status flags can be retrieved and (in some cases) modi-
fied using fcntl(). The full list of file creation flags and file sta-
tus flags is as follows:
O_APPEND
The file is opened in append mode. Before each write(), the
file offset is positioned at the end of the file, as if with
lseek(). O_APPEND may lead to corrupted files on NFS file sys-
tems 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()) when input or output
becomes possible on this file descriptor. This feature is only
available for terminals, pseudo-terminals, sockets, and (since
Linux 2.6) pipes and FIFOs. See fcntl() for further details.
O_CLOEXEC (Since Linux 2.6.)
Enable the close-on-exec flag for the new file descriptor. Spec-
ifying this flag permits a program to avoid additional fcntl()
F_SETFD operations to set the FD_CLOEXEC flag. Additionally, use
of this flag is essential in some multithreaded programs since
using a separate fcntl() F_SETFD operation to set the FD_CLOEXEC
flag does not suffice to avoid race conditions where one thread
opens a file descriptor at the same time as another thread does a
fork() plus execve().
O_CREAT
If the file does not exist it will be created. The owner (user
ID) of the file is set to the effective user ID of the process.
The group ownership (group ID) is set either to the effective
group ID of the process or to the group ID of the parent direc-
tory (depending on file system type and mount options, and the
mode of the parent directory, see the mount options bsdgroups and
sysvgroups described in mount()).
mode specifies the permissions to use in case a new file is cre-
ated. This argument must be supplied when O_CREAT is specified
in flags; if O_CREAT is not specified, then mode is ignored. The
effective permissions are modified by the process’s umask in the
usual way: The permissions of the created file are
(mode & ~umask). Note that this mode only applies 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 user (file owner) has read, write and execute per-
mission
S_IRUSR user has read permission
S_IWUSR user has write permission
S_IXUSR user has execute permission
S_IRWXG group has read, write and execute permission
S_IRGRP group has read permission
S_IWGRP group has write permission
S_IXGRP group has execute permission
S_IRWXO others have read, write and execute permission
S_IROTH others have read permission
S_IWOTH others have write permission
S_IXOTH others have execute permission
O_DIRECT (Since Linux 2.4.)
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 I/O is synchronous, that is, at the completion of a read()
or write(), data is guaranteed to have been transferred. See
NOTES below for further discussion.
A semantically similar (but deprecated) interface for block
devices is described in raw().
O_DIRECTORY
If pathname is not a directory, cause the open to fail. This
flag is Linux-specific, and was added in kernel version 2.1.,
to avoid denial-of-service problems if opendir() is called on a
FIFO or tape device, but should not be used outside of the imple-
mentation of opendir().
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() will fail. The behavior of O_EXCL is undefined if O_CREAT
is not specified.
When these two flags are specified, symbolic links are not fol-
lowed: if pathname is a symbolic link, then open() fails regard-
less of where the symbolic link points to.
O_EXCL is only supported on NFS when using NFSv3 or later on ker-
nel 2.6 or later. In environments where NFS 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 file system (e.g., incorporating hostname and
PID), and use link() to make a link to the lockfile. If link()
returns , the lock is successful. Otherwise, use stat() on the
unique file to check if its link count has increased to , 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 _LARGE-
FILE64_SOURCE macro must be defined in order to obtain this defi-
nition. Setting the _FILE_OFFSET_BITS feature test macro to
(rather than using O_LARGEFILE) is the preferred method of
obtaining method of accessing large files on -bit systems (see
feature_test_macros()).
O_NOATIME (Since Linux 2.6.)
Do not update the file last access time (st_atime in the inode)
when the file is read(). 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 effec-
tive on all file systems. One example is NFS, where the server
maintains the access time.
O_NOCTTY
If pathname refers to a terminal device — see tty() — 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. This is a
FreeBSD extension, which was added to Linux in version 2.1..
Symbolic links in earlier components of the pathname will still
be followed.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in non-blocking mode. Neither
the open() nor any subsequent operations on the file descriptor
which is returned will cause the calling process to wait. For
the handling of FIFOs (named pipes), see also fifo(). For a
discussion of the effect of O_NONBLOCK in conjunction with manda-
tory file locks and with file leases, see fcntl().
O_SYNC The file is opened for synchronous I/O. Any write()s on the
resulting file descriptor will block the calling process until
the data has been physically written to the underlying hardware.
But see NOTES below.
O_TRUNC
If the file already exists and is a regular file and the open
mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be
truncated to length . If the file is a FIFO or terminal device
file, the O_TRUNC flag is ignored. Otherwise the effect of
O_TRUNC is unspecified.
Some of these optional flags can be altered using fcntl() after the
file has been opened.
creat() is equivalent to open() with flags equal to
O_CREAT|O_WRONLY|O_TRUNC.
#include <unistd.h>
int fsync(int fd);
int fdatasync(int fd);
Feature Test Macro Requirements for glibc (see feature_test_macros()):
fsync(): _BSD_SOURCE || _XOPEN_SOURCE
|| /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
fdatasync(): _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >=
DESCRIPTION
fsync() transfers ("flushes") all modified in-core data of (i.e., modi-
fied buffer cache pages for) the file referred to by the file descriptor
fd to the disk device (or other permanent storage device) where that
file resides. The call blocks until the device reports that the trans-
fer has completed. It also flushes metadata information associated with
the file (see stat()).
Calling fsync() does not necessarily ensure that the entry in the direc-
tory containing the file has also reached disk. For that an explicit
fsync() on a file descriptor for the directory is also needed.
fdatasync() is similar to fsync(), but does not flush modified metadata
unless that metadata is needed in order to allow a subsequent data
retrieval to be correctly handled. For example, changes to st_atime or
st_mtime (respectively, time of last access and time of last modifica-
tion; see stat()) do not require flushing because they are not neces-
sary for a subsequent data read to be handled correctly. On the other
hand, a change to the file size (st_size, as made by say ftruncate()),
would require a metadata flush.
The aim of fdatasync() is to reduce disk activity for applications that
do not require all metadata to be synchronized with the disk.