linux open函数详解
头文件
#include<sys/types.h> /*提供类型pid_t,size_t的定义*/
#include<sys/stat.h>
#include<fcntl.h>
函数原型
int open(const char *path, int oflags,mode_t mode);
函数说明
open建立了一条到文件或设备的访问路径。
open函数一般用于打开或者创建文件，在打开或创建文件时可以制定文件的属性及用户的权限等各种参数。
第一个参数path表示：路径名或者文件名。路径名为绝对路径名（如C:/cpp/a.cpp），文件则是在当前工作目录下的。
第二个参数oflags表示：打开文件所采取的动作。
可能值：必须指定下面某一种:
O_RDONLY(只读),
O_WRONLY（只写）,
O_RDWR（可读可写）
打开/创建文件时，至少得使用上述三个常量中的一个，以下常量是选用的：
O_APPEND 每次写操作都写入文件的末尾
O_CREAT 如果指定文件不存在，则创建这个文件
O_EXCL 如果要创建的文件已存在，则返回 -1，并且修改errno的值
O_TRUNC 如果文件存在，并且以只写/读写方式打开，则清空文件全部内容
O_NOCTTY 如果路径名指向终端设备，不要把这个设备用作控制终端。
O_NONBLOCK 如果路径名指向 FIFO/块文件/字符文件，则把文件的打开和后继 I/O
设置为非阻塞模式（nonblocking mode）
第三个参数mode表示：设置文件访问权限的初始值。（与用户掩码umask变量有关，实际的访问权限有mode &~umask确定）
S_IRUSR,S_IWUSER,S_IXUSR,S_IRGRP,S_IWGRP,S_IXGRP,S_IROTH,S_IWOTH,S_IXOTH.其中R：读，W：写，X：执行，USR：文件所属的用户，GRP：文件所属的组，OTH：其他用户。
注：第三个参数是在第二个参数中有O_CREAT时才用作用。若没有，则第三个参数可以忽略。
返回值:如果操作成功，它将返回一个文件描述符，如果失败，返回－1
我是搬运工，以下来自man手册 man 2 open
OPEN(2)
NAME
open, openat, creat - open and possibly create a file
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);
int openat(int dirfd, const char *pathname, int flags);
int openat(int dirfd, const char *pathname, int flags, mode_t mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
openat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
DESCRIPTION
The open() system call opens the file specified by pathname. If the specified file does not exist, it may optionally (if O_CREAT is specified in flags) be created by open().
The return value of open() is a file descriptor, a small, nonnegative integer that is used in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.) to refer to the open file.
The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process.
By default, the new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled); the 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(2)).
A call to open() creates a new open file description, an entry in the system-wide table of open files. The open file description records the file offset and the file status flags (see
below). A file descriptor is a reference to an open file description; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file. For further
details on open file descriptions, see NOTES.
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_CLOEXEC, O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW,
O_TMPFILE, 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 creation flags affect the
semantics of the open operation itself, while the file status flags affect the semantics of subsequent I/O operations. The file status flags can be retrieved and (in some cases) modified;
see fcntl(2) for details.
The full list of file creation flags and file status flags is as follows:
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 cre‐
ated 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. Oth‐
erwise, 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 argu‐
ment). 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 subse‐
quent 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, "/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 underlying 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 argument 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 filesys‐
tem, making it permanent, using code like the following:
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 sub‐
ject 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 terminal
device file, the O_TRUNC flag is ignored. Otherwise, the effect of O_TRUNC is unspecified.
creat()
A call to creat() is equivalent to calling open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.
openat()
The openat() system call operates in exactly the same way as open(), except for the differences described here.
If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory
of the calling process, as is done by open() for a relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like open()).
If pathname is absolute, then dirfd is ignored.
RETURN VALUE
open(), openat(), and creat() return the new file descriptor, or -1 if an error occurred (in which case, errno is set appropriately).
ERRORS
open(), openat(), and creat() can fail with the following errors:
EACCES The requested access to the file is not allowed, or search permission is denied for one of the directories in the path prefix of pathname, or the file did not exist yet and write
access to the parent directory is not allowed. (See also path_resolution(7).)
EDQUOT Where O_CREAT is specified, the file does not exist, and the user's quota of disk blocks or inodes on the filesystem has been exhausted.
EEXIST pathname already exists and O_CREAT and O_EXCL were used.
EFAULT pathname points outside your accessible address space.
EFBIG See EOVERFLOW.
EINTR While blocked waiting to complete an open of a slow device (e.g., a FIFO; see fifo(7)), the call was interrupted by a signal handler; see signal(7).
EINVAL The filesystem does not support the O_DIRECT flag. See NOTES for more information.
EINVAL Invalid value in flags.
EINVAL O_TMPFILE was specified in flags, but neither O_WRONLY nor O_RDWR was specified.
EINVAL O_CREAT was specified in flags and the final component ("basename") of the new file's pathname is invalid (e.g., it contains characters not permitted by the underlying filesystem).
EISDIR pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or O_RDWR is set).
EISDIR pathname refers to an existing directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version does not provide the O_TMPFILE functionality.
ELOOP Too many symbolic links were encountered in resolving pathname.
ELOOP pathname was a symbolic link, and flags specified O_NOFOLLOW but not O_PATH.
EMFILE The per-process limit on the number of open file descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).
ENAMETOOLONG
pathname was too long.
ENFILE The system-wide limit on the total number of open files has been reached.
ENODEV pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel bug; in this situation ENXIO must be returned.)
ENOENT O_CREAT is not set and the named file does not exist. Or, a directory component in pathname does not exist or is a dangling symbolic link.
ENOENT pathname refers to a nonexistent directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version does not provide the O_TMPFILE functionality.
ENOMEM The named file is a FIFO, but memory for the FIFO buffer can't be allocated because the per-user hard limit on memory allocation for pipes has been reached and the caller is not privi‐
leged; see pipe(7).
ENOMEM Insufficient kernel memory was available.
ENOSPC pathname was to be created but the device containing pathname has no room for the new file.
ENOTDIR
A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was specified and pathname was not a directory.
ENXIO O_NONBLOCK | O_WRONLY is set, the named file is a FIFO, and no process has the FIFO open for reading.
ENXIO The file is a device special file and no corresponding device exists.
EOPNOTSUPP
The filesystem containing pathname does not support O_TMPFILE.
EOVERFLOW
pathname refers to a regular file that is too large to be opened. The usual scenario here is that an application compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64 tried to
open a file whose size exceeds (1<<31)-1 bytes; see also O_LARGEFILE above. This is the error specified by POSIX.1; in kernels before 2.6.24, Linux gave the error EFBIG for this case.
EPERM The O_NOATIME flag was specified, but the effective user ID of the caller did not match the owner of the file and the caller was not privileged.
EPERM The operation was prevented by a file seal; see fcntl(2).
EROFS pathname refers to a file on a read-only filesystem and write access was requested.
ETXTBSY
pathname refers to an executable image which is currently being executed and write access was requested.
EWOULDBLOCK
The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see fcntl(2)).
The following additional errors can occur for openat():
EBADF dirfd is not a valid file descriptor.
ENOTDIR
pathname is a relative pathname and dirfd is a file descriptor referring to a file other than a directory.
VERSIONS
openat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.
CONFORMING TO
open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
openat(): POSIX.1-2008.
The O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-specific. One must define _GNU_SOURCE to obtain their definitions.
The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not specified in POSIX.1-2001, but are specified in POSIX.1-2008. Since glibc 2.12, one can obtain their definitions by defining either
_POSIX_C_SOURCE with a value greater than or equal to 200809L or _XOPEN_SOURCE with a value greater than or equal to 700. In glibc 2.11 and earlier, one obtains the definitions by defining
_GNU_SOURCE.
As noted in feature_test_macros(7), feature test macros such as _POSIX_C_SOURCE, _XOPEN_SOURCE, and _GNU_SOURCE must be defined before including any header files.
NOTES
Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily have the intention to read or write. This is typically used to open devices in order to get a file
descriptor for use with ioctl(2).
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations. On many systems the file is actually truncated.
Note that open() can open device special files, but creat() cannot create them; use mknod(2) instead.
If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of last status change, and time of last modification; see stat(2)) are set to
the current time, and so are the st_ctime and st_mtime fields of the parent directory. Otherwise, if the file is modified because of the O_TRUNC flag, its st_ctime and st_mtime fields are
set to the current time.
The files in the /proc/[pid]/fd directory show the open file descriptors of the process with the PID pid. The files in the /proc/[pid]/fdinfo directory show even more information about these
files descriptors. See proc(5) for further details of both of these directories.
Open file descriptions
The term open file description is the one used by POSIX to refer to the entries in the system-wide table of open files. In other contexts, this object is variously also called an "open file
object", a "file handle", an "open file table entry", or—in kernel-developer parlance—a struct file.
When a file descriptor is duplicated (using dup(2) or similar), the duplicate refers to the same open file description as the original file descriptor, and the two file descriptors conse‐
quently share the file offset and file status flags. Such sharing can also occur between processes: a child process created via fork(2) inherits duplicates of its parent's file descriptors,
and those duplicates refer to the same open file descriptions.
Each open() of a file creates a new open file description; thus, there may be multiple open file descriptions corresponding to a file inode.
On Linux, one can use the kcmp(2) KCMP_FILE operation to test whether two file descriptors (in the same process or in two different processes) refer to the same open file description.
Synchronized I/O
The POSIX.1-2008 "synchronized I/O" option specifies different variants of synchronized I/O, and specifies the open() flags O_SYNC, O_DSYNC, and O_RSYNC for controlling the behavior. Regard‐
less of whether an implementation supports this option, it must at least support the use of O_SYNC for regular files.
Linux implements O_SYNC and O_DSYNC, but not O_RSYNC. (Somewhat incorrectly, glibc defines O_RSYNC to have the same value as O_SYNC.)
O_SYNC provides synchronized I/O file integrity completion, meaning write operations will flush data and all associated metadata to the underlying hardware. O_DSYNC provides synchronized I/O
data integrity completion, meaning write operations will flush data to the underlying hardware, but will only flush metadata updates that are required to allow a subsequent read operation to
complete successfully. Data integrity completion can reduce the number of disk operations that are required for applications that don't need the guarantees of file integrity completion.
To understand the difference between the two types of completion, consider two pieces of file metadata: the file last modification timestamp (st_mtime) and the file length. All write opera‐
tions will update the last file modification timestamp, but only writes that add data to the end of the file will change the file length. The last modification timestamp is not needed to
ensure that a read completes successfully, but the file length is. Thus, O_DSYNC would only guarantee to flush updates to the file length metadata (whereas O_SYNC would also always flush the
last modification timestamp metadata).
Before Linux 2.6.33, Linux implemented only the O_SYNC flag for open(). However, when that flag was specified, most filesystems actually provided the equivalent of synchronized I/O data
integrity completion (i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).
Since Linux 2.6.33, proper O_SYNC support is provided. However, to ensure backward binary compatibility, O_DSYNC was defined with the same value as the historical O_SYNC, and O_SYNC was
defined as a new (two-bit) flag value that includes the O_DSYNC flag value. This ensures that applications compiled against new headers get at least O_DSYNC semantics on pre-2.6.33 kernels.
C library/kernel differences
Since version 2.26, the glibc wrapper function for open() employs the openat() system call, rather than the kernel's open() system call. For certain architectures, this is also true in glibc
versions before 2.26.
NFS
There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC and O_NDELAY.
On NFS filesystems with UID mapping enabled, open() may return a file descriptor but, for example, read(2) requests are denied with EACCES. This is because the client performs open() by
checking the permissions, but UID mapping is performed by the server upon read and write requests.
FIFOs
Opening the read or write end of a FIFO blocks until the other end is also opened (by another process or thread). See fifo(7) for further details.
File access mode
Unlike the other values that can be specified in flags, the access mode values O_RDONLY, O_WRONLY, and O_RDWR do not specify individual bits. Rather, they define the low order two bits of
flags, and are defined respectively as 0, 1, and 2. In other words, the combination O_RDONLY | O_WRONLY is a logical error, and certainly does not have the same meaning as O_RDWR.
Linux reserves the special, nonstandard access mode 3 (binary 11) in flags to mean: check for read and write permission on the file and return a file descriptor that can't be used for reading
or writing. This nonstandard access mode is used by some Linux drivers to return a file descriptor that is to be used only for device-specific ioctl(2) operations.
Rationale for openat() and other directory file descriptor APIs
openat() and the other system calls and library functions that take a directory file descriptor argument (i.e., execveat(2), faccessat(2), fanotify_mark(2), fchmodat(2), fchownat(2),
fstatat(2), futimesat(2), linkat(2), mkdirat(2), mknodat(2), name_to_handle_at(2), readlinkat(2), renameat(2), statx(2), symlinkat(2), unlinkat(2), utimensat(2), mkfifoat(3), and scandi‐
rat(3)) address two problems with the older interfaces that preceded them. Here, the explanation is in terms of the openat() call, but the rationale is analogous for the other interfaces.
First, openat() allows an application to avoid race conditions that could occur when using open() to open files in directories other than the current working directory. These race conditions
result from the fact that some component of the directory prefix given to open() could be changed in parallel with the call to open(). Suppose, for example, that we wish to create the file
dir1/dir2/xxx.dep if the file dir1/dir2/xxx exists. The problem is that between the existence check and the file-creation step, dir1 or dir2 (which might be symbolic links) could be modified
to point to a different location. Such races can be avoided by opening a file descriptor for the target directory, and then specifying that file descriptor as the dirfd argument of (say)
fstatat(2) and openat(). The use of the dirfd file descriptor also has other benefits:
* the file descriptor is a stable reference to the directory, even if the directory is renamed; and
* the open file descriptor prevents the underlying filesystem from being dismounted, just as when a process has a current working directory on a filesystem.
Second, openat() allows the implementation of a per-thread "current working directory", via file descriptor(s) maintained by the application. (This functionality can also be obtained by
tricks based on the use of /proc/self/fd/dirfd, but less efficiently.)
O_DIRECT
The O_DIRECT flag may impose alignment restrictions on the length and address of user-space buffers and the file offset of I/Os. In Linux alignment restrictions vary by filesystem and kernel
version and might be absent entirely. However there is currently no filesystem-independent interface for an application to discover these restrictions for a given file or filesystem. Some
filesystems provide their own interfaces for doing so, for example the XFS_IOC_DIOINFO operation in xfsctl(3).
Under Linux 2.4, transfer sizes, and the alignment of the user buffer and the file offset must all be multiples of the logical block size of the filesystem. Since Linux 2.6.0, alignment to
the logical block size of the underlying storage (typically 512 bytes) suffices. The logical block size can be determined using the ioctl(2) BLKSSZGET operation or from the shell using the
command:
blockdev --getss
O_DIRECT I/Os should never be run concurrently with the fork(2) system call, if the memory buffer is a private mapping (i.e., any mapping created with the mmap(2) MAP_PRIVATE flag; this
includes memory allocated on the heap and statically allocated buffers). Any such I/Os, whether submitted via an asynchronous I/O interface or from another thread in the process, should be
completed before fork(2) is called. Failure to do so can result in data corruption and undefined behavior in parent and child processes. This restriction does not apply when the memory buf‐
fer for the O_DIRECT I/Os was created using shmat(2) or mmap(2) with the MAP_SHARED flag. Nor does this restriction apply when the memory buffer has been advised as MADV_DONTFORK with mad‐
vise(2), ensuring that it will not be available to the child after fork(2).
The O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar to those of Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and sizes.
FreeBSD 4.x introduced a flag of the same name, but without alignment restrictions.
O_DIRECT support was added under Linux in kernel version 2.4.10. Older Linux kernels simply ignore this flag. Some filesystems may not implement the flag, in which case open() fails with
the error EINVAL if it is used.
Applications should avoid mixing O_DIRECT and normal I/O to the same file, and especially to overlapping byte regions in the same file. Even when the filesystem correctly handles the
coherency issues in this situation, overall I/O throughput is likely to be slower than using either mode alone. Likewise, applications should avoid mixing mmap(2) of files with direct I/O to
the same files.
The behavior of O_DIRECT with NFS will differ from local filesystems. Older kernels, or kernels configured in certain ways, may not support this combination. The NFS protocol does not sup‐
port passing the flag to the server, so O_DIRECT I/O will bypass the page cache only on the client; the server may still cache the I/O. The client asks the server to make the I/O synchronous
to preserve the synchronous semantics of O_DIRECT. Some servers will perform poorly under these circumstances, especially if the I/O size is small. Some servers may also be configured to
lie to clients about the I/O having reached stable storage; this will avoid the performance penalty at some risk to data integrity in the event of server power failure. The Linux NFS client
places no alignment restrictions on O_DIRECT I/O.
In summary, O_DIRECT is a potentially powerful tool that should be used with caution. It is recommended that applications treat use of O_DIRECT as a performance option which is disabled by
default.
"The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and was probably designed by a deranged monkey on some serious mind-controlling sub‐
stances."—Linus