open, openat, creat - open and possibly create a file

#include <fcntl.h>
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, ...
 /* mode_t mode */ );
/* Documented separately, in 
openat2(2):
 */
int openat2(int dirfd, const char *pathname,
 const struct open_how *how, size_t size);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _ATFILE_SOURCE

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 an index to an entry in the process's table of open file descriptors. The file descriptor 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 ORed 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 to be applied when a new file is created. If neither O_CREAT nor O_TMPFILE is specified in flags, then mode is ignored (and can thus be specified as 0, or simply omitted). The mode argument must be supplied if O_CREAT or O_TMPFILE is specified in flags; if it is not supplied, some arbitrary bytes from the stack will be applied as the file mode.

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 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 Linux 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 the trailing component (i.e., basename) of 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 in Linux 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 I/O operations on the file descriptor which is returned will cause the calling process to wait.

Note that the setting of this flag has no effect on the operation of poll(2), select(2), epoll(7), and similar, since those interfaces merely inform the caller about whether a file descriptor is "ready", meaning that an I/O operation performed on the file descriptor with the O_NONBLOCK flag clear would not block.

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 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 filesystem, 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'... */
linkat(fd, "", AT_FDCWD, "/path/for/file", AT_EMPTY_PATH);
/* If the caller doesn't have the CAP_DAC_READ_SEARCH
   capability (needed to use AT_EMPTY_PATH with linkat(2)),
   and there is a proc(5) filesystem mounted, then the
   linkat(2) call above can be replaced with:
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 tmpfs 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.

The dirfd argument is used in conjunction with the pathname argument as follows:

• If the pathname given in pathname is absolute, then dirfd is ignored.

• If the pathname given in 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 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). In this case, dirfd must be a directory that was opened for reading (O_RDONLY) or using the O_PATH flag.

If the pathname given in pathname is relative, and dirfd is not a valid file descriptor, an error (EBADF) results. (Specifying an invalid file descriptor number in dirfd can be used as a means to ensure that pathname is absolute.)

openat2(2)

The openat2(2) system call is an extension of openat(), and provides a superset of the features of openat(). It is documented separately, in openat2(2).

On success, open(), openat(), and creat() return the new file descriptor (a nonnegative integer). On error, -1 is returned and errno is set to indicate the error.

chmod(2), chown(2), close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mmap(2), mount(2), open_by_handle_at(2), openat2(2), read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3), acl(5), fifo(7), inode(7), path_resolution(7), symlink(7)