open

title: open(2)

summary: open and possibly create a file

text:
# open(2)

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

## SYNOPSIS

```
#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);
```

```
/* 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)):

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 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-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 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 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 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 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 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.

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.

### 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).

## RETURN VALUE
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.

## 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).)

EACCES

Where O_CREAT is specified, the protected_fifos or protected_regular sysctl is enabled, the file already exists and is a FIFO or regular file, the owner of the file is neither the current user nor the owner of the containing directory, and the containing directory is both world- or group-writable and sticky. For details, see the descriptions of /proc/sys/fs/protected_fifos and /proc/sys/fs/protected_regular in proc(5).

EBUSY

O_EXCL was specified in flags and pathname refers to a block device that is in use by the system (e.g., it is mounted).

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).

EINVAL

The final component ("basename") of 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.

ENOENT

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 privileged; 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.

ENXIO

The file is a UNIX domain socket.

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.

ETXTBSY

pathname refers to a file that is currently in use as a swap file, and the O_TRUNC flag was specified.

ETXTBSY

pathname refers to a file that is currently being read by the kernel (e.g., for module/firmware loading), 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.

openat2(2) is Linux-specific.

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 is sometimes used in cases where one wants to open but does not necessarily have the intention to read or write. For example, this may be used to open a device 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 file descriptors. See proc(5) for further details of both of these directories.

The Linux header file <asm/fcntl.h> doesn't define O_ASYNC; the (BSD-derived) FASYNC synonym is defined instead.

### 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 consequently 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. Regardless 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_RSYNC is defined in the Linux header file <asm/fcntl.h> on HP PA-RISC, but it is not used.)

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 operations 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), fspick(2), fstatat(2), futimesat(2), linkat(2), mkdirat(2), move_mount(2), mknodat(2), name_to_handle_at(2), open_tree(2), openat2(2), readlinkat(2), renameat(2), statx(2), symlinkat(2), unlinkat(2), utimensat(2), mkfifoat(3), and scandirat(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.)

The dirfd argument for these APIs can be obtained by using open() or openat() to open a directory (with either the O_RDONLY or the O_PATH flag). Alternatively, such a file descriptor can be obtained by applying dirfd(3) to a directory stream created using opendir(3).

When these APIs are given a dirfd argument of AT_FDCWD or the specified pathname is absolute, then they handle their pathname argument in the same way as the corresponding conventional APIs. However, in this case, several of the APIs have a flags argument that provides access to functionality that is not available with the corresponding conventional APIs.

### 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, 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 buffer 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 madvise(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 support 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.

## BUGS
Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling open(); use fcntl(2) to enable this flag.

One must check for two different error codes, EISDIR and ENOENT, when trying to determine whether the kernel supports O_TMPFILE functionality.

When both O_CREAT and O_DIRECTORY are specified in flags and the file specified by pathname does not exist, open() will create a regular file (i.e., O_DIRECTORY is ignored).

## SEE ALSO
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)


package: man-pages

title: open_by_handle_at(2)

summary: obtain handle

text:
# open_by_handle_at(2)

## NAME
name_to_handle_at, open_by_handle_at - obtain handle for a pathname and open file via a handle

## SYNOPSIS

```
#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <fcntl.h>
```

```
int name_to_handle_at(int dirfd, const char *pathname,
                      struct file_handle *handle,
                      int *mount_id, int flags);
int open_by_handle_at(int mount_fd, struct file_handle *handle,
                      int flags);
```

## DESCRIPTION
The name_to_handle_at() and open_by_handle_at() system calls split the functionality of openat(2) into two parts: name_to_handle_at() returns an opaque handle that corresponds to a specified file; open_by_handle_at() opens the file corresponding to a handle returned by a previous call to name_to_handle_at() and returns an open file descriptor.

### name_to_handle_at()
The name_to_handle_at() system call returns a file handle and a mount ID corresponding to the file specified by the dirfd and pathname arguments. The file handle is returned via the argument handle, which is a pointer to a structure of the following form:

```
struct file_handle {

    unsigned int  handle_bytes;   /* Size of f_handle [in, out] */

    int           handle_type;    /* Handle type [out] */

    unsigned char f_handle[0];    /* File identifier (sized by

                                     caller) [out] */
};
```

It is the caller's responsibility to allocate the structure with a size large enough to hold the handle returned in f_handle. Before the call, the handle_bytes field should be initialized to contain the allocated size for f_handle. (The constant MAX_HANDLE_SZ, defined in <fcntl.h>, specifies the maximum expected size for a file handle. It is not a guaranteed upper limit as future filesystems may require more space.) Upon successful return, the handle_bytes field is updated to contain the number of bytes actually written to f_handle.

The caller can discover the required size for the file_handle structure by making a call in which handle->handle_bytes is zero; in this case, the call fails with the error EOVERFLOW and handle->handle_bytes is set to indicate the required size; the caller can then use this information to allocate a structure of the correct size (see EXAMPLES below). Some care is needed here as EOVERFLOW can also indicate that no file handle is available for this particular name in a filesystem which does normally support file-handle lookup. This case can be detected when the EOVERFLOW error is returned without handle_bytes being increased.

Other than the use of the handle_bytes field, the caller should treat the file_handle structure as an opaque data type: the handle_type and f_handle fields are needed only by a subsequent call to open_by_handle_at().

The flags argument is a bit mask constructed by ORing together zero or more of AT_EMPTY_PATH and AT_SYMLINK_FOLLOW, described below.

Together, the pathname and dirfd arguments identify the file for which a handle is to be obtained. There are four distinct cases:

- If pathname is a nonempty string containing an absolute pathname, then a handle is returned for the file referred to by that pathname. In this case, dirfd is ignored.

- If pathname is a nonempty string containing a relative pathname and dirfd has the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the caller, and a handle is returned for the file to which it refers.

- If pathname is a nonempty string containing a relative pathname and dirfd is a file descriptor referring to a directory, then pathname is interpreted relative to the directory referred to by dirfd, and a handle is returned for the file to which it refers. (See openat(2) for an explanation of why "directory file descriptors" are useful.)

- If pathname is an empty string and flags specifies the value AT_EMPTY_PATH, then dirfd can be an open file descriptor referring to any type of file, or AT_FDCWD, meaning the current working directory, and a handle is returned for the file to which it refers.

The mount_id argument returns an identifier for the filesystem mount that corresponds to pathname. This corresponds to the first field in one of the records in /proc/self/mountinfo. Opening the pathname in the fifth field of that record yields a file descriptor for the mount point; that file descriptor can be used in a subsequent call to open_by_handle_at(). mount_id is returned both for a successful call and for a call that results in the error EOVERFLOW.

By default, name_to_handle_at() does not dereference pathname if it is a symbolic link, and thus returns a handle for the link itself. If AT_SYMLINK_FOLLOW is specified in flags, pathname is dereferenced if it is a symbolic link (so that the call returns a handle for the file referred to by the link).

name_to_handle_at() does not trigger a mount when the final component of the pathname is an automount point. When a filesystem supports both file handles and automount points, a name_to_handle_at() call on an automount point will return with error EOVERFLOW without having increased handle_bytes. This can happen since Linux 4.13 with NFS when accessing a directory which is on a separate filesystem on the server. In this case, the automount can be triggered by adding a "/" to the end of the pathname.

### open_by_handle_at()
The open_by_handle_at() system call opens the file referred to by handle, a file handle returned by a previous call to name_to_handle_at().

The mount_fd argument is a file descriptor for any object (file, directory, etc.) in the mounted filesystem with respect to which handle should be interpreted. The special value AT_FDCWD can be specified, meaning the current working directory of the caller.

The flags argument is as for open(2). If handle refers to a symbolic link, the caller must specify the O_PATH flag, and the symbolic link is not dereferenced; the O_NOFOLLOW flag, if specified, is ignored.

The caller must have the CAP_DAC_READ_SEARCH capability to invoke open_by_handle_at().

## RETURN VALUE
On success, name_to_handle_at() returns 0, and open_by_handle_at() returns a file descriptor (a nonnegative integer).

In the event of an error, both system calls return -1 and set errno to indicate the error.

## ERRORS
name_to_handle_at() and open_by_handle_at() can fail for the same errors as openat(2). In addition, they can fail with the errors noted below.

name_to_handle_at() can fail with the following errors:

EFAULT

pathname, mount_id, or handle points outside your accessible address space.

EINVAL

flags includes an invalid bit value.

EINVAL

handle->handle_bytes is greater than MAX_HANDLE_SZ.

ENOENT

pathname is an empty string, but AT_EMPTY_PATH was not specified in flags.

ENOTDIR

The file descriptor supplied in dirfd does not refer to a directory, and it is not the case that both flags includes AT_EMPTY_PATH and pathname is an empty string.

EOPNOTSUPP

The filesystem does not support decoding of a pathname to a file handle.

EOVERFLOW

The handle->handle_bytes value passed into the call was too small. When this error occurs, handle->handle_bytes is updated to indicate the required size for the handle.

open_by_handle_at() can fail with the following errors:

EBADF

mount_fd is not an open file descriptor.

EFAULT

handle points outside your accessible address space.

EINVAL

handle->handle_bytes is greater than MAX_HANDLE_SZ or is equal to zero.

ELOOP

handle refers to a symbolic link, but O_PATH was not specified in flags.

EPERM

The caller does not have the CAP_DAC_READ_SEARCH capability.

ESTALE

The specified handle is not valid. This error will occur if, for example, the file has been deleted.

## VERSIONS
These system calls first appeared in Linux 2.6.39. Library support is provided in glibc since version 2.14.

## CONFORMING TO
These system calls are nonstandard Linux extensions.

FreeBSD has a broadly similar pair of system calls in the form of getfh() and openfh().

## NOTES
A file handle can be generated in one process using name_to_handle_at() and later used in a different process that calls open_by_handle_at().

Some filesystem don't support the translation of pathnames to file handles, for example, /proc, /sys, and various network filesystems.

A file handle may become invalid ("stale") if a file is deleted, or for other filesystem-specific reasons. Invalid handles are notified by an ESTALE error from open_by_handle_at().

These system calls are designed for use by user-space file servers. For example, a user-space NFS server might generate a file handle and pass it to an NFS client. Later, when the client wants to open the file, it could pass the handle back to the server. This sort of functionality allows a user-space file server to operate in a stateless fashion with respect to the files it serves.

If pathname refers to a symbolic link and flags does not specify AT_SYMLINK_FOLLOW, then name_to_handle_at() returns a handle for the link (rather than the file to which it refers). The process receiving the handle can later perform operations on the symbolic link by converting the handle to a file descriptor using open_by_handle_at() with the O_PATH flag, and then passing the file descriptor as the dirfd argument in system calls such as readlinkat(2) and fchownat(2).

### Obtaining a persistent filesystem ID
The mount IDs in /proc/self/mountinfo can be reused as filesystems are unmounted and mounted. Therefore, the mount ID returned by name_to_handle_at() (in *mount_id) should not be treated as a persistent identifier for the corresponding mounted filesystem. However, an application can use the information in the mountinfo record that corresponds to the mount ID to derive a persistent identifier.

For example, one can use the device name in the fifth field of the mountinfo record to search for the corresponding device UUID via the symbolic links in /dev/disks/by-uuid. (A more comfortable way of obtaining the UUID is to use the libblkid(3) library.) That process can then be reversed, using the UUID to look up the device name, and then obtaining the corresponding mount point, in order to produce the mount_fd argument used by open_by_handle_at().

## EXAMPLES
The two programs below demonstrate the use of name_to_handle_at() and open_by_handle_at(). The first program (t_name_to_handle_at.c) uses name_to_handle_at() to obtain the file handle and mount ID for the file specified in its command-line argument; the handle and mount ID are written to standard output.

The second program (t_open_by_handle_at.c) reads a mount ID and file handle from standard input. The program then employs open_by_handle_at() to open the file using that handle. If an optional command-line argument is supplied, then the mount_fd argument for open_by_handle_at() is obtained by opening the directory named in that argument. Otherwise, mount_fd is obtained by scanning /proc/self/mountinfo to find a record whose mount ID matches the mount ID read from standard input, and the mount directory specified in that record is opened. (These programs do not deal with the fact that mount IDs are not persistent.)

The following shell session demonstrates the use of these two programs:

```
$ echo 'Can you please think about it?' > cecilia.txt
$ ./t_name_to_handle_at cecilia.txt > fh
$ ./t_open_by_handle_at < fh
open_by_handle_at: Operation not permitted
$ sudo ./t_open_by_handle_at < fh      # Need CAP_SYS_ADMIN
Read 31 bytes
$ rm cecilia.txt
```

Now we delete and (quickly) re-create the file so that it has the same content and (by chance) the same inode. Nevertheless, open_by_handle_at() recognizes that the original file referred to by the file handle no longer exists.

```
$ stat --printf="%i\n" cecilia.txt     # Display inode number
4072121
$ rm cecilia.txt
$ echo 'Can you please think about it?' > cecilia.txt
$ stat --printf="%i\n" cecilia.txt     # Check inode number
4072121
$ sudo ./t_open_by_handle_at < fh
open_by_handle_at: Stale NFS file handle
```

### Program source: t_name_to_handle_at.c

```
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \

                        } while (0)
int
main(int argc, char *argv[])
{

    struct file_handle *fhp;

    int mount_id, fhsize, flags, dirfd;

    char *pathname;

    if (argc != 2) {

        fprintf(stderr, "Usage: %s pathname\n", argv[0]);

        exit(EXIT_FAILURE);

    }

    pathname = argv[1];

    /* Allocate file_handle structure. */

    fhsize = sizeof(*fhp);

    fhp = malloc(fhsize);

    if (fhp == NULL)

        errExit("malloc");

    /* Make an initial call to name_to_handle_at() to discover

       the size required for file handle. */

    dirfd = AT_FDCWD;           /* For name_to_handle_at() calls */

    flags = 0;                  /* For name_to_handle_at() calls */

    fhp->handle_bytes = 0;

    if (name_to_handle_at(dirfd, pathname, fhp,

                &mount_id, flags) != -1 || errno != EOVERFLOW) {

        fprintf(stderr, "Unexpected result from name_to_handle_at()\n");

        exit(EXIT_FAILURE);

    }

    /* Reallocate file_handle structure with correct size. */

    fhsize = sizeof(*fhp) + fhp->handle_bytes;

    fhp = realloc(fhp, fhsize);         /* Copies fhp->handle_bytes */

    if (fhp == NULL)

        errExit("realloc");

    /* Get file handle from pathname supplied on command line. */

    if (name_to_handle_at(dirfd, pathname, fhp, &mount_id, flags) == -1)

        errExit("name_to_handle_at");

    /* Write mount ID, file handle size, and file handle to stdout,

       for later reuse by t_open_by_handle_at.c. */

    printf("%d\n", mount_id);

    printf("%u %d   ", fhp->handle_bytes, fhp->handle_type);

    for (int j = 0; j < fhp->handle_bytes; j++)

        printf(" %02x", fhp->f_handle[j]);

    printf("\n");

    exit(EXIT_SUCCESS);
}
```

### Program source: t_open_by_handle_at.c

```
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \

                        } while (0)
/* Scan /proc/self/mountinfo to find the line whose mount ID matches

   'mount_id'. (An easier way to do this is to install and use the

   'libmount' library provided by the 'util-linux' project.)

   Open the corresponding mount path and return the resulting file

   descriptor. */
static int
open_mount_path_by_id(int mount_id)
{

    char *linep;

    size_t lsize;

    char mount_path[PATH_MAX];

    int mi_mount_id, found;

    ssize_t nread;

    FILE *fp;

    fp = fopen("/proc/self/mountinfo", "r");

    if (fp == NULL)

        errExit("fopen");

    found = 0;

    linep = NULL;

    while (!found) {

        nread = getline(&linep, &lsize, fp);

        if (nread == -1)

            break;

        nread = sscanf(linep, "%d %*d %*s %*s %s",

                       &mi_mount_id, mount_path);

        if (nread != 2) {

            fprintf(stderr, "Bad sscanf()\n");

            exit(EXIT_FAILURE);

        }

        if (mi_mount_id == mount_id)

            found = 1;

    }

    free(linep);

    fclose(fp);

    if (!found) {

        fprintf(stderr, "Could not find mount point\n");

        exit(EXIT_FAILURE);

    }

    return open(mount_path, O_RDONLY);
}
int
main(int argc, char *argv[])
{

    struct file_handle *fhp;

    int mount_id, fd, mount_fd, handle_bytes;

    ssize_t nread;

    char buf[1000];
#define LINE_SIZE 100

    char line1[LINE_SIZE], line2[LINE_SIZE];

    char *nextp;

    if ((argc > 1 && strcmp(argv[1], "--help") == 0) || argc > 2) {

        fprintf(stderr, "Usage: %s [mount-path]\n", argv[0]);

        exit(EXIT_FAILURE);

    }

    /* Standard input contains mount ID and file handle information:

         Line 1: <mount_id>

         Line 2: <handle_bytes> <handle_type>   <bytes of handle in hex>

    */

    if ((fgets(line1, sizeof(line1), stdin) == NULL) ||

           (fgets(line2, sizeof(line2), stdin) == NULL)) {

        fprintf(stderr, "Missing mount_id / file handle\n");

        exit(EXIT_FAILURE);

    }

    mount_id = atoi(line1);

    handle_bytes = strtoul(line2, &nextp, 0);

    /* Given handle_bytes, we can now allocate file_handle structure. */

    fhp = malloc(sizeof(*fhp) + handle_bytes);

    if (fhp == NULL)

        errExit("malloc");

    fhp->handle_bytes = handle_bytes;

    fhp->handle_type = strtoul(nextp, &nextp, 0);

    for (int j = 0; j < fhp->handle_bytes; j++)

        fhp->f_handle[j] = strtoul(nextp, &nextp, 16);

    /* Obtain file descriptor for mount point, either by opening

       the pathname specified on the command line, or by scanning

       /proc/self/mounts to find a mount that matches the 'mount_id'

       that we received from stdin. */

    if (argc > 1)

        mount_fd = open(argv[1], O_RDONLY);

    else

        mount_fd = open_mount_path_by_id(mount_id);

    if (mount_fd == -1)

        errExit("opening mount fd");

    /* Open file using handle and mount point. */

    fd = open_by_handle_at(mount_fd, fhp, O_RDONLY);

    if (fd == -1)

        errExit("open_by_handle_at");

    /* Try reading a few bytes from the file. */

    nread = read(fd, buf, sizeof(buf));

    if (nread == -1)

        errExit("read");

    printf("Read %zd bytes\n", nread);

    exit(EXIT_SUCCESS);
}
```

## SEE ALSO
open(2), libblkid(3), blkid(8), findfs(8), mount(8)

The libblkid and libmount documentation in the latest util-linux release at https://www.kernel.org/pub/linux/utils/util-linux/


title: openat2(2)

text:
# openat2(2)

## NAME
openat2 - open and possibly create a file (extended)

## SYNOPSIS

```
#include <fcntl.h>          /* Definition of O_* and S_* constants */
#include <linux/openat2.h>  /* Definition of RESOLVE_* constants */
#include <sys/syscall.h>    /* Definition of SYS_* constants */
#include <unistd.h>
```

```
long syscall(SYS_openat2, int dirfd, const char *pathname,
             struct open_how *how, size_t size);
```

Note: glibc provides no wrapper for openat2(), necessitating the use of syscall(2).

## DESCRIPTION
The openat2() system call is an extension of openat(2) and provides a superset of its functionality.

The openat2() system call opens the file specified by pathname. If the specified file does not exist, it may optionally (if O_CREAT is specified in how.flags) be created.

As with openat(2), if pathname is a relative pathname, then it is interpreted relative to the directory referred to by the file descriptor dirfd (or the current working directory of the calling process, if dirfd is the special value AT_FDCWD). If pathname is an absolute pathname, then dirfd is ignored (unless how.resolve contains RESOLVE_IN_ROOT, in which case pathname is resolved relative to dirfd).

Rather than taking a single flags argument, an extensible structure (how) is passed to allow for future extensions. The size argument must be specified as sizeof(struct open_how).

### The open_how structure
The how argument specifies how pathname should be opened, and acts as a superset of the flags and mode arguments to openat(2). This argument is a pointer to a structure of the following form:

```
struct open_how {

    u64 flags;    /* O_* flags */

    u64 mode;     /* Mode for O_{CREAT,TMPFILE} */

    u64 resolve;  /* RESOLVE_* flags */

    /* ... */
};
```

Any future extensions to openat2() will be implemented as new fields appended to the above structure, with a zero value in a new field resulting in the kernel behaving as though that extension field was not present. Therefore, the caller must zero-fill this structure on initialization. (See the "Extensibility" section of the NOTES for more detail on why this is necessary.)

The fields of the open_how structure are as follows:

flags

This field specifies the file creation and file status flags to use when opening the file. All of the O_* flags defined for openat(2) are valid openat2() flag values.

Whereas openat(2) ignores unknown bits in its flags argument, openat2() returns an error if unknown or conflicting flags are specified in how.flags.

mode

This field specifies the mode for the new file, with identical semantics to the mode argument of openat(2).

Whereas openat(2) ignores bits other than those in the range 07777 in its mode argument, openat2() returns an error if how.mode contains bits other than 07777. Similarly, an error is returned if openat2() is called with a nonzero how.mode and how.flags does not contain O_CREAT or O_TMPFILE.

resolve

This is a bit-mask of flags that modify the way in which all components of pathname will be resolved. (See path_resolution(7) for background information.)

The primary use case for these flags is to allow trusted programs to restrict how untrusted paths (or paths inside untrusted directories) are resolved. The full list of resolve flags is as follows:

RESOLVE_BENEATH

Do not permit the path resolution to succeed if any component of the resolution is not a descendant of the directory indicated by dirfd. This causes absolute symbolic links (and absolute values of pathname) to be rejected.

Currently, this flag also disables magic-link resolution (see below). However, this may change in the future. Therefore, to ensure that magic links are not resolved, the caller should explicitly specify RESOLVE_NO_MAGICLINKS.

RESOLVE_IN_ROOT

Treat the directory referred to by dirfd as the root directory while resolving pathname. Absolute symbolic links are interpreted relative to dirfd. If a prefix component of pathname equates to dirfd, then an immediately following .. component likewise equates to dirfd (just as /.. is traditionally equivalent to /). If pathname is an absolute path, it is also interpreted relative to dirfd.

The effect of this flag is as though the calling process had used chroot(2) to (temporarily) modify its root directory (to the directory referred to by dirfd). However, unlike chroot(2) (which changes the filesystem root permanently for a process), RESOLVE_IN_ROOT allows a program to efficiently restrict path resolution on a per-open basis.

Currently, this flag also disables magic-link resolution. However, this may change in the future. Therefore, to ensure that magic links are not resolved, the caller should explicitly specify RESOLVE_NO_MAGICLINKS.

RESOLVE_NO_MAGICLINKS

Disallow all magic-link resolution during path resolution.

Magic links are symbolic link-like objects that are most notably found in proc(5); examples include /proc/[pid]/exe and /proc/[pid]/fd/*. (See symlink(7) for more details.)

Unknowingly opening magic links can be risky for some applications. Examples of such risks include the following:

- If the process opening a pathname is a controlling process that currently has no controlling terminal (see credentials(7)), then opening a magic link inside /proc/[pid]/fd that happens to refer to a terminal would cause the process to acquire a controlling terminal.

- In a containerized environment, a magic link inside /proc may refer to an object outside the container, and thus may provide a means to escape from the container.

Because of such risks, an application may prefer to disable magic link resolution using the RESOLVE_NO_MAGICLINKS flag.

If the trailing component (i.e., basename) of pathname is a magic link, how.resolve contains RESOLVE_NO_MAGICLINKS, and how.flags contains both O_PATH and O_NOFOLLOW, then an O_PATH file descriptor referencing the magic link will be returned.

RESOLVE_NO_SYMLINKS

Disallow resolution of symbolic links during path resolution. This option implies RESOLVE_NO_MAGICLINKS.

If the trailing component (i.e., basename) of pathname is a symbolic link, how.resolve contains RESOLVE_NO_SYMLINKS, and how.flags contains both O_PATH and O_NOFOLLOW, then an O_PATH file descriptor referencing the symbolic link will be returned.

Note that the effect of the RESOLVE_NO_SYMLINKS flag, which affects the treatment of symbolic links in all of the components of pathname, differs from the effect of the O_NOFOLLOW file creation flag (in how.flags), which affects the handling of symbolic links only in the final component of pathname.

Applications that employ the RESOLVE_NO_SYMLINKS flag are encouraged to make its use configurable (unless it is used for a specific security purpose), as symbolic links are very widely used by end-users. Setting this flag indiscriminately—i.e., for purposes not specifically related to security—for all uses of openat2() may result in spurious errors on previously functional systems. This may occur if, for example, a system pathname that is used by an application is modified (e.g., in a new distribution release) so that a pathname component (now) contains a symbolic link.

RESOLVE_NO_XDEV

Disallow traversal of mount points during path resolution (including all bind mounts). Consequently, pathname must either be on the same mount as the directory referred to by dirfd, or on the same mount as the current working directory if dirfd is specified as AT_FDCWD.

Applications that employ the RESOLVE_NO_XDEV flag are encouraged to make its use configurable (unless it is used for a specific security purpose), as bind mounts are widely used by end-users. Setting this flag indiscriminately—i.e., for purposes not specifically related to security—for all uses of openat2() may result in spurious errors on previously functional systems. This may occur if, for example, a system pathname that is used by an application is modified (e.g., in a new distribution release) so that a pathname component (now) contains a bind mount.

RESOLVE_CACHED

Make the open operation fail unless all path components are already present in the kernel's lookup cache. If any kind of revalidation or I/O is needed to satisfy the lookup, openat2() fails with the error EAGAIN . This is useful in providing a fast-path open that can be performed without resorting to thread offload, or other mechanisms that an application might use to offload slower operations.

If any bits other than those listed above are set in how.resolve, an error is returned.

## RETURN VALUE
On success, a new file descriptor is returned. On error, -1 is returned, and errno is set to indicate the error.

## ERRORS
The set of errors returned by openat2() includes all of the errors returned by openat(2), as well as the following additional errors:

E2BIG

An extension that this kernel does not support was specified in how. (See the "Extensibility" section of NOTES for more detail on how extensions are handled.)

EAGAIN

how.resolve contains either RESOLVE_IN_ROOT or RESOLVE_BENEATH, and the kernel could not ensure that a ".." component didn't escape (due to a race condition or potential attack). The caller may choose to retry the openat2() call.

EAGAIN

RESOLVE_CACHED was set, and the open operation cannot be performed using only cached information. The caller should retry without RESOLVE_CACHED set in how.resolve .

EINVAL

An unknown flag or invalid value was specified in how.

EINVAL

mode is nonzero, but how.flags does not contain O_CREAT or O_TMPFILE.

EINVAL

size was smaller than any known version of struct open_how.

ELOOP

how.resolve contains RESOLVE_NO_SYMLINKS, and one of the path components was a symbolic link (or magic link).

ELOOP

how.resolve contains RESOLVE_NO_MAGICLINKS, and one of the path components was a magic link.

EXDEV

how.resolve contains either RESOLVE_IN_ROOT or RESOLVE_BENEATH, and an escape from the root during path resolution was detected.

EXDEV

how.resolve contains RESOLVE_NO_XDEV, and a path component crosses a mount point.

## VERSIONS
openat2() first appeared in Linux 5.6.

## CONFORMING TO
This system call is Linux-specific.

The semantics of RESOLVE_BENEATH were modeled after FreeBSD's O_BENEATH.

## NOTES

### Extensibility
In order to allow for future extensibility, openat2() requires the user-space application to specify the size of the open_how structure that it is passing. By providing this information, it is possible for openat2() to provide both forwards- and backwards-compatibility, with size acting as an implicit version number. (Because new extension fields will always be appended, the structure size will always increase.) This extensibility design is very similar to other system calls such as sched_setattr(2), perf_event_open(2), and clone3(2).

If we let usize be the size of the structure as specified by the user-space application, and ksize be the size of the structure which the kernel supports, then there are three cases to consider:

- If ksize equals usize, then there is no version mismatch and how can be used verbatim.

- If ksize is larger than usize, then there are some extension fields that the kernel supports which the user-space application is unaware of. Because a zero value in any added extension field signifies a no-op, the kernel treats all of the extension fields not provided by the user-space application as having zero values. This provides backwards-compatibility.

- If ksize is smaller than usize, then there are some extension fields which the user-space application is aware of but which the kernel does not support. Because any extension field must have its zero values signify a no-op, the kernel can safely ignore the unsupported extension fields if they are all-zero. If any unsupported extension fields are nonzero, then -1 is returned and errno is set to E2BIG. This provides forwards-compatibility.

Because the definition of struct open_how may change in the future (with new fields being added when system headers are updated), user-space applications should zero-fill struct open_how to ensure that recompiling the program with new headers will not result in spurious errors at runtime. The simplest way is to use a designated initializer:

```
struct open_how how = { .flags = O_RDWR,

                        .resolve = RESOLVE_IN_ROOT };
```

or explicitly using memset(3) or similar:

```
struct open_how how;
memset(&how, 0, sizeof(how));
how.flags = O_RDWR;
how.resolve = RESOLVE_IN_ROOT;
```

A user-space application that wishes to determine which extensions the running kernel supports can do so by conducting a binary search on size with a structure which has every byte nonzero (to find the largest value which doesn't produce an error of E2BIG).

## SEE ALSO
openat(2), path_resolution(7), symlink(7)


outb

title: outb(2)

summary: port I/O

text:
# outb(2)

## NAME
outb, outw, outl, outsb, outsw, outsl, inb, inw, inl, insb, insw, insl, outb_p, outw_p, outl_p, inb_p, inw_p, inl_p - port I/O

## SYNOPSIS

```
#include <sys/io.h>
```

```
unsigned char inb(unsigned short port);
unsigned char inb_p(unsigned short port);
unsigned short inw(unsigned short port);
unsigned short inw_p(unsigned short port);
unsigned int inl(unsigned short port);
unsigned int inl_p(unsigned short port);
```

```
void outb(unsigned char value, unsigned short port);
void outb_p(unsigned char value, unsigned short port);
void outw(unsigned short value, unsigned short port);
void outw_p(unsigned short value, unsigned short port);
void outl(unsigned int value, unsigned short port);
void outl_p(unsigned int value, unsigned short port);
```

```
void insb(unsigned short port, void *addr,
           unsigned long count);
void insw(unsigned short port, void *addr,
           unsigned long count);
void insl(unsigned short port, void *addr,
           unsigned long count);
void outsb(unsigned short port, const void *addr,
           unsigned long count);
void outsw(unsigned short port, const void *addr,
           unsigned long count);
void outsl(unsigned short port, const void *addr,
           unsigned long count);
```

## DESCRIPTION
This family of functions is used to do low-level port input and output. The out* functions do port output, the in* functions do port input; the b-suffix functions are byte-width and the w-suffix functions word-width; the _p-suffix functions pause until the I/O completes.

They are primarily designed for internal kernel use, but can be used from user space.

You must compile with -O or -O2 or similar. The functions are defined as inline macros, and will not be substituted in without optimization enabled, causing unresolved references at link time.

You use ioperm(2) or alternatively iopl(2) to tell the kernel to allow the user space application to access the I/O ports in question. Failure to do this will cause the application to receive a segmentation fault.

## CONFORMING TO
outb() and friends are hardware-specific. The value argument is passed first and the port argument is passed second, which is the opposite order from most DOS implementations.

## SEE ALSO
ioperm(2), iopl(2)


pause

title: pause(2)

summary: wait for signal

text:
# pause(2)

## NAME
pause - wait for signal

## SYNOPSIS

```
#include <unistd.h>
```

```
int pause(void);
```

## DESCRIPTION
pause() causes the calling process (or thread) to sleep until a signal is delivered that either terminates the process or causes the invocation of a signal-catching function.

## RETURN VALUE
pause() returns only when a signal was caught and the signal-catching function returned. In this case, pause() returns -1, and errno is set to EINTR.

## ERRORS
EINTR

a signal was caught and the signal-catching function returned.

## CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

## SEE ALSO
kill(2), select(2), signal(2), sigsuspend(2)


title: pciconfig_read(2)

summary: pci device information handling

text:
# pciconfig_read(2)

## NAME
pciconfig_read, pciconfig_write, pciconfig_iobase - pci device information handling

## SYNOPSIS

```
#include <pci.h>
```

```
int pciconfig_read(unsigned long bus, unsigned long dfn,
                   unsigned long off, unsigned long len,
                   unsigned char *buf);
int pciconfig_write(unsigned long bus, unsigned long dfn,
                   unsigned long off, unsigned long len,
                   unsigned char *buf);
int pciconfig_iobase(int which, unsigned long bus,
                   unsigned long devfn);
```

## DESCRIPTION
Most of the interaction with PCI devices is already handled by the kernel PCI layer, and thus these calls should not normally need to be accessed from user space.

pciconfig_read()

Reads to buf from device dev at offset off value.

pciconfig_write()

Writes from buf to device dev at offset off value.

pciconfig_iobase()

You pass it a bus/devfn pair and get a physical address for either the memory offset (for things like prep, this is 0xc0000000), the IO base for PIO cycles, or the ISA holes if any.

## RETURN VALUE
pciconfig_read()

On success, zero is returned. On error, -1 is returned and errno is set to indicate the error.

pciconfig_write()

On success, zero is returned. On error, -1 is returned and errno is set to indicate the error.

pciconfig_iobase()

Returns information on locations of various I/O regions in physical memory according to the which value. Values for which are: IOBASE_BRIDGE_NUMBER, IOBASE_MEMORY, IOBASE_IO, IOBASE_ISA_IO, IOBASE_ISA_MEM.

## ERRORS
EINVAL

len value is invalid. This does not apply to pciconfig_iobase().

EIO

I/O error.

ENODEV

For pciconfig_iobase(), "hose" value is NULL. For the other calls, could not find a slot.

ENOSYS

The system has not implemented these calls (CONFIG_PCI not defined).

EOPNOTSUPP

This return value is valid only for pciconfig_iobase(). It is returned if the value for which is invalid.

EPERM

User does not have the CAP_SYS_ADMIN capability. This does not apply to pciconfig_iobase().

## CONFORMING TO
These calls are Linux-specific, available since Linux 2.0.26/2.1.11.

## SEE ALSO
capabilities(7)