Invoking statx():¶

To access a file's status, no permissions are required on the file itself, but in the case of statx() with a pathname, execute (search) permission is required on all of the directories in pathname that lead to the file.

statx() uses pathname, dirfd, and flags to identify the target file in one of the following ways:

An absolute pathname

If pathname begins with a slash, then it is an absolute pathname that identifies the target file. In this case, dirfd is ignored.

A relative pathname

If pathname is a string that begins with a character other than a slash and dirfd is AT_FDCWD, then pathname is a relative pathname that is interpreted relative to the process's current working directory.

A directory-relative pathname

If pathname is a string that begins with a character other than a slash and dirfd is a file descriptor that refers to a directory, then pathname is a relative pathname that is interpreted relative to the directory referred to by dirfd. (See openat(2) for an explanation of why this is useful.)

By file descriptor

If pathname is an empty string and the AT_EMPTY_PATH flag is specified in flags (see below), then the target file is the one referred to by the file descriptor dirfd.

flags can be used to influence a pathname-based lookup. A value for flags is constructed by ORing together zero or more of the following constants:

AT_EMPTY_PATH

If pathname is an empty string, operate on the file referred to by dirfd (which may have been obtained using the open(2) O_PATH flag). In this case, dirfd can refer to any type of file, not just a directory.

If dirfd is AT_FDCWD, the call operates on the current working directory.

AT_NO_AUTOMOUNT

Don't automount the terminal ("basename") component of pathname if it is a directory that is an automount point. This allows the caller to gather attributes of an automount point (rather than the location it would mount). This flag has no effect if the mount point has already been mounted over.

The AT_NO_AUTOMOUNT flag can be used in tools that scan directories to prevent mass-automounting of a directory of automount points.

All of stat(2), lstat(2), and fstatat(2) act as though AT_NO_AUTOMOUNT was set.

AT_SYMLINK_NOFOLLOW

If pathname is a symbolic link, do not dereference it: instead return information about the link itself, like lstat(2).

flags can also be used to control what sort of synchronization the kernel will do when querying a file on a remote filesystem. This is done by ORing in one of the following values:

AT_STATX_SYNC_AS_STAT

Do whatever stat(2) does. This is the default and is very much filesystem-specific.

AT_STATX_FORCE_SYNC

Force the attributes to be synchronized with the server. This may require that a network filesystem perform a data writeback to get the timestamps correct.

AT_STATX_DONT_SYNC

Don't synchronize anything, but rather just take whatever the system has cached if possible. This may mean that the information returned is approximate, but, on a network filesystem, it may not involve a round trip to the server - even if no lease is held.

The mask argument to statx() is used to tell the kernel which fields the caller is interested in. mask is an ORed combination of the following constants:

Note that, in general, the kernel does not reject values in mask other than the above. (For an exception, see EINVAL in errors.) Instead, it simply informs the caller which values are supported by this kernel and filesystem via the statx.stx_mask field. Therefore, do not simply set mask to UINT_MAX (all bits set), as one or more bits may, in the future, be used to specify an extension to the buffer.

The returned information¶

The status information for the target file is returned in the statx structure pointed to by statxbuf. Included in this is stx_mask which indicates what other information has been returned. stx_mask has the same format as the mask argument and bits are set in it to indicate which fields have been filled in.

It should be noted that the kernel may return fields that weren't requested and may fail to return fields that were requested, depending on what the backing filesystem supports. (Fields that are given values despite being unrequested can just be ignored.) In either case, stx_mask will not be equal mask.

If a filesystem does not support a field or if it has an unrepresentable value (for instance, a file with an exotic type), then the mask bit corresponding to that field will be cleared in stx_mask even if the user asked for it and a dummy value will be filled in for compatibility purposes if one is available (e.g., a dummy UID and GID may be specified to mount under some circumstances).

A filesystem may also fill in fields that the caller didn't ask for if it has values for them available and the information is available at no extra cost. If this happens, the corresponding bits will be set in stx_mask.

Note: for performance and simplicity reasons, different fields in the statx structure may contain state information from different moments during the execution of the system call. For example, if stx_mode or stx_uid is changed by another process by calling chmod(2) or chown(2), stat() might return the old stx_mode together with the new stx_uid, or the old stx_uid together with the new stx_mode.

Apart from stx_mask (which is described above), the fields in the statx structure are:

stx_blksize

The "preferred" block size for efficient filesystem I/O. (Writing to a file in smaller chunks may cause an inefficient read-modify-rewrite.)

stx_attributes

Further status information about the file (see below for more information).

stx_nlink

The number of hard links on a file.

stx_uid

This field contains the user ID of the owner of the file.

stx_gid

This field contains the ID of the group owner of the file.

stx_mode

The file type and mode. See inode(7) for details.

stx_ino

The inode number of the file.

stx_size

The size of the file (if it is a regular file or a symbolic link) in bytes. The size of a symbolic link is the length of the pathname it contains, without a terminating null byte.

stx_blocks

The number of blocks allocated to the file on the medium, in 512-byte units. (This may be smaller than stx_size/512 when the file has holes.)

stx_attributes_mask

A mask indicating which bits in stx_attributes are supported by the VFS and the filesystem.

stx_atime

The file's last access timestamp.

stx_btime

The file's creation timestamp.

stx_ctime

The file's last status change timestamp.

stx_mtime

The file's last modification timestamp.

stx_dev_major and stx_dev_minor

The device on which this file (inode) resides.

stx_rdev_major and stx_rdev_minor

The device that this file (inode) represents if the file is of block or character device type.

stx_mnt_id

The mount ID of the mount containing the file. This is the same number reported by name_to_handle_at(2) and corresponds to the number in the first field in one of the records in /proc/self/mountinfo.

stx_dio_mem_align

The alignment (in bytes) required for user memory buffers for direct I/O (O_DIRECT) on this file, or 0 if direct I/O is not supported on this file.

STATX_DIOALIGN (stx_dio_mem_align and stx_dio_offset_align) is supported on block devices since Linux 6.1. The support on regular files varies by filesystem; it is supported by ext4, f2fs, and xfs since Linux 6.1.

stx_dio_offset_align

The alignment (in bytes) required for file offsets and I/O segment lengths for direct I/O (O_DIRECT) on this file, or 0 if direct I/O is not supported on this file. This will only be nonzero if stx_dio_mem_align is nonzero, and vice versa.

For further information on the above fields, see inode(7).

File attributes¶

The stx_attributes field contains a set of ORed flags that indicate additional attributes of the file. Note that any attribute that is not indicated as supported by stx_attributes_mask has no usable value here. The bits in stx_attributes_mask correspond bit-by-bit to stx_attributes.

The flags are as follows:

STATX_ATTR_COMPRESSED

The file is compressed by the filesystem and may take extra resources to access.

STATX_ATTR_IMMUTABLE

The file cannot be modified: it cannot be deleted or renamed, no hard links can be created to this file and no data can be written to it. See chattr(1).

STATX_ATTR_APPEND

The file can only be opened in append mode for writing. Random access writing is not permitted. See chattr(1).

STATX_ATTR_NODUMP

File is not a candidate for backup when a backup program such as dump(8) is run. See chattr(1).

STATX_ATTR_ENCRYPTED

A key is required for the file to be encrypted by the filesystem.

STATX_ATTR_VERITY (since Linux 5.5)

The file has fs-verity enabled. It cannot be written to, and all reads from it will be verified against a cryptographic hash that covers the entire file (e.g., via a Merkle tree).

STATX_ATTR_DAX (since Linux 5.8)

The file is in the DAX (cpu direct access) state. DAX state attempts to minimize software cache effects for both I/O and memory mappings of this file. It requires a file system which has been configured to support DAX.

DAX generally assumes all accesses are via CPU load / store instructions which can minimize overhead for small accesses, but may adversely affect CPU utilization for large transfers.

File I/O is done directly to/from user-space buffers and memory mapped I/O may be performed with direct memory mappings that bypass the kernel page cache.

While the DAX property tends to result in data being transferred synchronously, it does not give the same guarantees as the O_SYNC flag (see open(2)), where data and the necessary metadata are transferred together.

A DAX file may support being mapped with the MAP_SYNC flag, which enables a program to use CPU cache flush instructions to persist CPU store operations without an explicit fsync(2). See mmap(2) for more information.