Revised statx(2) man page for review

From: Michael Kerrisk (man-pages)
Date: Tue Apr 25 2017 - 07:14:42 EST

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Hello David, et al.,

I merged your statx(2) page, and edited somewhat heavily.
(The merged page source has been pushed to Git.)

Could you please carefully review the text below, in case
I added any errors.

There is one question in a FIXME below. Could you please
take a look at that also.

Your proposed page duplicated a lot of content from stat(2).
I like to avoid such redundancy, so I move the common pieces
into a new page, inode(7), and reworked stat(2) and statx(2).

Cheers,

Michael

NAME
statx - get file status (extended)

SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h> /* Definition of AT_* constants */

int statx(int dirfd, const char *pathname, int flags,
unsigned int mask, struct statx *buf);

Note: There is no glibc wrapper for renameat2(); see NOTES.

DESCRIPTION
This function returns information about a file, storing it in
the buffer pointed to by buf. The returned buffer is a strucâ
ture of the following type:

struct statx {
__u32 stx_mask; /* Mask of bits indicating
filled fields */
__u32 stx_blksize; /* Block size for filesystem I/O */
__u64 stx_attributes; /* Extra file attribute indicators */
__u32 stx_nlink; /* Number of hard links */
__u32 stx_uid; /* User ID of owner */
__u32 stx_gid; /* Group ID of owner */
__u16 stx_mode; /* File type and mode */
__u64 stx_ino; /* Inode number */
__u64 stx_size; /* Total size in bytes */
__u64 stx_blocks; /* Number of 512B blocks allocated */

/* The following fields are file timestamps */
struct statx_timestamp stx_atime; /* Last access */
struct statx_timestamp stx_btime; /* Creation */
struct statx_timestamp stx_ctime; /* Last status change */
struct statx_timestamp stx_mtime; /* Last modification */

/* If this file represents a device, then the next two
fields contain the ID of the device */
__u32 stx_rdev_major; /* Major ID */
__u32 stx_rdev_minor; /* Minor ID */

/* The next two fields contain the ID of the device
containing the filesystem where the file resides */
__u32 stx_dev_major; /* Major ID */
__u32 stx_dev_minor; /* Minor ID */
};

The file timestamps are structures of the following type:

struct statx_timestamp {
__s64 tv_sec; /* Seconds since the Epoch (UNIX time) */
__s32 tv_nsec; /* Nanoseconds before or since tv_sec */
};

(Note that reserved space and padding is omitted.)

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, exeâ
cute (search) permission is required on all of the directories
in pathname that lead to the file.

statx() uses pathname, dirfd, and flags 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 pathname interpreted relative to a directory file descriptor
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 pathâ
name that is interpreted relative to the directory
referred to by dirfd.

By file descriptor
If pathname is NULL, then the target file is the one
referred to by the file descriptor dirfd. dirfd may
refer to any type of file, not just a directory. (The
AT_EMPTY_PATH flag described below provides similar
functionality.)

âââââââââââââââââââââââââââââââââââââââââââââââââââââââ
âFIXME â
âââââââââââââââââââââââââââââââââââââââââââââââââââââââ
âIt appears that there are two different ways of â
âdoing the same thing: specifying the file to be â
âstat'ed via a file descriptor. Either, we specify â
â'pathname' as NULL, or we specify 'pathname' as an â
âempty string and include the AT_EMPTY_PATH flag. â
âWhat's the rationale for having two ways of doing â
âthis? â
âââââââââââââââââââââââââââââââââââââââââââââââââââââââ

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). If dirfd is AT_FDCWD, the
call operates on the current working directory. In this
case, dirfd can refer to any type of file, not just a
directory. This flag is Linux-specific; define
_GNU_SOURCE to obtain its definition.

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 can be used in tools that scan direcâ
tories to prevent mass-automounting of a directory of
automount points. The AT_NO_AUTOMOUNT flag has no
effect if the mount point has already been mounted over.
This flag is Linux-specific; define _GNU_SOURCE to
obtain its definition.

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 whatâ
ever 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 combinaâ
tion of the following constants:

STATX_TYPE Want stx_mode & S_IFMT
STATX_MODE Want stx_mode & ~S_IFMT
STATX_NLINK Want stx_nlink
STATX_UID Want stx_uid
STATX_GID Want stx_gid
STATX_ATIME Want stx_atime
STATX_MTIME Want stx_mtime
STATX_CTIME Want stx_ctime
STATX_INO Want stx_ino
STATX_SIZE Want stx_size
STATX_BLOCKS Want stx_blocks
STATX_BASIC_STATS [All of the above]
STATX_BTIME Want stx_btime
STATX_ALL [All currently available fields]

Note the kernel does not reject values in mask other than the
above. 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 specâ
ify an extension to the buffer.

The returned information
The status information for the target file is returned in the
statx structure pointed to by buf. 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.
In either case, stx_mask will not be equal mask.

If a filesystem does not support a field or if it has an unrepâ
resentable 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 availâ
able (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 difâ
ferent 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_mode
The file type and mode. See inode(7) for details.

stx_size
The size of the file (if it is a regular file or a symâ
bolic link) in bytes. The size of a symbolic link is
the length of the pathname it contains, without a termiâ
nating 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_blksize
The "preferred" block size for efficient filesystem I/O.
(Writing to a file in smaller chunks may cause an inefâ
ficient read-modify-rewrite.)

stx_nlink
The number of hard links on a file.

stx_uid
The user ID of the file's owner.

stx_gid
The ID of the group that may access the file.

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

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.

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:

STATX_ATTR_COMPRESSED
The file is compressed by the fs 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.

RETURN VALUE
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.

ERRORS
EACCES Search permission is denied for one of the directories
in the path prefix of pathname. (See also path_resoluâ
tion(7).)

EBADF dirfd is not a valid open file descriptor.

EFAULT Bad address.

EINVAL Invalid flag specified in flags.

ELOOP Too many symbolic links encountered while traversing the
pathname.

ENAMETOOLONG
pathname is too long.

ENOENT A component of pathname does not exist, or pathname is
an empty string and AT_EMPTY_PATH was not specified in
flags.

ENOMEM Out of memory (i.e., kernel memory).

ENOTDIR
A component of the path prefix of pathname is not a
directory or pathname is relative and dirfd is a file
descriptor referring to a file other than a directory.

VERSIONS
statx() was added to Linux in kernel 4.11.

CONFORMING TO
statx() is Linux specific.

NOTES
Glibc does not (yet) provide a wrapper for the statx() system
call; call it using syscall(2).

SEE ALSO
ls(1), stat(1), access(2), chmod(2), chown(2), stat(2), readâ
link(2), utime(2), capabilities(7), inode(7), symlink(7)

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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