statx manpage

[David Howells <dhowells@redhat.com>]

Christoph Hellwig <hch@infradead.org> wrote:

> This would be great to have in 4.11 together with the initial statx
> implementation.  But until I see documentation and testcases for statx
> I don't really feel comfortable reviewing anything related to it.

Well, since you asked for documentation, here's a manual page for you to
review:-)

Note that as it isn't in glibc yet, I've left out all the
set-this-and-that-#define-to-make-it-appear stuff except where it is pertinent
to particular constants.

I don't suppose you know where the documentation on writing xfstests tests is?
xfstests-dev/doc/ only contains an old and out of date changelog.

David
---
'\" t
.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
.\" Parts Copyright (c) 1995 Nicolai Langfeldt (janl@ifi.uio.no), 1/1/95
.\" and Copyright (c) 2006, 2007, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (c) 2017 David Howells <dhowells@redhat.com>
.\"
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.TH STATX 2 2017-03-07 "Linux" "Linux Programmer's Manual"
.SH NAME
statx \- Get file status (extended)
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.br
.B #include <sys/stat.h>
.br
.B #include <unistd.h>
.br
.BR "#include <fcntl.h>           " "/* Definition of AT_* constants */"
.sp
.BI "int statx(int " dirfd ", const char *" pathname ", int " flags ","
.BI "          unsigned int " mask ", struct statx *" buf );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.ad l
.PD 0
.sp
.RS 4
<unknown as yet>
.RE
.PD
.ad
.SH DESCRIPTION
.PP
This function returns information about a file, storing it in the buffer
pointed to by
.IR buf .
The buffer is filled in according to the following type:
.PP
.in +4n
.nf
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
    struct statx_timestamp stx_atime;  -- Time of last access
    struct statx_timestamp stx_btime;  -- Time of creation
    struct statx_timestamp stx_ctime;  -- Time of last status change
    struct statx_timestamp stx_mtime;  -- Time of last modification
    __u32 stx_rdev_major;    } Device number if device file
    __u32 stx_rdev_minor;    }
    __u32 stx_dev_major;      } Device number of containing file
    __u32 stx_dev_minor;      }
};
.fi
.in
.PP
Where the timestamps are defined as:
.PP
.in +4n
.nf
struct statx_timestamp {
    __s64 tv_sec;    -- Number of seconds before or since 1970
    __s32 tv_nsec;   -- Number of nanoseconds before or since tv_sec
};
.fi
.in
.PP
(Note that reserved space and padding is ommitted)
.SS
Invoking \fBstatx\fR():
.PP
To access a file's status, no permissions are required on the file itself, but
in the case of
.BR statx ()
with a path, execute (search) permission is required on all of the directories
in
.I pathname
that lead to the file.
.PP
.BR statx ()
uses
.IR pathname ", " dirfd " and " flags
to locate the target file in one of a variety of ways:
.TP
[*] By absolute path.
.I pathname
points to an absolute path and
.I dirfd
is ignored.  The file is looked up by name, starting from the root of the
filesystem as seen by the calling process.
.TP
[*] By cwd-relative path.
.I pathname
points to a relative path and
.IR dirfd " is " AT_FDCWD .
The file is looked up by name, starting from the current working directory.
.TP
[*] By dir-relative path.
.I pathname
points to relative path and
.I dirfd
indicates a file descriptor pointing to a directory.  The file is looked up by
name, starting from the directory specified by
.IR dirfd .
.TP
[*] By file descriptor.
.IR pathname " is " NULL " and " dirfd
indicates a file descriptor.  The file attached to the file descriptor is
queried directly.  The file descriptor may point to any type of file, not just
a directory.
.PP
.I flags
can be used to influence a path-based lookup.  A value for
.I flags
is constructed by OR'ing together zero or more of the following constants:
.TP
.BR AT_EMPTY_PATH " (since Linux 2.6.39)"
.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
If
.I pathname
is an empty string, operate on the file referred to by
.IR dirfd
(which may have been obtained using the
.BR open (2)
.B O_PATH
flag).
If
.I dirfd
is
.BR AT_FDCWD ,
the call operates on the current working directory.
In this case,
.I dirfd
can refer to any type of file, not just a directory.
This flag is Linux-specific; define
.B _GNU_SOURCE
.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed
to obtain its definition.
.TP
.BR AT_NO_AUTOMOUNT " (since Linux 2.6.38)"
Don't automount the terminal ("basename") component of
.I 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 directories
to prevent mass-automounting of a directory of automount points.
The
.B AT_NO_AUTOMOUNT
flag has no effect if the mount point has already been mounted over.
This flag is Linux-specific; define
.B _GNU_SOURCE
.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed
to obtain its definition.
.TP
.B AT_SYMLINK_NOFOLLOW
If
.I pathname
is a symbolic link, do not dereference it:
instead return information about the link itself, like
.BR lstat ().
.PP
.I flags
can also be used to control what sort of synchronisation the kernel will do
when querying a file on a remote filesystem.  This is done by OR'ing in one of
the following values:
.TP
AT_STATX_SYNC_AS_STAT
Do whatever
.BR stat ()
does.  This is the default and is very much filesystem specific.
.TP
AT_STATX_FORCE_SYNC
Force the attributes to be synchronised with the server.  This may require that
a network filesystem perform a data writeback to get the timestamps correct.
.TP
AT_STATX_DONT_SYNC
Don't synchronise 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.
.PP
The
.I mask
argument to
.BR statx ()
is used to tell the kernel which fields the caller is interested in
.I mask
is an OR'ed combination of the following constants:
.PP
.in +4n
.TS
lB l.
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{,_ns}
STATX_MTIME	Want stx_mtime{,_ns}
STATX_CTIME	Want stx_ctime{,_ns}
STATX_INO	Want stx_ino
STATX_SIZE	Want stx_size
STATX_BLOCKS	Want stx_blocks
STATX_BASIC_STATS	[The stuff in the normal stat struct]
STATX_BTIME	Want stx_btime{,_ns}
STATX_ALL	[All currently available stuff]
.TE
.in
.PP
.B "Do not"
simply set
.I mask
to UINT_MAX as one or more bits may, in future, be used to specify an extension
to the buffer.
.SS
The returned information
.PP
The status information for the target file is returned in the
.I statx
structure pointed to by
.IR buf .
Included in this is
.I stx_mask
which indicates what other information has been returned.
.I stx_mask
has the same format as the mask argument and bits are set in it to indicate
which fields have been filled in.
.PP
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,
.I stx_mask
will not be equal
.IR mask .
.PP
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
.I 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).
.PP
A filesystem may also fill in fields that the caller didn't ask for if it has
values for them available at no extra cost.  If this happens, the corresponding
bits will be set in
.IR stx_mask .
.PP

.\" Background: inode attributes are modified with i_mutex held, but
.\" read by stat() without taking the mutex.
.I Note:
For performance and simplicity reasons, different fields in the
.I statx
structure may contain state information from different moments
during the execution of the system call.
For example, if
.IR stx_mode
or
.IR stx_uid
is changed by another process by calling
.BR chmod (2)
or
.BR chown (2),
.BR stat ()
might return the old
.I stx_mode
together with the new
.IR stx_uid ,
or the old
.I stx_uid
together with the new
.IR stx_mode .
.PP
Apart from stx_mask (which is described above), the fields in the
.I statx
structure are:
.TP
.I stx_mode
The file type and mode.  This is described in more detail below.
.TP
.I 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.
.TP
.I stx_blocks
The number of blocks allocated to the file on the medium, in 512-byte units.
(This may be smaller than
.IR stx_size /512
when the file has holes.)
.TP
.I stx_blksize
The "preferred" blocksize for efficient filesystem I/O.  (Writing to a file in
smaller chunks may cause an inefficient read-modify-rewrite.)
.TP
.I stx_nlink
The number of hard links on a file.
.TP
.I stx_uid
The user ID of the file's owner.
.TP
.I stx_gid
The ID of the group that may access the file.
.TP
.IR stx_dev_major " and "  stx_dev_minor
The device on which this file (inode) resides.
.TP
.IR stx_rdev_major " and "  stx_rdev_minor
The device that this file (inode) represents if the file is of block or
character device type.
.TP
.I stx_attributes
Further status information about the file.  This consists of zero or more of
the following constants OR'ed together:
.in +4n
.TS
lB l.
STATX_ATTR_COMPRESSED	File is compressed by the fs
STATX_ATTR_IMMUTABLE	File is marked immutable
STATX_ATTR_APPEND	File is append-only
STATX_ATTR_NODUMP	File is not to be dumped
STATX_ATTR_ENCRYPTED	File requires key to decrypt in fs
.TE
.in
.TP
.I stx_atime
The file's last access timestamp.
This field is changed by file accesses, for example, by
.BR execve (2),
.BR mknod (2),
.BR pipe (2),
.BR utime (2),
and
.BR read (2)
(of more than zero bytes).
Other routines, such as
.BR mmap (2),
may or may not update it.
.TP
.I stx_btime
The file's creation timestamp.  This is set on file creation and not changed
subsequently.
.TP
.I stx_ctime
The file's last status change timestamp.  This field is changed by writing or
by setting inode information (i.e., owner, group, link count, mode, etc.).
.TP
.I stx_mtime
The file's last modification timestamp.  This is changed by file modifications,
for example, by
.BR mknod (2),
.BR truncate (2),
.BR utime (2),
and
.BR write (2)
(of more than zero bytes).  Moreover, the modification time of a directory is
changed by the creation or deletion of files in that directory.  This field is
.I not
changed for changes in owner, group, hard link count, or mode.



.PP
Not all of the Linux filesystems implement all of the timestamp fields.  Some
filesystems allow mounting in such a way that file and/or directory accesses do
not cause an update of the
.I stx_atime
field.
(See
.IR noatime ,
.IR nodiratime ,
and
.I relatime
in
.BR mount (8),
and related information in
.BR mount (2).)
In addition,
.I stx_atime
is not updated if a file is opened with the
.BR O_NOATIME ;
see
.BR open (2).

.SS File type and mode
.PP
The
.I stx_mode
field contains the combined file type and mode.  POSIX refers to the bits in
this field corresponding to the mask
.B S_IFMT
(see below) as the
.IR "file type" ,
the 12 bits corresponding to the mask 07777 as the
.IR "file mode bits"
and the least significant 9 bits (0777) as the
.IR "file permission bits" .
.IP
The following mask values are defined for the file type of the
.I stx_mode
field:
.in +4n
.TS
lB l l.
S_IFMT	0170000	bit mask for the file type bit field

S_IFSOCK	0140000	socket
S_IFLNK	0120000	symbolic link
S_IFREG	0100000	regular file
S_IFBLK	0060000	block device
S_IFDIR	0040000	directory
S_IFCHR	0020000	character device
S_IFIFO	0010000	FIFO
.TE
.in
.IP
Note that
.I stx_mode
has two mask flags covering it: one for the type and one for the mode bits.
.PP
To test for a regular file (for example), one could write:
.nf
.in +4n
statx(AT_FDCWD, pathname, 0, STATX_BASIC_STATS, &sb);
if ((sb.stx_mode & S_IFMT) == S_IFREG) {
    /* Handle regular file */
}
.in
.fi
.PP
Because tests of the above form are common, additional macros are defined by
POSIX to allow the test of the file type in
.I stx_mode
to be written more concisely:
.RS 4
.TS
lB l.
\fBS_ISREG\fR(m)	Is it a regular file?
\fBS_ISDIR\fR(m)	Is it a directory?
\fBS_ISCHR\fR(m)	Is it a character device?
\fBS_ISBLK\fR(m)	Is it a block device?
\fBS_ISFIFO\fR(m)	Is it a FIFO (named pipe)?
\fBS_ISLNK\fR(m)	Is it a symbolic link?  (Not in POSIX.1-1996.)
\fBS_ISSOCK\fR(m)	Is it a socket?  (Not in POSIX.1-1996.)
.TE
.RE
.PP
The preceding code snippet could thus be rewritten as:

.nf
.in +4n
statx(AT_FDCWD, pathname, 0, STATX_BASIC_STATS, &sb);
if (S_ISREG(sb.stx_mode)) {
    /* Handle regular file */
}
.in
.fi
.PP
The definitions of most of the above file type test macros
are provided if any of the following feature test macros is defined:
.BR _BSD_SOURCE
(in glibc 2.19 and earlier),
.BR _SVID_SOURCE
(in glibc 2.19 and earlier),
or
.BR _DEFAULT_SOURCE
(in glibc 2.20 and later).
In addition, definitions of all of the above macros except
.BR S_IFSOCK
and
.BR S_ISSOCK ()
are provided if
.BR _XOPEN_SOURCE
is defined.
The definition of
.BR S_IFSOCK
can also be exposed by defining
.BR _XOPEN_SOURCE
with a value of 500 or greater.

The definition of
.BR S_ISSOCK ()
is exposed if any of the following feature test macros is defined:
.BR _BSD_SOURCE
(in glibc 2.19 and earlier),
.BR _DEFAULT_SOURCE
(in glibc 2.20 and later),
.BR _XOPEN_SOURCE
with a value of 500 or greater, or
.BR _POSIX_C_SOURCE
with a value of 200112L or greater.
.PP
The following mask values are defined for
the file mode component of the
.I stx_mode
field:
.in +4n
.TS
lB l l.
S_ISUID	  04000	set-user-ID bit
S_ISGID	  02000	set-group-ID bit (see below)
S_ISVTX	  01000	sticky bit (see below)

S_IRWXU	  00700	owner has read, write, and execute permission
S_IRUSR	  00400	owner has read permission
S_IWUSR	  00200	owner has write permission
S_IXUSR	  00100	owner 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	T{
others (not in group) have read, write, and execute permission
T}
S_IROTH	  00004	others have read permission
S_IWOTH	  00002	others have write permission
S_IXOTH	  00001	others have execute permission
.TE
.in
.P
The set-group-ID bit
.RB ( S_ISGID )
has several special uses.
For a directory, it indicates that BSD semantics is to be used
for that directory: files created there inherit their group ID from
the directory, not from the effective group ID of the creating process,
and directories created there will also get the
.B S_ISGID
bit set.
For a file that does not have the group execution bit
.RB ( S_IXGRP )
set,
the set-group-ID bit indicates mandatory file/record locking.
.P
The sticky bit
.RB ( S_ISVTX )
on a directory means that a file
in that directory can be renamed or deleted only by the owner
of the file, by the owner of the directory, and by a privileged
process.


.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EINVAL
Invalid flag specified in
.IR flags .
.TP
.B EACCES
Search permission is denied for one of the directories
in the path prefix of
.IR pathname .
(See also
.BR path_resolution (7).)
.TP
.B EBADF
.I dirfd
is not a valid open file descriptor.
.TP
.B EFAULT
Bad address.
.TP
.B ELOOP
Too many symbolic links encountered while traversing the path.
.TP
.B ENAMETOOLONG
.I pathname
is too long.
.TP
.B ENOENT
A component of
.I pathname
does not exist, or
.I pathname
is an empty string.
.TP
.B ENOMEM
Out of memory (i.e., kernel memory).
.TP
.B ENOTDIR
A component of the path prefix of
.I pathname
is not a directory or
.I pathname
is relative and
.I dirfd
is a file descriptor referring to a file other than a directory.
.SH VERSIONS
.BR statx ()
was added to Linux in kernel 4.11;
library support is not yet added to glibc.
.SH SEE ALSO
.BR ls (1),
.BR stat (1),
.BR access (2),
.BR chmod (2),
.BR chown (2),
.BR readlink (2),
.BR utime (2),
.BR capabilities (7),
.BR symlink (7)