stat.2: NOTES: add "C library/kernel ABI differences" subheading

[thirdparty/man-pages.git] / man2 / stat.2

'\" 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>
.\"
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.\" preserved on all copies.
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.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
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.\" have taken the same level of care in the production of this manual,
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.\"
.\" Modified by Michael Haardt <michael@moria.de>
.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1995-05-18 by Todd Larason <jtl@molehill.org>
.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1995-01-09 by Richard Kettlewell <richard@greenend.org.uk>
.\" Modified 1998-05-13 by Michael Haardt <michael@cantor.informatik.rwth-aachen.de>
.\" Modified 1999-07-06 by aeb & Albert Cahalan
.\" Modified 2000-01-07 by aeb
.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2007-06-08 mtk: Added example program
.\" 2007-07-05 mtk: Added details on underlying system call interfaces
.\"
.TH STAT 2 2014-08-19 "Linux" "Linux Programmer's Manual"
.SH NAME
stat, fstat, lstat, fstatat \- get file status
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.br
.B #include <sys/stat.h>
.br
.B #include <unistd.h>
.sp
.BI "int stat(const char *" pathname ", struct stat *" buf );
.br
.BI "int fstat(int " fd ", struct stat *" buf );
.br
.BI "int lstat(const char *" pathname ", struct stat *" buf );
.sp
.BR "#include <fcntl.h>           " "/* Definition of AT_* constants */"
.B #include <sys/stat.h>
.sp
.BI "int fstatat(int " dirfd ", const char *" pathname ", struct stat *" \
buf ,
.BI "            int " flags );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.ad l
.PD 0
.sp
.BR lstat ():
.RS 4
/* glibc 2.19 and earlier */ _BSD_SOURCE ||
.br
/* Since glibc 2.20 */_DEFAULT_SOURCE ||
.br
_XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
.br
|| /* Since glibc 2.10: */ _POSIX_C_SOURCE\ >=\ 200112L
.RE
.sp
.BR fstatat ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.10:
_XOPEN_SOURCE\ >=\ 700 || _POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_ATFILE_SOURCE
.RE
.PD
.ad
.SH DESCRIPTION
.PP
These functions return information about a file, in the buffer pointed to by
.IR stat .
No permissions are required on the file itself, but\(emin the case of
.BR stat (),
.BR fstatat (),
and
.BR lstat ()\(emexecute
(search) permission is required on all of the directories in
.I pathname
that lead to the file.
.PP
.BR stat ()
and
.BR fstatat ()
retrieve information about the file pointed to by
.IR pathname ;
the differences for
.BR fstatat ()
are described below.

.BR lstat ()
is identical to
.BR stat (),
except that if
.I pathname
is a symbolic link, then it returns information about the link itself,
not the file that it refers to.

.BR fstat ()
is identical to
.BR stat (),
except that the file about which information is to be retrieved
is specified by the file descriptor
.IR fd .
.PP
All of these system calls return a
.I stat
structure, which contains the following fields:
.PP
.in +4n
.nf
struct stat {
    dev_t     st_dev;         /* ID of device containing file */
    ino_t     st_ino;         /* inode number */
    mode_t    st_mode;        /* protection */
    nlink_t   st_nlink;       /* number of hard links */
    uid_t     st_uid;         /* user ID of owner */
    gid_t     st_gid;         /* group ID of owner */
    dev_t     st_rdev;        /* device ID (if special file) */
    off_t     st_size;        /* total size, in bytes */
    blksize_t st_blksize;     /* blocksize for filesystem I/O */
    blkcnt_t  st_blocks;      /* number of 512B blocks allocated */

    /* Since Linux 2.6, the kernel supports nanosecond
       precision for the following timestamp fields.
       For the details before Linux 2.6, see NOTES. */

    struct timespec st_atim;  /* time of last access */
    struct timespec st_mtim;  /* time of last modification */
    struct timespec st_ctim;  /* time of last status change */

#define st_atime st_atim.tv_sec      /* Backward compatibility */
#define st_mtime st_mtim.tv_sec
#define st_ctime st_ctim.tv_sec
};
.fi
.in

.I Note:
the order of fields in the
.I stat
structure varies somewhat
across architectures.
In addition,
the definition above does not show the padding bytes
that may be present between some fields on various architectures.
Consult the the glibc and kernel source code
if you need to know the details.

The
.I st_dev
field describes the device on which this file resides.
(The
.BR major (3)
and
.BR minor (3)
macros may be useful to decompose the device ID in this field.)

The
.I st_rdev
field describes the device that this file (inode) represents.

The
.I st_size
field gives 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.

The
.I st_blocks
field indicates the number of blocks allocated to the file, 512-byte units.
(This may be smaller than
.IR st_size /512
when the file has holes.)

The
.I st_blksize
field gives the "preferred" blocksize for efficient filesystem I/O.
(Writing to a file in smaller chunks may cause
an inefficient read-modify-rewrite.)
.PP
Not all of the Linux filesystems implement all of the time fields.
Some filesystem types allow mounting in such a way that file
and/or directory accesses do not cause an update of the
.I st_atime
field.
(See
.IR noatime ,
.IR nodiratime ,
and
.I relatime
in
.BR mount (8),
and related information in
.BR mount (2).)
In addition,
.I st_atime
is not updated if a file is opened with the
.BR O_NOATIME ;
see
.BR open (2).

The field
.I st_atime
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, like
.BR mmap (2),
may or may not update
.IR st_atime .

The field
.I st_mtime
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,
.I st_mtime
of a directory is changed by the creation or deletion of files
in that directory.
The
.I st_mtime
field is
.I not
changed for changes in owner, group, hard link count, or mode.

The field
.I st_ctime
is changed by writing or by setting inode information
(i.e., owner, group, link count, mode, etc.).
.PP
The following mask values are defined for the file type component of the
.I st_mode
field:
.in +4n
.TS
lB l l.
S_IFMT	0170000	bit mask for the file type bit fields

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
.PP
Thus, to test for a regular file (for example), one could write:

.nf
.in +4n
stat(pathname, &sb);
if ((sb.st_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 st_mode
to be written more concisely:
.RS 4
.TP 1.2i
.BR S_ISREG (m)
is it a regular file?
.TP
.BR S_ISDIR (m)
directory?
.TP
.BR S_ISCHR (m)
character device?
.TP
.BR S_ISBLK (m)
block device?
.TP
.BR S_ISFIFO (m)
FIFO (named pipe)?
.TP
.BR S_ISLNK (m)
symbolic link?  (Not in POSIX.1-1996.)
.TP
.BR S_ISSOCK (m)
socket?  (Not in POSIX.1-1996.)
.RE
.PP
The preceding code snippet could thus be rewritten as:

.nf
.in +4n
stat(pathname, &sb);
if (S_ISREG(sb.st_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 permissions component of the
.I st_mode
field:
.in +4n
.TS
lB l l.
S_ISUID	0004000	set-user-ID bit
S_ISGID	0002000	set-group-ID bit (see below)
S_ISVTX	0001000	sticky bit (see below)

S_IRWXU	  00700	mask for file owner permissions
S_IRUSR	  00400	owner has read permission
S_IWUSR	  00200	owner has write permission
S_IXUSR	  00100	owner has execute permission

S_IRWXG	  00070	mask for group permissions
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{
mask for permissions for others (not in group)
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.
.\"
.\"
.SS fstatat()
The
.BR fstatat ()
system call operates in exactly the same way as
.BR stat (),
except for the differences described here.

If the pathname given in
.I pathname
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
.I dirfd
(rather than relative to the current working directory of
the calling process, as is done by
.BR stat ()
for a relative pathname).

If
.I pathname
is relative and
.I dirfd
is the special value
.BR AT_FDCWD ,
then
.I pathname
is interpreted relative to the current working
directory of the calling process (like
.BR stat ()).

If
.I pathname
is absolute, then
.I dirfd
is ignored.

.I flags
can either be 0, or include one or more of the following flags ORed:
.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 ().
(By default,
.BR fstatat ()
dereferences symbolic links, like
.BR stat ().)
.PP
See
.BR openat (2)
for an explanation of the need for
.BR fstatat ().
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.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 fd
is bad.
.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.
.TP
.B EOVERFLOW
.I pathname
or
.I fd
refers to a file whose size, inode number,
or number of blocks cannot be represented in, respectively, the types
.IR off_t ,
.IR ino_t ,
or
.IR blkcnt_t .
This error can occur when, for example,
an application compiled on a 32-bit platform without
.I -D_FILE_OFFSET_BITS=64
calls
.BR stat ()
on a file whose size exceeds
.I (1<<31)-1
bytes.
.PP
The following additional errors can occur for
.BR fstatat ():
.TP
.B EBADF
.I dirfd
is not a valid file descriptor.
.TP
.B EINVAL
Invalid flag specified in
.IR flags .
.TP
.B ENOTDIR
.I pathname
is relative and
.I dirfd
is a file descriptor referring to a file other than a directory.
.SH VERSIONS
.BR fstatat ()
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.
.SH CONFORMING TO
.BR stat (),
.BR fstat (),
.BR lstat ():
SVr4, 4.3BSD, POSIX.1-2001, POSIX.1.2008.
.\" SVr4 documents additional
.\" .BR fstat ()
.\" error conditions EINTR, ENOLINK, and EOVERFLOW.  SVr4
.\" documents additional
.\" .BR stat ()
.\" and
.\" .BR lstat ()
.\" error conditions EINTR, EMULTIHOP, ENOLINK, and EOVERFLOW.

.BR fstatat ():
POSIX.1-2008.

According to POSIX.1-2001,
.BR lstat ()
on a symbolic link need return valid information only in the
.I st_size
field and the file-type component of the
.IR st_mode
field of the
.IR stat
structure.
POSIX.1-2008 tightens the specification, requiring
.BR lstat ()
to return valid information in all fields except the permission bits in
.IR st_mode .

Use of the
.I st_blocks
and
.I st_blksize
fields may be less portable.
(They were introduced in BSD.
The interpretation differs between systems,
and possibly on a single system when NFS mounts are involved.)
If you need to obtain the definition of the
.IR blkcnt_t
or
.IR blksize_t
types from
.IR <sys/stat.h> ,
then define
.BR _XOPEN_SOURCE
with the value 500 or greater (before including
.I any
header files).
.LP
POSIX.1-1990 did not describe the
.BR S_IFMT ,
.BR S_IFSOCK ,
.BR S_IFLNK ,
.BR S_IFREG ,
.BR S_IFBLK ,
.BR S_IFDIR ,
.BR S_IFCHR ,
.BR S_IFIFO ,
.B S_ISVTX
constants, but instead demanded the use of
the macros
.BR S_ISDIR (),
and so on.
The
.BR S_IF*
constants are present in POSIX.1-2001 and later.

The
.BR S_ISLNK ()
and
.BR S_ISSOCK ()
macros are not in
POSIX.1-1996, but both are present in POSIX.1-2001;
the former is from SVID 4, the latter from SUSv2.
.LP
UNIX\ V7 (and later systems) had
.BR S_IREAD ,
.BR S_IWRITE ,
.BR S_IEXEC ,
where POSIX
prescribes the synonyms
.BR S_IRUSR ,
.BR S_IWUSR ,
.BR S_IXUSR .
.SS Other systems
Values that have been (or are) in use on various systems:
.ad l
.TS
l l l l l.
hex	name	ls	octal	description
f000	S_IFMT		170000	mask for file type
0000			000000	T{
SCO out-of-service inode; BSD unknown type; SVID-v2 and XPG2
have both 0 and 0100000 for ordinary file
T}
1000	S_IFIFO	p|	010000	FIFO (named pipe)
2000	S_IFCHR	c	020000	character special (V7)
3000	S_IFMPC		030000	multiplexed character special (V7)
4000	S_IFDIR	d/	040000	directory (V7)
5000	S_IFNAM		050000	T{
XENIX named special file with two subtypes, distinguished by
\fIst_rdev\fP values 1, 2
T}
0001	S_INSEM	s	000001	XENIX semaphore subtype of IFNAM
0002	S_INSHD	m	000002	XENIX shared data subtype of IFNAM
6000	S_IFBLK	b	060000	block special (V7)
7000	S_IFMPB		070000	multiplexed block special (V7)
8000	S_IFREG	-	100000	regular (V7)
9000	S_IFCMP		110000	VxFS compressed
9000	S_IFNWK	n	110000	network special (HP-UX)
a000	S_IFLNK	l@	120000	symbolic link (BSD)
b000	S_IFSHAD		130000	T{
Solaris shadow inode for ACL (not seen by user space)
T}
c000	S_IFSOCK	s=	140000	socket (BSD; also "S_IFSOC" on VxFS)
d000	S_IFDOOR	D>	150000	Solaris door
e000	S_IFWHT	w%	160000	BSD whiteout (not used for inode)
0200	S_ISVTX		001000	T{
sticky bit: save swapped text even after use (V7)
.br
reserved (SVID-v2)
.br
On nondirectories: don't cache this file (SunOS)
.br
On directories: restricted deletion flag (SVID-v4.2)
T}
0400	S_ISGID		002000	T{
set-group-ID on execution (V7)
.br
for directories: use BSD semantics for propagation of GID
T}
0400	S_ENFMT		002000	T{
System V file locking enforcement (shared with S_ISGID)
T}
0800	S_ISUID		004000	set-user-ID on execution (V7)
0800	S_CDF		004000	T{
directory is a context dependent file (HP-UX)
T}
.TE
.ad

A sticky command appeared in Version 32V AT&T UNIX.
.SH NOTES
On Linux,
.BR lstat ()
will generally not trigger automounter action, whereas
.BR stat ()
will (but see
.BR fstatat (2)).

For most files under the
.I /proc
directory,
.BR stat ()
does not return the file size in the
.I st_size
field; instead the field is returned with the value 0.
.SS Timestamp fields
Older kernels and older standards did not support nanosecond timestamp
fields.
Instead, there were three timestamp
.RI fields\(em st_atime ,
.IR st_mtime ,
and
.IR st_ctime \(emtyped
as
.IR time_t
that recorded timestamps with one-second precision.

Since kernel 2.5.48, the
.I stat
structure supports nanosecond resolution for the three file timestamp fields.
The nanosecond components of each timestamp are available
via names of the form
.IR st_atim.tv_nsec
if the
.B _BSD_SOURCE
or
.B _SVID_SOURCE
feature test macro is defined.
Nanosecond timestamps are nowadays standardized,
starting with POSIX.1-2008, and, starting with version 2.12,
glibc also exposes the nanosecond component names if
.BR _POSIX_C_SOURCE
is defined with the value 200809L or greater, or
.BR _XOPEN_SOURCE
is defined with the value 700 or greater.
If none of the aforementioned macros are defined,
then the nanosecond values are exposed with names of the form
.IR st_atimensec .

Nanosecond timestamps are supported on XFS, JFS, Btrfs, and
ext4 (since Linux 2.6.23).
.\" commit ef7f38359ea8b3e9c7f2cae9a4d4935f55ca9e80
Nanosecond timestamps are not supported in ext2, ext3, and Reiserfs.
On filesystems that do not support subsecond timestamps,
the nanosecond fields are returned with the value 0.
.SS Underlying kernel interface
.SS C library/kernel ABI differences
Over time, increases in the size of the
.I stat
structure have led to three successive versions of
.BR stat ():
.IR sys_stat ()
(slot
.IR __NR_oldstat ),
.IR sys_newstat ()
(slot
.IR __NR_stat ),
and
.I sys_stat64()
(new in kernel 2.4; slot
.IR __NR_stat64 ).
The glibc
.BR stat ()
wrapper function hides these details from applications,
invoking the most recent version of the system call provided by the kernel,
and repacking the returned information if required for old binaries.
Similar remarks apply for
.BR fstat ()
and
.BR lstat ().
.\"
.\" A note from Andries Brouwer, July 2007
.\"
.\" > Is the story not rather more complicated for some calls like
.\" > stat(2)?
.\"
.\" Yes and no, mostly no. See /usr/include/sys/stat.h .
.\"
.\" The idea is here not so much that syscalls change, but that
.\" the definitions of struct stat and of the types dev_t and mode_t change.
.\" This means that libc (even if it does not call the kernel
.\" but only calls some internal function) must know what the
.\" format of dev_t or of struct stat is.
.\" The communication between the application and libc goes via
.\" the include file <sys/stat.h> that defines a _STAT_VER and
.\" _MKNOD_VER describing the layout of the data that user space
.\" uses. Each (almost each) occurrence of stat() is replaced by
.\" an occurrence of xstat() where the first parameter of xstat()
.\" is this version number _STAT_VER.
.\"
.\" Now, also the definitions used by the kernel change.
.\" But glibc copes with this in the standard way, and the
.\" struct stat as returned by the kernel is repacked into
.\" the struct stat as expected by the application.
.\" Thus, _STAT_VER and this setup cater for the application-libc
.\" interface, rather than the libc-kernel interface.
.\"
.\" (Note that the details depend on gcc being used as c compiler.)

The underlying system call employed by the glibc
.BR fstatat ()
wrapper function is actually called
.BR fstatat64 ().
.SH EXAMPLE
The following program calls
.BR stat ()
and displays selected fields in the returned
.I stat
structure.
.nf

#include <sys/types.h>
#include <sys/stat.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char *argv[])
{
    struct stat sb;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <pathname>\\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (stat(argv[1], &sb) == \-1) {
        perror("stat");