newfstatat (2)
Leading comments
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> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the c...
NAME
stat, fstat, lstat, fstatat - get file statusSYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h> int stat(const char *pathname, struct stat *buf);
int fstat(int fd, struct stat *buf);
int lstat(const char *pathname, struct stat *buf); #include <fcntl.h> /* Definition of AT_* constants */ #include <sys/stat.h> int fstatat(int dirfd, const char *pathname, struct stat *buf, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
lstat():
-
/* glibc 2.19 and earlier */ _BSD_SOURCE ||
/* Since glibc 2.20 */_DEFAULT_SOURCE ||
_XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.10: */ _POSIX_C_SOURCE >= 200112L
fstatat():
-
- Since glibc 2.10:
- _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
- _ATFILE_SOURCE
DESCRIPTION
These functions return information about a file, in the buffer pointed to by buf. No permissions are required on the file itself, but---in the case of stat(), fstatat(), and lstat()---execute (search) permission is required on all of the directories in pathname that lead to the file.
stat()
and
fstatat()
retrieve information about the file pointed to by
pathname;
the differences for
fstatat()
are described below.
lstat()
is identical to
stat(),
except that if
pathname
is a symbolic link, then it returns information about the link itself,
not the file that it refers to.
fstat()
is identical to
stat(),
except that the file about which information is to be retrieved
is specified by the file descriptor
fd.
All of these system calls return a stat structure, which contains the following fields:
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 };
Note: the order of fields in the 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 glibc and kernel source code if you need to know the details.
Note: For performance and simplicity reasons, different fields in the stat structure may contain state information from different moments during the execution of the system call. For example, if st_mode or st_uid is changed by another process by calling chmod(2) or chown(2), stat() might return the old st_mode together with the new st_uid, or the old st_uid together with the new st_mode.
The st_dev field describes the device on which this file resides. (The major(3) and minor(3) macros may be useful to decompose the device ID in this field.)
The st_rdev field describes the device that this file (inode) represents.
The 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 st_blocks field indicates the number of blocks allocated to the file, 512-byte units. (This may be smaller than st_size/512 when the file has holes.)
The 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.)
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
st_atime
field.
(See
noatime,
nodiratime,
and
relatime
in
mount(8),
and related information in
mount(2).)
In addition,
st_atime
is not updated if a file is opened with the
O_NOATIME;
see
open(2).
The field
st_atime
is changed by file accesses, for example, by
execve(2),
mknod(2),
pipe(2),
utime(2),
and
read(2)
(of more than zero bytes).
Other routines, like
mmap(2),
may or may not update
st_atime.
The field
st_mtime
is changed by file modifications, for example, by
mknod(2),
truncate(2),
utime(2),
and
write(2)
(of more than zero bytes).
Moreover,
st_mtime
of a directory is changed by the creation or deletion of files
in that directory.
The
st_mtime
field is
not
changed for changes in owner, group, hard link count, or mode.
The field
st_ctime
is changed by writing or by setting inode information
(i.e., owner, group, link count, mode, etc.).
POSIX refers to the st_mode bits corresponding to the mask S_IFMT (see below) as the file type, the 12 bits corresponding to the mask 07777 as the file mode bits and the least significant 9 bits (0777) as the file permission bits.
The following mask values are defined for the file type of the st_mode field:
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 |
Thus, to test for a regular file (for example), one could write:
stat(pathname, &sb); if ((sb.st_mode & S_IFMT) == S_IFREG) { /* Handle regular file */ }
Because tests of the above form are common, additional macros are defined by POSIX to allow the test of the file type in st_mode to be written more concisely:
-
- S_ISREG(m)
- is it a regular file?
- S_ISDIR(m)
- directory?
- S_ISCHR(m)
- character device?
- S_ISBLK(m)
- block device?
- S_ISFIFO(m)
- FIFO (named pipe)?
- S_ISLNK(m)
- symbolic link? (Not in POSIX.1-1996.)
- S_ISSOCK(m)
- socket? (Not in POSIX.1-1996.)
The preceding code snippet could thus be rewritten as:
stat(pathname, &sb); if (S_ISREG(sb.st_mode)) { /* Handle regular file */ }
The definitions of most of the above file type test macros
are provided if any of the following feature test macros is defined:
_BSD_SOURCE
(in glibc 2.19 and earlier),
_SVID_SOURCE
(in glibc 2.19 and earlier),
or
_DEFAULT_SOURCE
(in glibc 2.20 and later).
In addition, definitions of all of the above macros except
S_IFSOCK
and
S_ISSOCK()
are provided if
_XOPEN_SOURCE
is defined.
The definition of
S_IFSOCK
can also be exposed by defining
_XOPEN_SOURCE
with a value of 500 or greater.
The definition of
S_ISSOCK()
is exposed if any of the following feature test macros is defined:
_BSD_SOURCE
(in glibc 2.19 and earlier),
_DEFAULT_SOURCE
(in glibc 2.20 and later),
_XOPEN_SOURCE
with a value of 500 or greater, or
_POSIX_C_SOURCE
with a value of 200112L or greater.
The following mask values are defined for the file mode component of the st_mode field:
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 |
others (not in group) 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 |
fstatat()
The fstatat() system call operates in exactly the same way as stat(), 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 stat() 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 stat()).
If pathname is absolute, then dirfd is ignored.
flags can either be 0, or include one or more of the following flags ORed:
- AT_EMPTY_PATH (since Linux 2.6.39)
- 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 (since Linux 2.6.38)
- 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 directories 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(). (By default, fstatat() dereferences symbolic links, like stat().)
See openat(2) for an explanation of the need for fstatat().
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_resolution(7).)
- EBADF
- fd is bad.
- EFAULT
- Bad address.
- ELOOP
- Too many symbolic links encountered while traversing the path.
- ENAMETOOLONG
- pathname is too long.
- ENOENT
- A component of pathname does not exist, or pathname is an empty string.
- ENOMEM
- Out of memory (i.e., kernel memory).
- ENOTDIR
- A component of the path prefix of pathname is not a directory.
- EOVERFLOW
- pathname or fd refers to a file whose size, inode number, or number of blocks cannot be represented in, respectively, the types off_t, ino_t, or blkcnt_t. This error can occur when, for example, an application compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64 calls stat() on a file whose size exceeds (1<<31)-1 bytes.
The following additional errors can occur for fstatat():
- EBADF
- dirfd is not a valid file descriptor.
- EINVAL
- Invalid flag specified in flags.
- ENOTDIR
- pathname is relative and dirfd is a file descriptor referring to a file other than a directory.
VERSIONS
fstatat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.CONFORMING TO
stat(), fstat(), lstat(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1.2008.fstatat(): POSIX.1-2008.
According to POSIX.1-2001, lstat() on a symbolic link need return valid information only in the st_size field and the file type of the st_mode field of the stat structure. POSIX.1-2008 tightens the specification, requiring lstat() to return valid information in all fields except the mode bits in st_mode.
Use of the st_blocks and 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 blkcnt_t or blksize_t types from <sys/stat.h>, then define _XOPEN_SOURCE with the value 500 or greater (before including any header files).
POSIX.1-1990 did not describe the
S_IFMT,
S_IFSOCK,
S_IFLNK,
S_IFREG,
S_IFBLK,
S_IFDIR,
S_IFCHR,
S_IFIFO,
S_ISVTX
constants, but instead demanded the use of
the macros
S_ISDIR(),
and so on.
The
S_IF*
constants are present in POSIX.1-2001 and later.
The
S_ISLNK()
and
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.
UNIX V7 (and later systems) had S_IREAD, S_IWRITE, S_IEXEC, where POSIX prescribes the synonyms S_IRUSR, S_IWUSR, S_IXUSR.
Other systems
Values that have been (or are) in use on various systems:hex | name | ls | octal | description |
f000 | S_IFMT | 170000 | mask for file type | |
0000 | 000000 |
SCO out-of-service inode; BSD unknown type; SVID-v2 and XPG2
have both 0 and 0100000 for ordinary file
| ||
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 |
XENIX named special file with two subtypes, distinguished by
st_rdev values 1, 2
| |
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 |
Solaris shadow inode for ACL (not seen by user space)
| |
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 |
sticky bit: save swapped text even after use (V7)
reserved (SVID-v2) On nondirectories: don't cache this file (SunOS) On directories: restricted deletion flag (SVID-v4.2) | |
0400 | S_ISGID | 002000 |
set-group-ID on execution (V7)
for directories: use BSD semantics for propagation of GID | |
0400 | S_ENFMT | 002000 |
System V file locking enforcement (shared with S_ISGID)
| |
0800 | S_ISUID | 004000 | set-user-ID on execution (V7) | |
0800 | S_CDF | 004000 |
directory is a context dependent file (HP-UX)
|
A sticky command appeared in Version 32V AT&T UNIX.
NOTES
On Linux, lstat() will generally not trigger automounter action, whereas stat() will (but see fstatat(2)).For most files under the /proc directory, stat() does not return the file size in the st_size field; instead the field is returned with the value 0.
Timestamp fields
Older kernels and older standards did not support nanosecond timestamp fields. Instead, there were three timestamp fields---st_atime, st_mtime, and st_ctime---typed as time_t that recorded timestamps with one-second precision.Since kernel 2.5.48, the stat structure supports nanosecond resolution for the three file timestamp fields. The nanosecond components of each timestamp are available via names of the form st_atim.tv_nsec if the _BSD_SOURCE or _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 _POSIX_C_SOURCE is defined with the value 200809L or greater, or _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 st_atimensec.
Nanosecond timestamps are supported on XFS, JFS, Btrfs, and ext4 (since Linux 2.6.23). 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.
C library/kernel differences
Over time, increases in the size of the stat structure have led to three successive versions of stat(): sys_stat() (slot __NR_oldstat), sys_newstat() (slot __NR_stat), and sys_stat64() (slot __NR_stat64) on 32-bit platforms such as i386. The first two versions were already present in Linux 1.0 (albeit with different names); the last was added in Linux 2.4. Similar remarks apply for fstat() and lstat().The kernel-internal versions of the stat structure dealt with by the different versions are, respectively:
-
- __old_kernel_stat
- The original structure, with rather narrow fields, and no padding.
- stat
- Larger st_ino field and padding added to various parts of the structure to allow for future expansion.
- stat64
- Even larger st_ino field, larger st_uid and st_gid fields to accommodate the Linux-2.4 expansion of UIDs and GIDs to 32 bits, and various other enlarged fields and further padding in the structure. (Various padding bytes were eventually consumed in Linux 2.6, with the advent of 32-bit device IDs and nanosecond components for the timestamp fields.)
The glibc
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.
On modern 64-bit systems, life is simpler: there is a single
stat()
system call and the kernel deals with a
stat
structure that contains fields of a sufficient size.
The underlying system call employed by the glibc
fstatat()
wrapper function is actually called
fstatat64()
or, on some architectures,
newfstatat().
EXAMPLE
The following program calls stat() and displays selected fields in the returned stat structure.#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"); exit(EXIT_FAILURE); } printf("File type: "); switch (sb.st_mode & S_IFMT) { case S_IFBLK: printf("block device\n"); break; case S_IFCHR: printf("character device\n"); break; case S_IFDIR: printf("directory\n"); break; case S_IFIFO: printf("FIFO/pipe\n"); break; case S_IFLNK: printf("symlink\n"); break; case S_IFREG: printf("regular file\n"); break; case S_IFSOCK: printf("socket\n"); break; default: printf("unknown?\n"); break; } printf("I-node number: %ld\n", (long) sb.st_ino); printf("Mode: %lo (octal)\n", (unsigned long) sb.st_mode); printf("Link count: %ld\n", (long) sb.st_nlink); printf("Ownership: UID=%ld GID=%ld\n", (long) sb.st_uid, (long) sb.st_gid); printf("Preferred I/O block size: %ld bytes\n", (long) sb.st_blksize); printf("File size: %lld bytes\n", (long long) sb.st_size); printf("Blocks allocated: %lld\n", (long long) sb.st_blocks); printf("Last status change: %s", ctime(&sb.st_ctime)); printf("Last file access: %s", ctime(&sb.st_atime)); printf("Last file modification: %s", ctime(&sb.st_mtime)); exit(EXIT_SUCCESS); }