fstatat (2)

#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

These functions return information about a file, in the buffer pointed to by stat. 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 the glibc and kernel source code if you need to know the details.

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.).

The following mask values are defined for the file type component of the st_mode field:

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 permissions component of the st_mode field:

The set-group-ID bit (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 S_ISGID bit set. For a file that does not have the group execution bit (S_IXGRP) set, the set-group-ID bit indicates mandatory file/record locking.

The sticky bit (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.

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().

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

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.

fstatat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.

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 component 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 permission 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:

A sticky command appeared in Version 32V AT&T UNIX.

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.

Underlying kernel interface

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() (new in kernel 2.4; slot __NR_stat64). 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. Similar remarks apply for fstat() and lstat().

The underlying system call employed by the glibc fstatat() wrapper function is actually called fstatat64().

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);
}

ls(1), stat(1), access(2), chmod(2), chown(2), readlink(2), utime(2), capabilities(7), symlink(7)

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