fstatat (2) - Linux Manuals

fstatat: get file status

Command to display fstatat manual in Linux: $ man 2 fstatat

NAME

stat, fstat, lstat, fstatat - get file status

SYNOPSIS

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int stat(const char *pathname, struct stat *statbuf);
int fstat(int fd, struct stat *statbuf);
int lstat(const char *pathname, struct stat *statbuf);

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

int fstatat(int dirfd, const char *pathname, struct stat *statbuf,
            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
    || /* Since glibc 2.10: */ _POSIX_C_SOURCE >= 200112L

fstatat():

Since glibc 2.10:: _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:: _ATFILE_SOURCE

DESCRIPTION

These functions return information about a file, in the buffer pointed to by statbuf. 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 the link 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.

The stat structure

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;        /* File type and mode */
    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;     /* Block size 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 fields in the stat structure are as follows:

st_dev: This 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.)
st_ino: This field contains the file's inode number.
st_mode: This field contains the file type and mode. See inode(7) for further information.
st_nlink: This field contains the number of hard links to the file.
st_uid: This field contains the user ID of the owner of the file.
st_gid: This field contains the ID of the group owner of the file.
st_rdev: This field describes the device that this file (inode) represents.
st_size: This 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.
st_blksize: This field gives the "preferred" block size for efficient filesystem I/O.
st_blocks: This field indicates the number of blocks allocated to the file, in 512-byte units. (This may be smaller than st_size/512 when the file has holes.)
st_atime: This is the time of the last access of file data.
st_mtime: This is the time of last modification of file data.
st_ctime: This is the file's last status change timestamp (time of last change to the inode).

For further information on the above fields, see inode(7).

fstatat()

The fstatat() system call is a more general interface for accessing file information which can still provide exactly the behavior of each of stat(), lstat(), and fstat().

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() and lstat() 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() and lstat()).

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). In this case, dirfd can refer to any type of file, not just a directory, and the behavior of fstatat() is similar to that of fstat(). If dirfd is AT_FDCWD, the call operates on the current working 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). Since Linux 4.14, also don't instantiate a nonexistent name in an on-demand directory such as used for automounter indirect maps. This flag has no effect if the mount point has already been mounted over.
: Both stat() and lstat() act as though AT_NO_AUTOMOUNT was set.
: The AT_NO_AUTOMOUNT can be used in tools that scan directories to prevent mass-automounting of a directory of automount points.
: 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 not a valid open file descriptor.
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 is a dangling symbolic link.
ENOENT: 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.
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.)

NOTES

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 suitable feature test macros are defined. Nanosecond timestamps were standardized in POSIX.1-2008, and, starting with version 2.12, glibc 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. Up to and including glibc 2.19, the definitions of the nanoseconds components are also defined if _BSD_SOURCE or _SVID_SOURCE is defined. If none of the aforementioned macros are defined, then the nanosecond values are exposed with names of the form st_atimensec.

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().

EXAMPLES

The following program calls lstat() and displays selected fields in the returned stat structure.

#include <sys/types.h> #include <sys/stat.h> #include <stdint.h> #include <time.h> #include <stdio.h> #include <stdlib.h> #include <sys/sysmacros.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 (lstat(argv[1], &sb) == -1) {
        perror("lstat");
        exit(EXIT_FAILURE);
    }

    printf("ID of containing device:  [%jx,%jx]\n",
            (uintmax_t) major(sb.st_dev),
            (uintmax_t) minor(sb.st_dev));

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: %ju\n", (uintmax_t) sb.st_ino);

printf("Mode: %jo (octal)\n",
(uintmax_t) sb.st_mode);

    printf("Link count:               %ju\n", (uintmax_t) sb.st_nlink);
    printf("Ownership:                UID=%ju   GID=%ju\n",
            (uintmax_t) sb.st_uid, (uintmax_t) sb.st_gid);

    printf("Preferred I/O block size: %jd bytes\n",
            (intmax_t) sb.st_blksize);
    printf("File size:                %jd bytes\n",
            (intmax_t) sb.st_size);
    printf("Blocks allocated:         %jd\n",
            (intmax_t) 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); }

COLOPHON

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