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stat() - Unix, Linux System Call

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NAME

stat, fstat, lstat - get file status

SYNOPSIS

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

int stat(const char *path, struct stat *buf);
int fstat(int filedes, struct stat *buf);
int lstat(const char *path, struct stat *buf);

DESCRIPTION

These functions return information about a file. No permissions are required on the file itself, but — in the case of stat() and lstat() — execute (search) permission is required on all of the directories in path that lead to the file.

stat() stats the file pointed to by path and fills in buf.

lstat() is identical to stat(), except that if path is a symbolic link, then the link itself is stat-ed, not the file that it refers to.

fstat() is identical to stat(), except that the file to be stat-ed is specified by the file descriptor filedes.

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 blocks allocated */
    time_t    st_atime;   /* time of last access */
    time_t    st_mtime;   /* time of last modification */
    time_t    st_ctime;   /* time of last status change */
};

The st_dev field describes the device on which this file resides.

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 symlink is the length of the pathname it contains, without a trailing 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, for example, when the file has holes.)

The st_blksize field gives the "preferred" blocksize for efficient file system 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 file system types allow mounting in such a way that file accesses do not cause an update of the st_atime field. (See ‘noatime’ in mount(8).)

The field st_atime is changed by file accesses, e.g. 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, e.g. 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 POSIX macros are defined to check the file type using the st_mode field:

TagDescription
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 following flags are defined for the st_mode field:

S_IFMT0170000bitmask for the file type bitfields
S_IFSOCK0140000socket
S_IFLNK0120000symbolic link
S_IFREG0100000regular file
S_IFBLK0060000block device
S_IFDIR0040000directory
S_IFCHR0020000character device
S_IFIFO0010000FIFO
S_ISUID0004000set UID bit
S_ISGID0002000set-group-ID bit (see below)
S_ISVTX0001000sticky bit (see below)
S_IRWXU00700mask for file owner permissions
S_IRUSR00400owner has read permission
S_IWUSR00200owner has write permission
S_IXUSR00100owner has execute permission
S_IRWXG00070mask for group permissions
S_IRGRP00040group has read permission
S_IWGRP00020group has write permission
S_IXGRP00010group has execute permission
S_IRWXO00007mask for permissions for others (not in group)
S_IROTH00004others have read permission
S_IWOTH00002others have write permission
S_IXOTH00001others have execute permission

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.

LINUX NOTES

Since kernel 2.5.48, the stat structure supports nanosecond resolution for the three file timestamp fields. Glibc exposes the nanosecond component of each field using names either of the form st_atim.tv_nsec, if the _BSD_SOURCE or _SVID_SOURCE feature test macro is defined, or of the form st_atimensec, if neither of these macros is defined. On file systems that do not support sub-second timestamps, these nanosecond fields are returned with the value 0.

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.

RETURN VALUE

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

ERRORS

TagDescription
EACCES Search permission is denied for one of the directories in the path prefix of path. (See also path_resolution(2).)
EBADF filedes is bad.
EFAULT Bad address.
ELOOP Too many symbolic links encountered while traversing the path.
ENAMETOOLONG
  File name too long.
ENOENT A component of the path path does not exist, or the path is an empty string.
ENOMEM Out of memory (i.e. kernel memory).
ENOTDIR
  A component of the path is not a directory.

CONFORMING TO

These system calls conform to SVr4, 4.3BSD, POSIX.1-2001.

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.)

POSIX does not describe the S_IFMT, S_IFSOCK, S_IFLNK, S_IFREG, S_IFBLK, S_IFDIR, S_IFCHR, S_IFIFO, S_ISVTX bits, but instead demands the use of the macros S_ISDIR(), etc. 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:

hexnamelsoctaldescription
f000S_IFMT 170000mask for file type
0000  000000SCO out-of-service inode, BSD unknown type
    SVID-v2 and XPG2 have both 0 and 0100000 for ordinary file
1000S_IFIFOp|010000FIFO (named pipe)
2000S_IFCHRc020000character special (V7)
3000S_IFMPC 030000multiplexed character special (V7)
4000S_IFDIRd/040000directory (V7)
5000S_IFNAM 050000XENIX named special file
    with two subtypes, distinguished by st_rdev values 1, 2:
0001S_INSEMs000001XENIX semaphore subtype of IFNAM
0002S_INSHDm000002XENIX shared data subtype of IFNAM
6000S_IFBLKb060000block special (V7)
7000S_IFMPB 070000multiplexed block special (V7)
8000S_IFREG-100000regular (V7)
9000S_IFCMP 110000VxFS compressed
9000S_IFNWKn110000network special (HP-UX)
a000S_IFLNKl@120000symbolic link (BSD)
b000S_IFSHAD 130000Solaris shadow inode for ACL (not seen by userspace)
c000S_IFSOCKs=140000socket (BSD; also "S_IFSOC" on VxFS)
d000S_IFDOORD>150000Solaris door
e000S_IFWHTw%160000BSD whiteout (not used for inode)
0200S_ISVTX 001000‘sticky bit’: save swapped text even after use (V7)
    reserved (SVID-v2)
    On non-directories: don’t cache this file (SunOS)
    On directories: restricted deletion flag (SVID-v4.2)
0400S_ISGID 002000set-group-ID on execution (V7)
    for directories: use BSD semantics for propagation of GID
0400S_ENFMT 002000SysV file locking enforcement (shared with S_ISGID)
0800S_ISUID 004000set-user-ID on execution (V7)
0800S_CDF 004000directory is a context dependent file (HP-UX)

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

SEE ALSO


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