stat~~~访问文件状态的利器

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

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

lstat():

_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.10: */ _POSIX_C_SOURCE >= 200112L

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 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 file system I/O */

    blkcnt_t  st_blocks;  /* number of 512B 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  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 file system I/O. (Writing to a file in smaller chunks may cause an inefficient read-modify-rewrite.)

Not all of the Linux file systems implement all of the time fields. Some file system 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 POSIX macros are defined to check the file type using the st_mode field:

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:

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.

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 path. (See also path_resolution(7).)

EBADF

fd is bad.

EFAULT

Bad address.

ELOOP

Too many symbolic links encountered while traversing the path.

ENAMETOOLONG

path is too long.

ENOENT

A component of path does not exist, or path is an empty string.

ENOMEM

Out of memory (i.e., kernel memory).

ENOTDIR

A component of the path prefix of  path is not a directory.

EOVERFLOW

path 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.

Conforming To

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

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.-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(), etc. 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.

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 of the form st_atim.tv_nsec if the _BSD_SOURCE or _SVID_SOURCEfeature test macro is defined. These fields are specified in POSIX.1-2008, and, starting with version 2.12, glibc also exposes these field 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. On file systems that do not support subsecond timestamps, the nanosecond fields are returned with the value 0.

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.

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().

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);

}

See Also

access(2), chmod(2), chown(2), fstatat(2), readlink(2), utime(2), capabilities(7), symlink(7)

Referenced By

acl(5), amd.conf(5), attr(5), collectd.conf(5), csh(1), dpns_readdirx(3), dump(8), dvips(1), euidaccess(3), ev(3),explain(1), explain(3), explain_stat(3), explain_stat_or_die(3), fakeroot-tcp(1), fallocate(2), fastrm(1),fcdircacheloadfile(3), fgetln(3), file(1), find(1), fsync(2), ftok(3), fts(3), ftw(3), futimesat(2), getfilecon(3),getseuserbyname(3), getxattr(2), glob(3), guestfish(1), guestfs(3), inotify(7), lam_rfposix(2), lfc_readdirxc(3),libinn(3), libinnhist(3), librrd(3), libssh2_sftp_stat_ex(3), link(2), listxattr(2), llvm-ar(1), lslk(8), lsof(8), makedev(3),mgp(1), mirrordir(1), mkdir(2), mkfifo(3), mtree(8), muttrc(5), nfslogsum(8), nhfsstone(8), obsolete(2),path_resolution(2), pax(1), perl561delta(1), perl56delta(1), perlfunc(1), pipe(7), pivot_root(2), proc(5), rdist(1),rdup(1), rdup(8), readline(3), removexattr(2), samefile(1), sec(1), setfilecon(3), setmode(3), setxattr(2), sh(1), spax(1),spu_create(2), spufs(2), spufs(7), star(1), stat(1), statfs(2), strmode(3), sudo(8), syscalls(2), time(7), umask(2),ustat(2), utimensat(2), watchdog(8), xbiff(1), xdvi(1), xfs_db(8), xfs_io(8), zfs(8), zshmodules(1)