Unix学习笔记-----Unix编程练习实例-----lstat()&& stat 结构体详解

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man 2 lstat  
STAT(2) Linux Programmer’s Manual STAT(2)  
  
  
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  
  
  
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 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 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:  
  
  
S\_IFMT 0170000 bit mask for the file type bit fields  
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  
S\_ISUID 0004000 set UID bit  
S\_ISGID 0002000 set-group-ID bit (see below)  
S\_ISVTX 0001000 sticky bit (see below)  
S\_IRWXU 00700 mask for file owner permissions  
S\_IRUSR 00400 owner has read permission  
S\_IWUSR 00200 owner has write permission  
S\_IXUSR 00100 owner has execute permission  
S\_IRWXG 00070 mask for group permissions  
S\_IRGRP 00040 group has read permission  
S\_IWGRP 00020 group has write permission  
S\_IXGRP 00010 group has execute permission  
S\_IRWXO 00007 mask for permissions for others (not in group)  
S\_IROTH 00004 others have read permission  
S\_IWOTH 00002 others have write permission  
S\_IXOTH 00001 others 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.  
  
  
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  
File name 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  
(stat()) path refers to a file whose size cannot be represented in the type off\_t. This can occur when an application compiled on a 32-bit platform without  
\-D\_FILE\_OFFSET\_BITS=64 calls stat() on a file whose size exceeds (2<<31)-1 bits.  
  
  
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:  
  
  
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 userspace)  
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 non-directories: 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  
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.  
  
  
On Linux, lstat() will generally not trigger automounter action, whereas stat() will.  
  
  
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\_SUCCESS);  
\}  
  
  
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)  
  
  
COLOPHON  
This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.ker-  
nel.org/doc/man-pages/.  
  
  
Linux 2009-04-21 STAT(2)