本文详细介绍了Linux系统中文件和目录的操作方法，包括文件状态获取（stat、fstat、lstat）、权限设置（chmod、fchmod）、文件所有者变更（chown、fchown、lchown）、文件截断（truncate、ftruncate）等实用函数的使用方法。同时，还探讨了符号链接的创建和解析，以及文件时间戳的调整。
文件和目录
stat,fstat,lstat函数.原型:<sys/stat.h> //获取有关文件的struct stat信息.
int stat(const char *restrict pathname,struct stat *restrict buf); //返回与此命名文件有关信息结构
int fstat(int filedes,struct stat *buf); //获取已经在描述符上打开文件的有关信息.和stat类似.
int lstat(const char *restrict pathname,struct stat *restrict buf); //返回该符号链接的有关信息,而不是由符号链接引用文件的信息.
先看以一数据结构(来自/usr/include/asm/stat.h)
#ifdef __i386__
struct stat { //display file or file system status.
unsigned long st_dev; //设备号
unsigned long st_ino; //i_node节点号
unsigned short st_mode; //文件类型和模式(权限)
unsigned short st_nlink; //链接数
unsigned short st_uid; //拥有者用户ID
unsigned short st_gid; //拥有者的组ID
unsigned long st_rdev; //存储文件数据的设备号
unsigned long st_size; //文件大小,专为普通文件
unsigned long st_blksize; //完整的I/O块的大小
unsigned long st_blocks; //磁盘上的块数号
unsigned long st_atime; //最后访问时间
unsigned long st_atime_nsec; //
unsigned long st_mtime; //最后创建时间
unsigned long st_mtime_nsec;
unsigned long st_ctime; //最后修改时间
unsigned long st_ctime_nsec;
unsigned long __unused4;
unsigned long __unused5;
};
看了这个结构,以上三个函数含义就很明显了.
access函数.
当open打开一文件时,内核内部对文件有效用户id和有效组ID进行测试.有时,进程也希望按其实际用户ID和实际组ID测试其访问能力.
原型:<unistd.h>
int access(const char* pathname,int mode); //成功返回0,出错返回-1.
access mode 来自于usr/include/unistd.h. R_OK,W_OK,X_OK,F_OK.
umask函数.设置文件模式屏蔽字,并返回旧的值.
mode_t umask(mode_t cmask); //返回以前的值.
注意修改后,当前进程再创建文件就会根据新的掩码,屏蔽一些值.与chmod设置不同.
chmod和fchmod函数.修改文件权限.
int chmod(const char* pathname,mode_t mode);
int fchmod(int filedes,mode_t mode); //两个函数成功返回0,出错返回-1
这两个函数很简单,第一个是指定文件操作,没有打开.第二个对已经打开的文件描述符进行设置权限.
为了改变一个文件权限,进程有效用户ID必须是文件拥有者ID,或者是超级用户.
chown,fchown和lchown函数.这三个函数也很简单,改变文件的拥有者用户ID和组ID.
int chown(const char *pathname,uid_t owner,gid_t group);
int fchown(int filedes,uid_t owner,gid_t group);
int lchown(const char* pathname,uid_t owner,gid_t group); //三个函数成功返回0,出错返回-1
如果owner和group任意一个为-1,则对应的ID不变.
truncate和ftruncate函数.截短文件.原型:
int truncate(const char *pathname,off_t length);
int ftruncate(int filedes,off_t length); //两个函数成功返回0,出错返回-1
link,unlink,remove和rename函数.文件系统的一些操作函数.原型:
int link(const char *existingpath,const char *newpath); //成功返回0,出错返回-1.newpath存在则出错.注意这里是硬链接.
int unlink(const char *pathname); //删除目录项,并将引用文件的链接数减1.出错返回-1,并对原文件不作任何修改.
int remove(const char* pathname); //对于文件与unlink相同,对于目录和rmdir功能相同.
int rename(const char* oldname,const char* newname);//根据oldname和newname作不同反应,或出错.
符号链接.
symlink可创建一个符号链接.
int symlink(const char *actualpath,const char *sympath); //成功返回 0,出错返回-1
actualpath是新的目录项,actualpath和sympath可以不在同一个文件系统.
readlink提供一个返回符号链接本身的内容,也就是一个跟踪目录.
ssize_t readlink(const char *restrict pathname,char *restrict buf,size_t bufsize);//成功返回读到的字节数,出错返回-1
返回内容在buf内部,这个函数集合了,open,read和close功能.
文件时间
一个文件有三个时间,创建时间,修改时间,和访问时间.
通过utime可以修改访问时间和修改时间.原型:<utime.h>
int utime(const char* pathname,const struct utimebuf *times);
struct utimbuf //来自/usr/include/utime.h
{ //这个结构提供修改文件时间的集合
__time_t actime;
__time_t modtime;
}
mkdir和rmdir函数.
int mkdir(const char*pathname,mode_t mode);//创建一个空目录
int rmdir(const char *pathname); //删除一个空目录,不能删除有文件目录
读目录
对于某个目录具有访问权限的任一用户都可以读该目录,但是,为了防止文件系统产生混乱,只有内核才能写目录.这里又有一个新头文件出现<dirent.h>
DIR* opendir(const char *pathname);//成功返回指针,出错返回NULL
struct dirent *readdir(DIR *dp);
void rewinddir(DIR *dp);
int closedir(DIR *dp);
long telldir(DIR *dp);
void seekdir(DIR *dp,long loc);
struct dirent //来自/usr/include/bits/dirent.h
{
#ifndef __USE_FILE_OFFSET64
__ino_t d_ino;
__off_t d_off;
#else
__ino64_t d_ino; //i_node节点号
__off64_t d_off;
#endif
unsigned short int d_reclen;
unsigned char d_type;
char d_name[256]; /*We must not include limits.h*///目录名
}
DIR结构是内核用的,对用户不公开使用.
chdir,fchdir和getcwd函数.
int chdir(const char *pathname);
int fchdir(int filedes);
以上两上函数用来改为当前工作目录,分别用char *和int,即路径名和路径描述符来更改当前目录.
char *getcwd(char *buf,size_t size); //返回值成功则返回buf,出错返回NULL
该函数获取完整的目录名.
/* Copyright (C) 1991, 1992, 1995-2004, 2005, 2006, 2007
Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA. */
/*
* POSIX Standard: 5.6 File Characteristics <sys/stat.h>
*/
#ifndef _SYS_STAT_H
#define _SYS_STAT_H 1
#include <features.h> //一些特性宏
#include <bits/types.h> /* For __mode_t and __dev_t. */
#if defined __USE_XOPEN || defined __USE_XOPEN2K || defined __USE_MISC \
|| defined __USE_ATFILE
# if defined __USE_XOPEN || defined __USE_XOPEN2K
# define __need_time_t
# endif
# if defined __USE_MISC || defined __USE_ATFILE
# define __need_timespec
# endif
# include <time.h> /* For time_t resp. timespec. */
#endif
#if defined __USE_XOPEN || defined __USE_XOPEN2K
/* The Single Unix specification says that some more types are
available here. */
# ifndef __dev_t_defined //定义一些必备的基本数据类型
typedef __dev_t dev_t;
# define __dev_t_defined
# endif
# ifndef __gid_t_defined
typedef __gid_t gid_t;
# define __gid_t_defined
# endif
# ifndef __ino_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __ino_t ino_t;
# else
typedef __ino64_t ino_t;
# endif
# define __ino_t_defined
# endif
# ifndef __mode_t_defined
typedef __mode_t mode_t;
# define __mode_t_defined
# endif
# ifndef __nlink_t_defined
typedef __nlink_t nlink_t;
# define __nlink_t_defined
# endif
# ifndef __off_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __off_t off_t;
# else
typedef __off64_t off_t;
# endif
# define __off_t_defined
# endif
# ifndef __uid_t_defined
typedef __uid_t uid_t;
# define __uid_t_defined
# endif
#endif /* X/Open */
#ifdef __USE_UNIX98
# ifndef __blkcnt_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __blkcnt_t blkcnt_t;
# else
typedef __blkcnt64_t blkcnt_t;
# endif
# define __blkcnt_t_defined
# endif
# ifndef __blksize_t_defined
typedef __blksize_t blksize_t;
# define __blksize_t_defined
# endif
#endif /* Unix98 */
__BEGIN_DECLS
#include <bits/stat.h>
#if defined __USE_BSD || defined __USE_MISC || defined __USE_XOPEN
# define S_IFMT __S_IFMT //这里定义一些文件属性判断宏,可以直接用
# define S_IFDIR __S_IFDIR
# define S_IFCHR __S_IFCHR
# define S_IFBLK __S_IFBLK
# define S_IFREG __S_IFREG
# ifdef __S_IFIFO
# define S_IFIFO __S_IFIFO
# endif
# ifdef __S_IFLNK
# define S_IFLNK __S_IFLNK
# endif
# if (defined __USE_BSD || defined __USE_MISC || defined __USE_UNIX98) \
&& defined __S_IFSOCK
# define S_IFSOCK __S_IFSOCK
# endif
#endif
/* Test macros for file types. */
#define __S_ISTYPE(mode, mask) (((mode) & __S_IFMT) == (mask))
#define S_ISDIR(mode) __S_ISTYPE((mode), __S_IFDIR) //这里定义一些文件属性判断宏,可以直接用
#define S_ISCHR(mode) __S_ISTYPE((mode), __S_IFCHR)
#define S_ISBLK(mode) __S_ISTYPE((mode), __S_IFBLK)
#define S_ISREG(mode) __S_ISTYPE((mode), __S_IFREG)
#ifdef __S_IFIFO
# define S_ISFIFO(mode) __S_ISTYPE((mode), __S_IFIFO)
#endif
#ifdef __S_IFLNK
# define S_ISLNK(mode) __S_ISTYPE((mode), __S_IFLNK)
#endif
#if defined __USE_BSD && !defined __S_IFLNK
# define S_ISLNK(mode) 0
#endif
#if (defined __USE_BSD || defined __USE_UNIX98) \
&& defined __S_IFSOCK
# define S_ISSOCK(mode) __S_ISTYPE((mode), __S_IFSOCK)
#endif
/* These are from POSIX.1b. If the objects are not implemented using separate
distinct file types, the macros always will evaluate to zero. Unlike the
other S_* macros the following three take a pointer to a `struct stat'
object as the argument. */
#ifdef __USE_POSIX199309
# define S_TYPEISMQ(buf) __S_TYPEISMQ(buf)
# define S_TYPEISSEM(buf) __S_TYPEISSEM(buf)
# define S_TYPEISSHM(buf) __S_TYPEISSHM(buf)
#endif
/* Protection bits. */
#define S_ISUID __S_ISUID /* Set user ID on execution. */
#define S_ISGID __S_ISGID /* Set group ID on execution. */
#if defined __USE_BSD || defined __USE_MISC || defined __USE_XOPEN
/* Save swapped text after use (sticky bit). This is pretty well obsolete. */
# define S_ISVTX __S_ISVTX
#endif
#define S_IRUSR __S_IREAD /* Read by owner. */ //判断可读
#define S_IWUSR __S_IWRITE /* Write by owner. */ //可写
#define S_IXUSR __S_IEXEC /* Execute by owner. */ //可执行
/* Read, write, and execute by owner. */
#define S_IRWXU (__S_IREAD|__S_IWRITE|__S_IEXEC)
#if defined __USE_MISC && defined __USE_BSD
# define S_IREAD S_IRUSR
# define S_IWRITE S_IWUSR
# define S_IEXEC S_IXUSR
#endif
#define S_IRGRP (S_IRUSR >> 3) /* Read by group. */
#define S_IWGRP (S_IWUSR >> 3) /* Write by group. */
#define S_IXGRP (S_IXUSR >> 3) /* Execute by group. */
/* Read, write, and execute by group. */
#define S_IRWXG (S_IRWXU >> 3)
#define S_IROTH (S_IRGRP >> 3) /* Read by others. */
#define S_IWOTH (S_IWGRP >> 3) /* Write by others. */
#define S_IXOTH (S_IXGRP >> 3) /* Execute by others. */
/* Read, write, and execute by others. */
#define S_IRWXO (S_IRWXG >> 3)
#ifdef __USE_BSD
/* Macros for common mode bit masks. */
# define ACCESSPERMS (S_IRWXU|S_IRWXG|S_IRWXO) /* 0777 */
# define ALLPERMS (S_ISUID|S_ISGID|S_ISVTX|S_IRWXU|S_IRWXG|S_IRWXO)/* 07777 */
# define DEFFILEMODE (S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH)/* 0666*/
# define S_BLKSIZE 512 /* Block size for `st_blocks'. */
#endif
#ifndef __USE_FILE_OFFSET64
/* Get file attributes for FILE and put them in BUF. */
extern int stat (__const char *__restrict __file,
struct stat *__restrict __buf) __THROW __nonnull ((1, 2));
/* Get file attributes for the file, device, pipe, or socket
that file descriptor FD is open on and put them in BUF. */
extern int fstat (int __fd, struct stat *__buf) __THROW __nonnull ((2));
#else
# ifdef __REDIRECT_NTH
extern int __REDIRECT_NTH (stat, (__const char *__restrict __file,
struct stat *__restrict __buf), stat64)
__nonnull ((1, 2));
extern int __REDIRECT_NTH (fstat, (int __fd, struct stat *__buf), fstat64)
__nonnull ((2));
# else
# define stat stat64
# define fstat fstat64
# endif
#endif
#ifdef __USE_LARGEFILE64
extern int stat64 (__const char *__restrict __file,
struct stat64 *__restrict __buf) __THROW __nonnull ((1, 2));
extern int fstat64 (int __fd, struct stat64 *__buf) __THROW __nonnull ((2));
#endif
#ifdef __USE_ATFILE
/* Similar to stat, get the attributes for FILE and put them in BUF.
Relative path names are interpreted relative to FD unless FD is
AT_FDCWD. */
# ifndef __USE_FILE_OFFSET64
extern int fstatat (int __fd, __const char *__restrict __file,
struct stat *__restrict __buf, int __flag)
__THROW __nonnull ((2, 3));
# else
# ifdef __REDIRECT_NTH
extern int __REDIRECT_NTH (fstatat, (int __fd, __const char *__restrict __file,
struct stat *__restrict __buf,
int __flag),
fstatat64) __nonnull ((2, 3));
# else
# define fstatat fstatat64
# endif
# endif
extern int fstatat64 (int __fd, __const char *__restrict __file,
struct stat64 *__restrict __buf, int __flag)
__THROW __nonnull ((2, 3));
#endif
#if defined __USE_BSD || defined __USE_XOPEN_EXTENDED
# ifndef __USE_FILE_OFFSET64
/* Get file attributes about FILE and put them in BUF.
If FILE is a symbolic link, do not follow it. */
extern int lstat (__const char *__restrict __file,
struct stat *__restrict __buf) __THROW __nonnull ((1, 2));
# else
# ifdef __REDIRECT_NTH
extern int __REDIRECT_NTH (lstat,
(__const char *__restrict __file,
struct stat *__restrict __buf), lstat64)
__nonnull ((1, 2));
# else
# define lstat lstat64
# endif
# endif
# ifdef __USE_LARGEFILE64
extern int lstat64 (__const char *__restrict __file,
struct stat64 *__restrict __buf)
__THROW __nonnull ((1, 2));
# endif
#endif
/* Set file access permissions for FILE to MODE.
If FILE is a symbolic link, this affects its target instead. */
extern int chmod (__const char *__file, __mode_t __mode)
__THROW __nonnull ((1));
#ifdef __USE_BSD
/* Set file access permissions for FILE to MODE.
If FILE is a symbolic link, this affects the link itself
rather than its target. */
extern int lchmod (__const char *__file, __mode_t __mode)
__THROW __nonnull ((1));
#endif
/* Set file access permissions of the file FD is open on to MODE. */
#if defined __USE_BSD || defined __USE_XOPEN_EXTENDED
extern int fchmod (int __fd, __mode_t __mode) __THROW;
#endif
#ifdef __USE_ATFILE
/* Set file access permissions of FILE relative to
the directory FD is open on. */
extern int fchmodat (int __fd, __const char *__file, __mode_t mode, int __flag)
__THROW __nonnull ((2)) __wur;
#endif /* Use ATFILE. */
/* Set the file creation mask of the current process to MASK,
and return the old creation mask. */
extern __mode_t umask (__mode_t __mask) __THROW;
#ifdef __USE_GNU
/* Get the current `umask' value without changing it.
This function is only available under the GNU Hurd. */
extern __mode_t getumask (void) __THROW;
#endif
/* Create a new directory named PATH, with permission bits MODE. */
extern int mkdir (__const char *__path, __mode_t __mode)
__THROW __nonnull ((1));
#ifdef __USE_ATFILE
/* Like mkdir, create a new directory with permission bits MODE. But
interpret relative PATH names relative to the directory associated
with FD. */
extern int mkdirat (int __fd, __const char *__path, __mode_t __mode)
__THROW __nonnull ((2));
#endif
/* Create a device file named PATH, with permission and special bits MODE
and device number DEV (which can be constructed from major and minor
device numbers with the `makedev' macro above). */
#if defined __USE_MISC || defined __USE_BSD || defined __USE_XOPEN_EXTENDED
extern int mknod (__const char *__path, __mode_t __mode, __dev_t __dev)
__THROW __nonnull ((1));
#endif
#ifdef __USE_ATFILE
/* Like mknod, create a new device file with permission bits MODE and
device number DEV. But interpret relative PATH names relative to
the directory associated with FD. */
extern int mknodat (int __fd, __const char *__path, __mode_t __mode,
__dev_t __dev) __THROW __nonnull ((2));
#endif
/* Create a new FIFO named PATH, with permission bits MODE. */
extern int mkfifo (__const char *__path, __mode_t __mode)
__THROW __nonnull ((1));
#ifdef __USE_ATFILE
/* Like mkfifo, create a new FIFO with permission bits MODE. But
interpret relative PATH names relative to the directory associated
with FD. */
extern int mkfifoat (int __fd, __const char *__path, __mode_t __mode)
__THROW __nonnull ((2));
#endif
#ifdef __USE_ATFILE
/* Set file access and modification times relative to directory file
descriptor. */
extern int utimensat (int __fd, __const char *__path,
__const struct timespec __times[2],
int __flags)
__THROW __nonnull ((2));
#endif
#ifdef __USE_GNU
/* XXX This will change to the macro for the next 2008 POSIX revision. */
/* Set file access and modification times of the file associated with FD. */
extern int futimens (int __fd, __const struct timespec __times[2]) __THROW;
#endif
/* To allow the `struct stat' structure and the file type `mode_t'
bits to vary without changing shared library major version number,
the `stat' family of functions and `mknod' are in fact inline
wrappers around calls to `xstat', `fxstat', `lxstat', and `xmknod',
which all take a leading version-number argument designating the
data structure and bits used. <bits/stat.h> defines _STAT_VER with
the version number corresponding to `struct stat' as defined in
that file; and _MKNOD_VER with the version number corresponding to
the S_IF* macros defined therein. It is arranged that when not
inlined these function are always statically linked; that way a
dynamically-linked executable always encodes the version number
corresponding to the data structures it uses, so the `x' functions
in the shared library can adapt without needing to recompile all
callers. */
#ifndef _STAT_VER
# define _STAT_VER 0
#endif
#ifndef _MKNOD_VER
# define _MKNOD_VER 0
#endif
/* Wrappers for stat and mknod system calls. */
#ifndef __USE_FILE_OFFSET64
extern int __fxstat (int __ver, int __fildes, struct stat *__stat_buf)
__THROW __nonnull ((3));
extern int __xstat (int __ver, __const char *__filename,
struct stat *__stat_buf) __THROW __nonnull ((2, 3));
extern int __lxstat (int __ver, __const char *__filename,
struct stat *__stat_buf) __THROW __nonnull ((2, 3));
extern int __fxstatat (int __ver, int __fildes, __const char *__filename,
struct stat *__stat_buf, int __flag)
__THROW __nonnull ((3, 4));
#else
# ifdef __REDIRECT_NTH
extern int __REDIRECT_NTH (__fxstat, (int __ver, int __fildes,
struct stat *__stat_buf), __fxstat64)
__nonnull ((3));
extern int __REDIRECT_NTH (__xstat, (int __ver, __const char *__filename,
struct stat *__stat_buf), __xstat64)
__nonnull ((2, 3));
extern int __REDIRECT_NTH (__lxstat, (int __ver, __const char *__filename,
struct stat *__stat_buf), __lxstat64)
__nonnull ((2, 3));
extern int __REDIRECT_NTH (__fxstatat, (int __ver, int __fildes,
__const char *__filename,
struct stat *__stat_buf, int __flag),
__fxstatat64) __nonnull ((3, 4));
# else
# define __fxstat __fxstat64
# define __xstat __xstat64
# define __lxstat __lxstat64
# endif
#endif
#endif /* sys/stat.h */
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