文件系统的其他操作

文件系统除了基本的I/O操作外，还有很多特征和性质，比如文件读、写、执行权限，文件所有者、所属组等等。

1. 函数stat、fstat、fstatat、lstat

1.1 定义:

(位于 /usr/include/sys/stat.h,四个函数成功返回0,出错返回-1)

/* Get file attributes for FILE and put them in BUF.  */int stat (const char *__restrict __file,struct stat *__restrict__buf);/* Get file attributes for the file, device, pipe, or socket   that file descriptor FD is open on and put them in BUF.  */int fstat (int __fd, struct stat *__buf);/* 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.  */int fstatat (int __fd, const char *__restrict __file, struct stat *__restrict __buf, int __flag);/* 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);

1.2 使用:

stat函数的第一个参数为文件的带路径名，stat函数返回此文件的属性-struct stat ，并传递给第二个参数 buf.

fstat函数获得在文件描述符fd上打开文件的属性-struct stat,并传递给第二个参数buf.

fstatat函数为一个相对于当前打开目录(__fd参数指向)的路径名返回文件属性.
lstat函数获取文件属性–struct stat，并传递给参数buf.

1.2.1 结构体stat的定义:

(位于/usr/bits/stat.h，文件权限和文件类型位于st_mode中,由于兼容32位和64位平台原因结构体中多了很多宏)

struct stat  {    __dev_t st_dev;     /* Device.  */#ifndef __x86_64__    unsigned short int __pad1;#endif#if defined __x86_64__ || !defined __USE_FILE_OFFSET64    __ino_t st_ino;     /* File serial number.  */#else    __ino_t __st_ino;           /* 32bit file serial number.    */#endif#ifndef __x86_64__    __mode_t st_mode;           /* File mode.  */    __nlink_t st_nlink;         /* Link count.  */#else    __nlink_t st_nlink;     /* Link count.  */    __mode_t st_mode;       /* File mode.  */#endif    __uid_t st_uid;     /* User ID of the file's owner. */    __gid_t st_gid;     /* Group ID of the file's group.*/#ifdef __x86_64__    int __pad0;#endif    __dev_t st_rdev;        /* Device number, if device.  */#ifndef __x86_64__    unsigned short int __pad2;#endif#if defined __x86_64__ || !defined __USE_FILE_OFFSET64    __off_t st_size;            /* Size of file, in bytes.  */#else    __off64_t st_size;          /* Size of file, in bytes.  */#endif    __blksize_t st_blksize; /* Optimal block size for I/O.  */#if defined __x86_64__  || !defined __USE_FILE_OFFSET64    __blkcnt_t st_blocks;       /* Number 512-byte blocks allocated. */#else    __blkcnt64_t st_blocks;     /* Number 512-byte blocks allocated. */#endif#ifdef __USE_XOPEN2K8    /* Nanosecond resolution timestamps are stored in a format       equivalent to 'struct timespec'.  This is the type used       whenever possible but the Unix namespace rules do not allow the       identifier 'timespec' to appear in the <sys/stat.h> header.       Therefore we have to handle the use of this header in strictly       standard-compliant sources special.  */    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#else    __time_t st_atime;          /* Time of last access.  */    __syscall_ulong_t st_atimensec; /* Nscecs of last access.  */    __time_t st_mtime;          /* Time of last modification.  */    __syscall_ulong_t st_mtimensec; /* Nsecs of last modification.  */    __time_t st_ctime;          /* Time of last status change.  */    __syscall_ulong_t st_ctimensec; /* Nsecs of last status change.  */#endif#ifdef __x86_64__    __syscall_slong_t __glibc_reserved[3];#else# ifndef __USE_FILE_OFFSET64    unsigned long int __glibc_reserved4;    unsigned long int __glibc_reserved5;# else    __ino64_t st_ino;           /* File serial number.  */# endif#endif  };

2. 文件类型

文件类型包含在stat结构的st_mode成员中。

文件类型 宏 S_ISREG() 普通文件 S_ISDIR() 目录文件 S_ISCHR() 字符特殊文件 S_ISBLK() 块特殊文件 S_ISFIFO() 管道或FIFO S_IFLINK() 符号链接 S_ISSOCK() 套接字

2.1 打印文件类型demo

fileflags.c

#include "apue.h"int main(int argc, char *argv[]){    int i;    struct stat buf;    char *ptr;    for(i=1; i<argc;++i){        printf("%s: ",argv[i]);        if (lstat(argv[i], &buf )<0){            err_ret("lstat error");            continue;        }        if (S_ISREG(buf.st_mode))            ptr = "regular";        else if (S_ISDIR(buf.st_mode))                ptr = "directory";        else if ( S_ISCHR(buf.st_mode))                ptr = "character special";        else if (S_ISBLK(buf.st_mode))                ptr = "block special";        else if (S_ISFIFO(buf.st_mode))                ptr = "fifo";        else if( S_ISLNK(buf.st_mode))                ptr = "symbolic link";        else if (S_ISSOCK(buf.st_mode))                ptr = "socket";        else            ptr="** unkown mode **";        printf("%s\n", ptr);    }    exit(0);}

4. 文件访问权限

st_mode值也包含了对文件的访问权限位，每个文件有9个访问权限位。

st_mode屏蔽 含义 S_IRUSR 用户读 S_IWUSR 用户写 S_IXUSR 用户执行 S_IRGRP 组读 S_IWGRP 组写 S_IXGRP 组执行 S_IROTH 其他读 S_IWOTH 其他写 S_IXOTH 其他执行

5. 函数access和faccessat

用于验证实际用户能否访问给定的文件

5.1 定义

(两个函数的返回值:成功,返回0.出错返回 -1)

 #include <unistd.h>int access(const char *path, int amode);int faccessat(int fd, const char *path, int amode, int flag);

faccessat与access函数在下面2中情况下是相同的：一种是path参数为绝对路径，另一种是fd参数取值位AT_FDCWD而path参数为相对路径,否则,faccessat计算相对于打开目录(fd参数指向)的path

5.1.2 参数mode取值说明

mode 说明 R_OK 测试读权限 W_OK 测试写权限 X_OK 测试执行权限

### 5.2 access函数的使用demo
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