C小程序 - 把文件权限从八进制转换为字符串

mode可以从stat获得：
struct stat info;
stat(filename_ptr, &info);
int mode = info.st_mode; //获得的是八进制的字符串

void mode_to_letters( int mode, char str[] )
{
    strcpy( str, "----------" );           /* default=no perms */

    if ( S_ISDIR(mode) )  str[0] = 'd';    /* directory?       */
    if ( S_ISCHR(mode) )  str[0] = 'c';    /* char devices     */
    if ( S_ISBLK(mode) )  str[0] = 'b';    /* block device     */

    if ( mode & S_IRUSR ) str[1] = 'r';    /* 3 bits for user  */
    if ( mode & S_IWUSR ) str[2] = 'w';
    if ( (mode & S_ISUID) && (mode & S_IXUSR) )
    str[3] = 's';
    else if ( (mode & S_ISUID) && !(mode & S_IXUSR) )
    str[3] = 'S';
    else if ( mode & S_IXUSR )
    str[3] = 'x';

    if ( mode & S_IRGRP ) str[4] = 'r';    /* 3 bits for group */
    if ( mode & S_IWGRP ) str[5] = 'w';
    if ( (mode & S_ISGID) && (mode & S_IXGRP) )
    str[6] = 's';
    else if ( (mode & S_ISGID) && !(mode & S_IXGRP) )
    str[6] = 'S';
    else if ( mode & S_IXGRP )
    str[6] = 'x';

    if ( mode & S_IROTH ) str[7] = 'r';    /* 3 bits for other */
    if ( mode & S_IWOTH ) str[8] = 'w';
    if ( (mode & S_ISVTX) && (mode & S_IXOTH) )
    str[9] = 't';
    else if ( (mode & S_ISVTX) && !(mode & S_IXOTH) )
    str[9] = 'T';
    else if ( mode & S_IXOTH )
    str[9] = 'x';
}

类似于 S_IFDIR 这样的宏可以在 stat.h 中找到
stat.h:#define S_IFDIR  0040000