Android用readdir()函数读取exfat格式U盘文件返回的d_type为空的问题

野性酷女 2022-10-13 05:09 238阅读 0赞

### 一、前言 ###

`Android4.4`上面，用`readdir()`函数读取`exfat`格式的`U`盘文件后，在返回的`dirent`结构体中，有关读取文件的类型`d_type`值为空。

### 二、问题分析 ###

这个有个可能是`exfat`文件在挂载时，相应的结构体没有填充导致。

### 三、解决方法 ###

有两种解决方法，一种是用stat()函数，另一种从根源解决。

1.  `stat()`函数  
    具体方式参照`/frameworks/av/media/libmedia/MediaScanner.cpp`中对位置文件类型处理方式进行处理。代码如下：

MediaScanResult MediaScanner::doProcessDirectoryEntry(
            char *path, int pathRemaining, MediaScannerClient &client, bool noMedia,
            struct dirent* entry, char* fileSpot) { 
        struct stat statbuf;
        const char* name = entry->d_name;
    
        // ignore "." and ".."
        if (name[0] == '.' && (name[1] == 0 || (name[1] == '.' && name[2] == 0))) { 
            return MEDIA_SCAN_RESULT_SKIPPED;
        }
    
        int nameLength = strlen(name);
        if (nameLength + 1 > pathRemaining) { 
            // path too long!
            return MEDIA_SCAN_RESULT_SKIPPED;
        }
        strcpy(fileSpot, name);
    
    	/* 用stat()函数区分文件类型 */
        int type = entry->d_type;
        if (type == DT_UNKNOWN) { 
            // If the type is unknown, stat() the file instead.
            // This is sometimes necessary when accessing NFS mounted filesystems, but
            // could be needed in other cases well.
            if (stat(path, &statbuf) == 0) { 
                if (S_ISREG(statbuf.st_mode)) { 
                    type = DT_REG;
                } else if (S_ISDIR(statbuf.st_mode)) { 
                    type = DT_DIR;
                }
            } else { 
                ALOGD("stat() failed for %s: %s", path, strerror(errno) );
            }
        }
        if (type == DT_DIR) { 
            bool childNoMedia = noMedia;
            // set noMedia flag on directories with a name that starts with '.'
            // for example, the Mac ".Trashes" directory
            if (name[0] == '.')
                childNoMedia = true;
    
            // report the directory to the client
            if (stat(path, &statbuf) == 0) { 
                status_t status = client.scanFile(path, statbuf.st_mtime, 0,
                        true /*isDirectory*/, childNoMedia);
                if (status) { 
                    return MEDIA_SCAN_RESULT_ERROR;
                }
            }
    
            // and now process its contents
            strcat(fileSpot, "/");
            MediaScanResult result = doProcessDirectory(path, pathRemaining - nameLength - 1,
                    client, childNoMedia);
            if (result == MEDIA_SCAN_RESULT_ERROR) { 
                return MEDIA_SCAN_RESULT_ERROR;
            }
        } else if (type == DT_REG) { 
            stat(path, &statbuf);
            status_t status = client.scanFile(path, statbuf.st_mtime, statbuf.st_size,
                    false /*isDirectory*/, noMedia);
            if (status) { 
                return MEDIA_SCAN_RESULT_ERROR;
            }
        }
    
        return MEDIA_SCAN_RESULT_OK;
    }

用`stat()`函数的缺点是每次调用`readdir()`后，当`d_type`为空时，需要调用`stat()`函数进行判断，这样一来既繁琐又耗费时间。

1.  修改`fuse.c`文件  
    修改fuse.c文件可以从根源上解决，目前[GitHub\[点击进入\]][GitHub]上已经修改过，可以解决`d_type`为空的问题，需要修改的文件为`lib/fuse.c`，`commit`号为：`bdd2d4110fbc6d2059eb699efad2cca4a7eacccb`，代码如下：

static int fill_dir_plus(void *dh_, const char *name, const struct stat *statp,
    			 off_t off, enum fuse_fill_dir_flags flags)
    { 
    	struct fuse_dh *dh = (struct fuse_dh *) dh_;
    	struct fuse_entry_param e = { 
    		/* ino=0 tells the kernel to ignore readdirplus stat info */
    		.ino = 0,
    	};
    	struct fuse *f = dh->fuse;
    	int res;
    
    	if ((flags & ~FUSE_FILL_DIR_PLUS) != 0) { 
    		dh->error = -EIO;
    		return 1;
    	}
    
    	if (statp && (flags & FUSE_FILL_DIR_PLUS)) { 
    		e.attr = *statp;
    
    		if (!is_dot_or_dotdot(name)) { 
    			res = do_lookup(f, dh->nodeid, name, &e);
    			if (res) { 
    				dh->error = res;
    				return 1;
    			}
    		}
    	} else { 
    		e.attr.st_ino = FUSE_UNKNOWN_INO;
    		/* 处理d_type为空的情况 -- start */
    		if (statp) { 
    			e.attr.st_mode = statp->st_mode;
    			if (f->conf.use_ino)
    				e.attr.st_ino = statp->st_ino;
    		}
    		/* 处理d_type为空的情况 -- end */
    		if (!f->conf.use_ino && f->conf.readdir_ino) { 
    			e.attr.st_ino = (ino_t)
    				lookup_nodeid(f, dh->nodeid, name);
    		}
    	}
    
    	if (off) { 
    		size_t newlen;
    
    		if (dh->filled) { 
    			dh->error = -EIO;
    			return 1;
    		}
    
    		if (dh->first) { 
    			dh->error = -EIO;
    			return 1;
    		}
    		if (extend_contents(dh, dh->needlen) == -1)
    			return 1;
    
    		newlen = dh->len +
    			fuse_add_direntry_plus(dh->req, dh->contents + dh->len,
    					       dh->needlen - dh->len, name,
    					       &e, off);
    		if (newlen > dh->needlen)
    			return 1;
    		dh->len = newlen;
    	} else { 
    		dh->filled = 1;
    
    		if (fuse_add_direntry_to_dh(dh, name, &e.attr) == -1)
    			return 1;
    	}
    
    	return 0;
    }

修改完后局部编译模块，将生成的`bin`文件放置`Android`系统的`/system/bin`文件下即可。

[GitHub]: https://github.com/libfuse/libfuse/commit/bdd2d4110fbc6d2059eb699efad2cca4a7eacccb?branch=bdd2d4110fbc6d2059eb699efad2cca4a7eacccb&diff=split