tmpfs: temporary file system

tmpfs is an implementation of a temporary file system. It is a memory-based file system with the following advantages:

• Dynamically adjust virtual memory

• Fast reading and writing speed

Another feature of tmpfs is that it does not exist on an underlying block device, but is directly built on virtual memory. You do not need to use mkfs to format it, you can directly mount it to create a tmpfs file system. Since data is stored in memory, the data will be lost after a power outage.

In RT-Thread, there is also a ramfs. The differences between tmpfs and ramfs are shown in the following table:

characteristic	tmpfs	ramfs
Is the size fixed?	no	yes
Whether to support folder operations	yes	no
Volatile	yes	yes

Currently, tmpfs already exists as a component in RT-Thread, located at rt-thread\components\dfs\filesystems\tmpfs, and can be tailored and configured.

File Management

For the operation of tmpfs files, the application layer can directly use POSIX to access them. The access sequence diagram of the operation file is as follows:

image-20221101172133720

The operations on files are:

• open: open a file

• close: close the file

• read: read file data

• write: write data to a file

• lseek: Change the read and write pointer position when reading and writing a file

• getdents: Get directory entries

• stat: Get file status

Catalog Management

The POSIX interface can be used to manage directories. Its access sequence diagram is similar to the file access sequence diagram. The operations on the directory are:

• mkdir: Create a directory

• rmdir: Remove a directory

• opendir: open a directory

• readdir: read directory

• closedir: close directory

• dirent: read directory

• telldir: Get the read position of the directory stream

• seekdir: Set the next directory read location

• rewinddir: reset the directory read position to the beginning

MSH Commands

You can use the DFS MSH command to operate tmpfs. The commonly used MSH commands for file system operations are shown in the following table:

MSH Commands	describe
ls	Display information about files and directories
cd	Enter the specified directory
cp	Copying Files
rm	Deleting a file or directory
mv	Move or rename the file
echo	Write the specified content to the specified file. If the file exists, write it to the file. If the file does not exist, create a new file and write it to it.
cat	Display the contents of a file
pwd	Print out the current directory address
mkdir	Create a folder

How to use

The following is a sequence diagram of the application layer using the DFS virtual file system API to access tmpfs:

• Automatic file system initialization: After automatic initialization is turned on, both the virtual file system and the temporary file system will be automatically initialized without user execution.

• Use dfs_mount() at the application layer to mount the tmpfs file system.

• At the application layer, use the DFS virtual file system API to access tmpfs. For more API interfaces, see DFS Virtual File System .

image-20221102174841105

Usage Examples

Use QEMU to demonstrate the use of tmpfs, taking mounting it in the "/mnt/tmp" directory as an example.

In the rt-smart branch of the rt-thread source code, open the qemu-vexpress-a9 BSP, use menuconfig to configure tmpfs in the component, located in "RT-Thread Components → Device virtual file system", exit and save.

image-20221101155404497

Note: If multiple file systems are used in the same system, pay attention to modify the values ​​of the number of mountable file systems and the number of file system types on the same interface to support multiple file systems.

image-20221103101448013

The code for mounting the file system using dfs_mount in mnt.c is as follows:

    /* romfs 挂载在 / 下 */
    /* fatfs 挂载在 /mnt 下 */
    /* tmpfs 挂载在 /mnt/tmp 下 */
    if (dfs_mount(RT_NULL, "/mnt/tmp", "tmp", 0, NULL) != 0)
    {
        rt_kprintf("Dir /tmp mount failed!\n");
        return -1;
    }copymistakeCopy Success

After compiling with the command scons, enter qemu.batto run qemu, switch to the mnt directory and create a tmp directory (if you use fatfs for the first time, you need to format it with mkfs first)

msh />cd mnt            # 使用 cd 命令切换目录
msh /mnt>mkdir tmp      # 使用 mkdir 创建新目录
msh /mnt>ls             # 使用 ls 命令查看当前目录信息
Directory /mnt:
tmp                 <DIR>copymistakeCopy Success

Execute "ctrl+c" to exit qemu, and then execute again qemu.batto run qemu

image-20221101155816447

Use the file system's MSH commands to manipulate files in the tmpfs file system:

msh /mnt/tmp>echo "RT-Thread"         # 将字符串输出到标准输出
RT-Thread

msh /mnt/tmp>mkdir test               # 创建 test 目录
msh /mnt/tmp>echo "rtt" tmpfile.txt   # 使用 echo 命令将输入的字符串输出到指定输出位置
msh /mnt/tmp>ls                       # 使用 ls 命令查看当前目录信息
Directory /mnt/tmp:
tmpfile.txt         3
test                <DIR>

msh /mnt/tmp>cat tmpfile.txt          # 使用 cat 命令查看文件内容
rttcopymistakeCopy Success