26、 File system API (device sharing between applications; directory and file API)

One 、 Sharing between devices and Applications

terminal

1、 Multiple processes print in parallel , How to ensure no confusion

2、 Multiple processes read in parallel , There will be competition

GPU（CUDA）

1、 Every CUDA Applications are a series CUDA Of API call

2、 All drivers realize scheduling （ Isolation ）

Disks need to support data persistence

1、 Application data

（1） Executable files and dynamic link libraries

2、 User data

3、 system data

（1）manpage

（2） The system configuration

Two 、 file system ： Virtual disk

The goal is ：

1、 Provide reasonable API Enable multiple applications to share data

2、 Provide some isolation , Make malicious / Mistakes don't run away in large numbers

The storage device （ Byte sequence ） Virtualization of

1、 disk （I/O equipment ）= A sequence of bytes that can be read and written

2、 Virtual disk （ file ）= A dynamic byte sequence that can be read and written

（1） Name Management

        The name of the virtual disk , Retrieval and convenience

（2） Data management

        Random access （ Reading and writing ）

3、 ... and 、 Virtual disk ： Command management

Locality of information ： Set the virtual disk （ file ） Organized into a hierarchy

1、 Directory tree

Root of directory

1、Windows

C:\Program Files\

D:\User

2、Unix/Linux

（1） There is only one root

/home/xxx

3、 Splicing of directory tree

Unix： Allow any directory ” mount “ A directory tree represented by a device

（1） You can mount the device anywhere

mount system call 
int mount(const char *source, const char *target,
          const char *filesystemtype, unsigned long mountflags,
          const void *data);

mount /dev/sdb /mnt

umount mnt

Four 、Linux Startup process

Linux-minimal Running on the ‘initramfs’ Pattern

1、Linux-minimal Start process

export PATH=/bin
busybox mknod /dev/sda b 8 0
busybox mkdir -p /newroot
busybox mount -t ext2 /dev/sda /newroot    # take /dev/sda The device is mounted on /newroot
exec busybox switch_root /newroot/ /etc/init

2、 File mount

file = Virtual disk on disk

Mount the file = Virtual disk created on virtual disk

（1）Linux Treatment mode

        Create a loopback（ Loop ） equipment

        The device driver drives the device read/write Translated into documents read/write

        Use lsblk Check... In the system block devices（strace）

        strace View the process of mounting

ioctl(3, LOOP_CTL_GET_FREE)
ioctl(4, LOOP_SET_FD, 3)

5、 ... and 、 Directory management

1、mkdir

（1） Create a directory

（2） You can set permissions

2、rmdir

（1） Delete an empty directory

（2） No, ” Recursive delete “ System call

        rm -rf Traverse delete

6、 ... and 、 Hard links

demand ： There are multiple versions of the same runtime

Using a link can avoid a copy

7、 ... and 、 Soft link

Soft link ： Store a jump prompt in the file

1、 Soft link is also a file

（1） When referencing this document , Find another file

（2） Can cross file systems , You can link to a directory