[note] IPC traditional interprocess communication and binder interprocess communication principle

Tradition IPC

Two copy process ：

1. The message sender stores the message data to be sent into the memory buffer , Enter kernel state through system call .
2. Then kernel program allocates memory in kernel space , And open up a memory cache , call copy_from_user() Copy data from the user space memory cache to the kernel cache in kernel space .
3. When receiving data, the receiver process creates a memory buffer in its own user space .
4. Then the kernel program calls copy_to_user() Copy the data from the kernel cache to the memory cache of the receiver process .

This completes a data transmission between processes .

This traditional IPC There are two problems with communication ：
1. Poor performance , A data transfer needs to go through ： Memory cache --> Kernel cache --> Memory cache , need 2 Secondary data copy ;
2. The buffer for receiving data is provided by the data receiving process , But the receiving process doesn't know how much space is needed to store the data to be transferred , So we can only open up as much memory space as possible or call first API Receive the message header to get the small of the message body , These two methods are either a waste of space or a waste of time .

Binder IPC

Binder IPC The memory mapping involved in the mechanism is through mmap() To achieve .
mmap()： Is a memory mapping method in the operating system .
Memory mapping is simply to map a block of memory area of user space to kernel space . After the mapping relationship is established , The user's modification of this memory area can directly reflect the kernel space ; On the other hand, the kernel space modification of this area can also directly reflect the user space .
Memory mapping can reduce the number of data copies , Achieve efficient interaction between user space and kernel space .
The changes of the two spaces can be directly reflected in the mapped memory area , So as to be perceived by each other's space in time

Once complete Binder IPC The communication process is usually like this ：
1. First Binder The driver creates a data receiving buffer in kernel space ;
2. And then open a kernel cache in kernel space , Establish the mapping relationship between the kernel cache and the data receiving cache in the kernel , And the mapping relationship between the data receiving buffer and the user space address of the receiving process in the kernel ;
3. Sender process through system call copy from user() Put the data copy To the inside
Kernel cache in the kernel , Due to the memory mapping between the kernel cache and the user space of the receiving process , Therefore, it is equivalent to sending data to the user space of the receiving process , This completes a communication between processes .