9. Process communication

Process of communication ： Information exchange between processes .

• The memory address spaces owned by processes are independent of each other
• To ensure safety , One process cannot directly access the address space of another process

One 、 Shared memory

reason ：
Two processes cannot directly access each other's address space

Handle ：
The operating system will allocate a shared space for the two processes , The communication between the two processes will be completed through the shared space .

Be careful ：

• The two processes' access to the shared space must be mutually exclusive
• Mutually exclusive access is realized through the tools provided by the operating system
• The operating system is only responsible for providing shared space and synchronization and mutual exclusion tools （ Such as P、V operation ）

classification ：

1. Sharing based on data structure ：
   • Only fixed structures can be stored , If the length is 10 Array of
   • This sharing method is slow 、 More restrictions , Is a low-level means of communication
2. Store based sharing
   • Draw a shared memory area in memory , The form of data 、 The storage location is controlled by the process , Not the operating system .
   • Faster sharing , It is an advanced means of communication

Two 、 Pipeline communication

“ The Conduit ” It refers to a shared file used to connect the read-write process , also called pipe file .

The essence ：
Open a fixed size buffer in memory .

Be careful ：

1. Pipes can only be used Half duplex communication , Only one-way transmission can be realized in a certain period of time . If you want to realize two-way simultaneous communication , You need to set up two pipes .
2. Each process should Mutually exclusive To access the pipeline .
3. Data is written to the pipeline as a character stream , When Pipeline full when , Write progress Of write() The system call will be Blocking , Wait for the reading and writing process to take away the data . When the reading process takes all the data , The pipe becomes empty , here Reading process Of read() The system call will be Blocking .
4. If it's not full , You are not allowed to read . If you don't have time to read , You are not allowed to write .
5. Once the data in the pipeline is read out , Just abandoned from the pipeline , That means There can be at most one read process , Otherwise, wrong data may be read .

3、 ... and 、 The messaging

Data exchange between processes to Formatted message In units of . Process communication passed OS Provided “ Send a message / receive messages ” Two The original language Data exchange .