Interprocess communication

Why do processes need to communicate?

Shared data, data transmission, message notification, process control

What are the types of communication between processes?

First of all, according to the previous knowledge, the user address space between processes is independent of each other and cannot be accessed to each other. However, the kernel space is shared, so the communication between processes should pass through the kernel.

Taking Linux as an example, this paper introduces several common communication modes between Linux processes: shared memory, pipeline, message queue, semaphore and signal.

The Conduit

If you have studied linux commands, you must have seen this vertical bar.

$ ps auxf | grep mysql

The vertical line | in the above command is a pipe. Its function here is to take the output of the previous command (ps auxf) as the input of the next command (grep mysql).

It can be seen that pipeline data transmission is one-way. If we want to communicate with each other, we need at least two pipes.

The pipeline in the above example has no name, so we call it an anonymous pipeline, which will be destroyed when used up.

Another kind of pipeline is named pipeline, also known as FIFO, which means first in first out.

Before using the named pipe, you need to create it with the mkfifo command and add the name of the pipe.

$ mkfifo myPipe

The pipe name is myPipe, which also exists in the form of a file in Linux:

w-rw-r--. 1 iron2222 iron2222    0 Dec  4 22:20 myPipe

We try to write data to this pipeline:

$ echo "hello" > myPipe //Write the data in
                        //You'll find it stopped and didn't respond

Because the command can exit normally only after the data in the pipeline is read:

$ cat < myPipe 
hello

Note that you must input the command on another terminal and keep the original terminal interface unchanged. In this way, it will be obvious that the original terminal command can exit normally after executing the command.

It can be seen from here that this way of pipeline is inefficient and not suitable for frequent data exchange.

Message queue

Communication mode of message queue: process A sends messages to process B. process A can directly return the messages after putting them in the corresponding message queue. Process B can read them when needed.

The essence of a message queue is a linked list of messages stored in the kernel. The sender and receiver should agree on the data type and size of the message body. After reading, the kernel will delete the message body.

Bad news: in the process of message queue communication, there is data copy overhead between user state and kernel state.

Shared memory

The mechanism of shared memory is to take out a virtual address space and map it to the same physical memory. High reading efficiency.

The disadvantage is that if two processes write data to a physical address at the same time, it will cause overwrite.

Semaphore

In order to prevent multiple processes competing for shared resources like shared memory, a protection mechanism is needed so that shared resources can only be accessed by one process at any time.

Semaphore is actually an integer counter, which is mainly used to realize mutual exclusion and synchronization between processes, rather than to cache the data of inter process communication.

The initial value is 1. If process A wants to use it, it becomes 0. If process B wants to use it, it becomes - 1, blocking process B.

It is equivalent to a flag that tells the process whether the shared resource is free.

signal

Signal is generally used for the communication between processes under abnormal conditions. It is an asynchronous signal, that is, a number.

For example, in Linux, dozens of signals are provided for different events, which represent different meanings:

$ kill -l
 1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL	 5) SIGTRAP
 6) SIGABRT	 7) SIGBUS	 8) SIGFPE	 9) SIGKILL	10) SIGUSR1
11) SIGSEGV	12) SIGUSR2	13) SIGPIPE	14) SIGALRM	15) SIGTERM
16) SIGSTKFLT	17) SIGCHLD	18) SIGCONT	19) SIGSTOP	20) SIGTSTP
21) SIGTTIN	22) SIGTTOU	23) SIGURG	24) SIGXCPU	25) SIGXFSZ
26) SIGVTALRM	27) SIGPROF	28) SIGWINCH	29) SIGIO	30) SIGPWR
31) SIGSYS	34) SIGRTMIN	35) SIGRTMIN+1	36) SIGRTMIN+2	37) SIGRTMIN+3
38) SIGRTMIN+4	39) SIGRTMIN+5	40) SIGRTMIN+6	41) SIGRTMIN+7	42) SIGRTMIN+8
43) SIGRTMIN+9	44) SIGRTMIN+10	45) SIGRTMIN+11	46) SIGRTMIN+12	47) SIGRTMIN+13
48) SIGRTMIN+14	49) SIGRTMIN+15	50) SIGRTMAX-14	51) SIGRTMAX-13	52) SIGRTMAX-12
53) SIGRTMAX-11	54) SIGRTMAX-10	55) SIGRTMAX-9	56) SIGRTMAX-8	57) SIGRTMAX-7
58) SIGRTMAX-6	59) SIGRTMAX-5	60) SIGRTMAX-4	61) SIGRTMAX-3	62) SIGRTMAX-2
63) SIGRTMAX-1	64) SIGRTMAX	

Take a common example:

In the shell terminal, we can use some shortcut keys to send signals to the process:

• Ctrl+C will generate SIGINT signal to terminate the process;
• Ctrl+Z will generate SIGTSTP signal to stop the process, but the process is not over.

You can also use the kill Command, but only if you know the PID number of the process.

• kill -9 1050, which means sending 9) SIGKILL signal to the process with PID 1050 and ending the process immediately.

The only asynchronous communication mechanism. The process needs to set corresponding listening and processing for the signal and perform relevant operations.

Socket

If you want to communicate with processes on different hosts across the network, you need to use Socket.

Summary and comparison of various communication modes