Table of contents
Why do we need inter-process communication and what can inter-process communication do?
1. Inter-process communication method
The difference between named pipes and unnamed pipes
Communication between threads
global variable
process characteristics
The system will allocate 0-4g virtual space for each process, 0-3g (user space) is unique to each process, and 3g-4g (kernel space) is shared by all processes.
When the CPU schedules a process, it will allocate a time slice (a few milliseconds to more than ten milliseconds) to the process. When the time slice is used up, the CPU will schedule other processes to realize the rotation of the process, thereby realizing multi-tasking operations.
Why do we need inter-process communication and what can inter-process communication do?
1).Data transmission
A process needs to send its data to another process;
2). Resource sharing
Share the same resource among multiple processes;
3).Notification event
A process needs to send a message to another process or group of processes informing them that some event has occurred;
4). Process control
Some processes want to completely control the execution of another process (such as the Debug process), and the control process hopes to be able to intercept all operations of another process and know its state changes in time.
1. Inter-process communication method
Early interprocess communication: unnamed pipes, named pipes, signals
system V IPC objects: shared memory, semaphore sets, message queues
BSD：socket
2. Nameless pipes
features
a. Anonymous can only be used for correspondence with kinship.
b. Half-duplex communication, with a fixed read end and write end.
(Simplex: one direction, TV, radio.
Half-duplex: It can be bidirectional, but can only be in one direction at the same time.
Full-duplex: can be two-way at the same time. )
c. The unnamed pipe can be regarded as a special file. For its reading and writing, file IO such as read and write can be used.
d. The unnamed pipe communicates with the file descriptor. When the pipe is established, two file descriptors are automatically created, fd[0] reads the pipe, and fd[1] writes the pipe.
functional interface
int pipe(int fd[2])
Function: create an unnamed pipe
Parameters: file descriptor fd[0]: read end fd[1] : write end
Return value: success 0
fail -1
Precautions
- When there is no data in the pipeline, the read operation is blocked; if there is no data in the pipeline, the write end is closed, and the read operation returns immediately;
- When the pipeline is full of 64k, the write is blocked, the 4k space is read, and the write continues;
- Only when the read end of the pipeline exists, it is meaningful to write data to the pipeline, which will cause the pipeline to break and the kernel to send the SIGPIPE signal.
practise
The parent-child process realizes the communication, the parent process cyclically inputs data from the terminal, and the child process cyclically prints, and ends when quit is input.
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
int main(int argc, char const *argv[])
{
int fd[2] = {0};//定义两个文件描述符
char buf[32] ="";
int pfd=pipe(fd);
if (pfd < 0)//1.先创建无名管道
{
perror("pipe err");
return -1;
}
printf("fd[0]:%d fd[1]:%d\n", fd[0],fd[1]);//读端、写端
pid_t pid = fork();//2.后创建父子进程，拿到同一个管道
if(pid<0)
{
perror("pipe err");
return -1;
}
else if (pid == 0)
{
while (1)
{
read(fd[0], buf, 32);
if (!strcmp(buf, "quit"))
break;
printf("buf:%s\n", buf);
}
}
else{
while (1)
{
scanf("%s", buf);
write(fd[1], buf, 32);
if (!strcmp(buf, "quit"))
break;
}
wait(NULL);//回收所有的子进程
}
close(fd[0]);
close(fd[1]);
return 0;
}
operation result
3. Famous pipeline
features
- Allows unrelated processes to communicate.
- Named pipes can be indicated by pathnames and are visible on the file system, but their contents are stored in memory.
- Well-known pipes for file IO operations
- FIFO
- The lseek() operation is not supported.
functional interface
int mkfifo(const char *filename,mode_t mode);
Function: Create a famous pipeline
Parameters: filename: name of the pipe file
mode: permission
Return value: Success: 0
Fail: -1, and set errno number
Note how errors are handled:
If the error is file exist, pay attention to add judgment, such as: if ( errno == EEXIST )
Precautions
- Write-only mode, write blocking until another process opens the read
- Read-only mode, read blocking until another process opens the write
- Readable and writable, if there is no data in the pipeline, read blocking.
4. Summary
The difference between named pipes and unnamed pipes
| nameless pipe |
famous pipeline |
|
| scenes to be used |
kinship |
unrelated process |
| features |
half duplex fixed read and write Treated as a special file, operated through file IO |
Visible in the file system, data stored in memory file IO operation Does not support lseek() operation, follows first-in first-out |
| function |
pipe() Direct read, write |
mkfifo() Open first, then read and write |
| Read and write characteristics |
When the unnamed pipe has no data, read blocking When the unnamed pipe is full of 64k, write blocking The read end is closed, continuing to write data will cause the pipe to break |
Write-only mode, write blocking until another process opens the read Read-only mode, read blocking until another process opens the write Readable and writable, when there is no data in the pipeline, read blocking |