Fcntl function

adopt fcntl Function to set the blocking property of the file
Set to block ： fcntl(fd, F_SETFL, 0);
Set to non blocking ： fcntl(fd, F_SETFL, O_NONBLOCK);

If it's a jam , There's no data in the pipeline ,read Will be waiting , It will not continue to run until there is data , Otherwise one Wait for
If it's non blocking ,read Function runtime , I will first check whether there is data in the pipeline , If there's data , That's right Read data in normal operation , If there is no data in the pipeline , be read The function immediately returns , Continue to run the following code

Case study ：

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    
	int fd_pipe[2];
	char buf[] = "hello world";
	pid_t pid;
	
	if (pipe(fd_pipe) < 0)
	{
    
		perror("fail to pipe");
		exit(1);
	}

	pid = fork();
	if (pid < 0)
	{
    
		perror("fail to fork");
		exit(0);
	}
	if (pid == 0)
	{
    
		while(1)
		{
    
			sleep(5);
			write(fd_pipe[1], buf, strlen(buf));
		}
	}
	else
	{
    
		// take fd_pipe[0] Set to block 
		//fcntl(fd_pipe[0], F_SETFL, 0);
		// take fd_pipe[0] Set to non blocking 
		fcntl(fd_pipe[0], F_SETFL, O_NONBLOCK);

		while(1)
		{
    
			memset(buf, 0, sizeof(buf));
			read(fd_pipe[0], buf, sizeof(buf));
			printf("buf=[%s]\n", buf);
			sleep(1);
		}
	}
	return 0;
}

Execution results ：