linux-famous pipe

1. The basics of named pipes

Named pipes are also called FIFO files. They are visible in the file system and can be read and written like other files!

Features of named pipes:

1. When the writing process writes data to the pipeline, if there is no process to read the data, the writing process will be blocked
2. When reading data in the pipeline, if there is no data, the reading process will be blocked
3. When the writing process is blocked, there is a reading process to read the data, then the writing process returns to normal

4. When the reading process is blocked, if the writing process writes data, the reading process will read the data, and then execute the following code normally

2. shell

2.1 Create a pipeline:

$ mkfifo /tmp/testpipe

2.2 Communication between command lines

cat < /tmp/testpipe &
echo "hegaozhi" > /tmp/testpipe

2.3  read

#########################################################################
# File Name: reader.sh
# Author: hegaozhi
# mail: [email protected]
# Created Time: 2020年01月03日 星期五 11时05分19秒
#########################################################################
#!/bin/bash
# filename: reader.sh
# 逐行读取管道中的内容
pipe=/tmp/testpipe
trap "rm -f $pipe" EXIT
if [[ ! -p $pipe ]]; then
    mkfifo $pipe
fi
while true
do
    if read line <$pipe; then
        if [[ "$line" == 'quit' ]]; then
            break
        else
            echo $line
        fi
    fi
done
echo "Stop reader...."

2.4 write

#########################################################################
# File Name: writer.sh
# Author: hegaozhi
# mail: [email protected]
# Created Time: 2020年01月03日 星期五 11时07分18秒
#########################################################################
#!/bin/bash
# writer.sh
# 把当前进程的pid写到管道
pipe=/tmp/testpipe
if [[ ! -p $pipe ]]; then
    echo "Reader not running"
    exit 1
fi

while true
do
    if [[ "$1" ]]; then
        echo "$1" >$pipe
    else
        echo "Hello from $$" >$pipe
    fi
    sleep 1
done

3. C language

3.1 reader.c

/*************************************************************************
    > File Name: reader.c
    > Author: hegaozhi
    > Mail: [email protected] 
    > Created Time: 2020年01月03日 星期五 11时25分07秒
 ************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <string.h>
#include <fcntl.h>
#include <sys/select.h>
#include <sys/stat.h>
#include <errno.h>
#include <unistd.h>

#define FIFO "/tmp/testpipe"
int main()
{
	int rfd;
	char str[32];
	int i ;
	printf("读程序\n");
	
	if(access(FIFO,F_OK))
    {
        int ret  = mkfifo(FIFO,0777);
        if(ret == -1)
        {
            printf("Create FIFO failed!\n");
            return -1;
        }
    }
    
	rfd = open( FIFO , O_RDWR);  //O_RDONLY 只读且阻塞 
	if(rfd==-1)
	{
	    printf("Open file error\n");
	    exit(1);
	}
	
	while(1)
	{
		memset(str,0,sizeof(str));
		if((i=read(rfd,str,sizeof(str)))>0)
			printf("read:%s\n",str);
	}
	close(rfd);
}

3.2 write

/*************************************************************************
    > File Name: writer.c
    > Author: hegaozhi
    > Mail: [email protected] 
    > Created Time: 2020年01月03日 星期五 11时25分49秒
 ************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <string.h>
 #include <unistd.h>
 
#define FIFO "/tmp/testpipe"

int main(int argc,char *argv[])
{
    if(argc < 2)
        return 0;
    int wfd;
    char str[32];
    char *str1 = "\n";
    if(access(FIFO,F_OK))
    {
        int ret  =mkfifo(FIFO,0777);
        if(ret == -1)
        {
            printf("Create FIFO failed!\n");
            return -1;
        }
    }

    sscanf(argv[1], "%s",str);
    strcat(str,str1);
    wfd  =open(FIFO, O_WRONLY|O_TRUNC);    //可读可写，存在数据则清空; O_WRONLY 只写且阻塞
	 
    if(wfd<=0)
        return 0;
        
    write(wfd, str, sizeof(str));
	 
    close(wfd);
    exit(0);
}