FIFO阻塞读写

作者: LiHui 分类: Linux C 发布时间: 2015-01-31 16:12

pipe一般都要和fork结合起来用,所以多进程都是基于一个主进程产生的,有一定的关联性,假如两个毫无相关的进程要进行进程间通信,命名管道可以实现;一个进程向FIFO写数据,另一个进程从FIFO读取数据

为了保持原子操作,可以通过阻塞方式来打开FIFO,就算只有一个读进程或者只有一个写进程存在,也会阻塞住等待对应的另一个进程的出现

打开FIFO的方式有下面几种:

open(const char *path, O_RDONLY);
open(const char *path, O_WRONLY);
open(const char *path, O_RDONLY | O_NONBLOCK);
open(const char *path, O_WRONLY | O_NONBLOCK);

如果带上O_NONBLOCK,那么表示非阻塞,这样读和写只要成功打开FIFO就返回,而不管另一面的写和读到底存在与否,这样假如读进程先打开FIFO,不管写进程有没有从FIFO写入数据,读进程打开FIFO成功就返回了,说不定根本就没读取到数据;显然假如要保持同步,理论上进行数据传输,假如读进程先打开了FIFO,如果写进程还没有写入数据,需要阻塞住等待写进程写入数据从而停止阻塞从而读出数据,假如写进程线打开了FIFO,如果读进程还没来读,也可以阻塞起来等待读进程打开了FIFO

下面就是两个独立的读写进程,来进行数据传输:

首先是写进程write.c:

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

#define MAXSIZE 20

int main()
{
    const int open_mode = O_WRONLY;
    const char *fifo_file = "/home/lihui/this_fifo";
    int fd;
    char ch[MAXSIZE];
    if (access(fifo_file, 0) == -1)
        if (mkfifo(fifo_file, 0777) == -1){
            printf("mkfifo failed!\n");            
            exit(1);
	}
    printf("Write pid: %d\n", getpid());
    printf("Open mode: %d\n", open_mode);
    printf("Input: ");
    scanf("%s", ch);

    fd = open(fifo_file, open_mode);
    if (fd == -1){
        printf("open fifo failed!\n");        
        exit(1);
    }
    if (write(fd, ch, strlen(ch)) == -1){
        printf("write fifo failed!\n");
        exit(1);
    }
    printf("Now write %d bytes!\n", strlen(ch));
    close(fd);
    return 0;
}

然后是读进程read.c

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

#define MAXSIZE 20

int main(){
    int open_mode = O_RDONLY;
    int fd;
    char buffer[MAXSIZE];
    const char *fifo_file = "/home/lihui/this_fifo";
    if (access(fifo_file, 0) == -1){
        printf("%s not found!\n", fifo_file);
        exit(1);
    }
    printf("Read pid: %d\n", getpid());
    printf("Open mode: %d\n", open_mode);
    fd = open(fifo_file, open_mode);
    if (fd == -1){
        printf("open fifo failed!\n");
        exit(1);
    }
    memset(buffer, 0, sizeof(buffer));
    if (read(fd, buffer, MAXSIZE) == -1){
        printf("read fifo failed!\n");
        exit(1);
    }
    printf("Now read: %s, %d bytes\n", buffer, strlen(buffer));
    return 0;
}

编译,执行,直接先执行读端,再来写

首先,读端会阻塞住:

[lihui@localhost ~]$ cc -o readfifo read.c 
[lihui@localhost ~]$ cc -o writefifo write.c 
[lihui@localhost ~]$ ./readfifo 
Read pid: 1668
Open mode: 0
光标

然后,执行写端:

[lihui@localhost ~]$ ./writefifo 
Write pid: 1677
Open mode: 1
Input: HelloWorld!
Now write 11 bytes!

再看读端:

[lihui@localhost ~]$ ./readfifo 
Read pid: 1668
Open mode: 0
Now read: HelloWorld!, 11 bytes

这样可以看到进程间通信,以及阻塞情况

在写程序过程中,出了2次问题:

1:写端ch先定义的字符串指针,结果coredump了,换成字符串数组就好了

2:读端的buffer没有memset,结果会出现下面结果:

[lihui@localhost ~]$ ./a.out 
Read pid: 1233
Open mode: 0
Now read: Helloworld!@, 12 bytes

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