linux 有名管道（FIFO）

﹏ヽ暗。殇╰゛Y 2022-05-16 22:07 242阅读 0赞

无名管道应用的一个重大限制是它没有名字，因此，只能用于具有亲缘关系的进程间通信，在有名管道（named pipe或FIFO）提出后，该限制得到了克服。FIFO不同于管道之处在于它提供一个路径名与之关联，以FIFO的文件形式存在于文件系统中。这样，即使与FIFO的创建进程不存在亲缘关系的进程，只要可以访问该路径，就能够彼此通过FIFO相互通信（能够访问该路径的进程以及FIFO的创建进程之间），因此，通过FIFO不相关的进程也能交换数据。值得注意的是，FIFO严格遵循先进先出（first in first out），对管道及FIFO的读总是从开始处返回数据，对它们的写则把数据添加到末尾。它们不支持诸如lseek()等文件定位操作。  
管道的缓冲区是有限的（管道制存在于内存中，在管道创建时，为缓冲区分配一个页面大小）  
管道所传送的是无格式字节流，这就要求管道的读出方和写入方必须事先约定好数据的格式，比如多少字节算作一个消息（或命令、或记录）等等

FIFO往往都是多个写进程，一个读进程。可以参考我之前的博客[http://blog.csdn.net/firefoxbug/article/details/7358715][http_blog.csdn.net_firefoxbug_article_details_7358715]

## FIFO的打开规则： ##

1.  如果当前打开操作是为读而打开FIFO时，若已经有相应进程为写而打开该FIFO，则当前打开操作将成功返回；否则，可能阻塞直到有相应进程为写而打开该FIFO（当前打开操作设置了阻塞标志）；或者，成功返回（当前打开操作没有设置阻塞标志）。
2.  如果当前打开操作是为写而打开FIFO时，如果已经有相应进程为读而打开该FIFO，则当前打开操作将成功返回；否则，可能阻塞直到有相应进程为读而打开该FIFO（当前打开操作设置了阻塞标志）；或者，返回ENXIO错误（当前打开操作没有设置阻塞标志）。

总之就是一句话，一旦设置了阻塞标志，调用mkfifo建立好之后，那么管道的两端读写必须分别打开，有任何一方未打开，则在调用open的时候就阻塞。

## 从FIFO中读取数据： ##

约定：如果一个进程为了从FIFO中读取数据而阻塞打开FIFO，那么称该进程内的读操作为设置了阻塞标志的读操作。（意思就是我现在要打开一个有名管道来读数据！）

如果有进程写打开FIFO，且当前FIFO内没有数据(可以理解为管道的两端都建立好了，但是写端还没开始写数据！)

1.  则对于设置了阻塞标志的读操作来说，将一直阻塞（就是block住了，等待数据。它并不消耗CPU资源，这种进程的同步方式对CPU而言是非常有效率的。）
2.  对于没有设置阻塞标志读操作来说则返回-1，当前errno值为EAGAIN，提醒以后再试。

对于设置了阻塞标志的读操作说（见上面的约定）  
造成阻塞的原因有两种

1.  FIFO内有数据，但有其它进程在读这些数据（对于各个读进程而言，这根有名管道是临界资源，大家得互相谦让，不能一起用。）
2.  FIFO内没有数据。解阻塞的原因则是FIFO中有新的数据写入，不论信写入数据量的大小，也不论读操作请求多少数据量。

读打开的阻塞标志只对本进程第一个读操作施加作用，如果本进程内有多个读操作序列，则在第一个读操作被唤醒并完成读操作后，其它将要执行的读操作将不再阻塞，即使在执行读操作时，FIFO中没有数据也一样,此时，读操作返回0。

注：如果FIFO中有数据，则设置了阻塞标志的读操作不会因为FIFO中的字节数小于请求读的字节数而阻塞，此时，读操作会返回FIFO中现有的数据量。

## 向FIFO中写入数据： ##

约定：如果一个进程为了向FIFO中写入数据而阻塞打开FIFO，那么称该进程内的写操作为设置了阻塞标志的写操作。（意思就是我现在要打开一个有名管道来写数据！）

对于设置了阻塞标志的写操作：

1.  当要写入的数据量不大于PIPE\_BUF时，linux将保证写入的原子性。如果此时管道空闲缓冲区不足以容纳要写入的字节数，则进入睡眠，直到当缓冲区中能够容纳要写入的字节数时，才开始进行一次性写操作。（PIPE\_BUF ==>> /usr/include/linux/limits.h）
2.  当要写入的数据量大于PIPE\_BUF时，linux将不再保证写入的原子性。FIFO缓冲区一有空闲区域，写进程就会试图向管道写入数据，写操作在写完所有请求写的数据后返回。

对于没有设置阻塞标志的写操作：

1.  当要写入的数据量大于PIPE\_BUF时，linux将不再保证写入的原子性。在写满所有FIFO空闲缓冲区后，写操作返回。
2.  当要写入的数据量不大于PIPE\_BUF时，linux将保证写入的原子性。如果当前FIFO空闲缓冲区能够容纳请求写入的字节数，写完后成功返回；如果当前FIFO空闲缓冲区不能够容纳请求写入的字节数，则返回EAGAIN错误，提醒以后再写；

简单描述下上面设置了阻塞标志的逻辑  
设置了阻塞标志

if (buf_to_write <= PIPE_BUF) 		//写入的数据量不大于PIPE_BUF时
 then
 	if ( buf_to_write > system_buf_left )	//保证写入的原子性,要么一次性把buf_to_write全都写完，要么一个字节都不写！
 	then
 		block ;
 		until ( buf_to_write <= system_buf_left );
 		goto write ;
 	else
 		write ;
 	fi
 else
 	write ; //不管怎样，就是不断写，知道把缓冲区写满了才阻塞
 fi

### 管道写端 pipe\_read.c ###

<table style="margin:5px 0px 10px; padding:0px; border:0px; font-size:13px; vertical-align:baseline; line-height:1.4em; background-color:rgb(255,255,255); border-collapse:collapse"> 
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 </div> </td> 
 <td style="margin:0px; padding:0px; border:0px; font-size:13px; vertical-align:top; line-height:1.4em; font-family:Monaco,'Lucida Console',monospace"> 
 <div style="margin:0px; padding:5px; border:0px; vertical-align:baseline; line-height:1.4em; color:rgb(248,248,248); background-color:rgb(12,16,33)"> 
 //pipe_read.c 
 
 
 #include &lt;stdio.h&gt; &nbsp; 
 
 #include &lt;stdlib.h&gt; &nbsp; 
 
 #include &lt;string.h&gt; &nbsp; 
 
 #include &lt;fcntl.h&gt; &nbsp; 
 
 #include &lt;limits.h&gt; &nbsp; 
 
 #include &lt;sys/types.h&gt; &nbsp; 
 
 #include &lt;sys/stat.h&gt; &nbsp; 
 &nbsp;&nbsp; 
 
 #define FIFO_NAME "/tmp/my_fifo" &nbsp; 
 
 #define BUFFER_SIZE PIPE_BUF &nbsp; 
 &nbsp;&nbsp; 
 
 int&nbsp;main 
 ( 
 )&nbsp;&nbsp; 
 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 int&nbsp;pipe_fd 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 int&nbsp;res 
 ;&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 int&nbsp;open_mode&nbsp; 
 =&nbsp;O_RDONLY 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 char&nbsp;buffer 
 [BUFFER_SIZE&nbsp; 
 +&nbsp; 
 1 
 ] 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 int&nbsp;bytes&nbsp; 
 =&nbsp; 
 0 
 ;&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/memset.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">memset</a> 
 (buffer 
 ,&nbsp; 
 '\0' 
 ,&nbsp; 
 sizeof 
 (buffer 
 ) 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">printf</a> 
 ( 
 "Process %d opeining FIFO O_RDONLY\n" 
 ,&nbsp;getpid 
 ( 
 ) 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; pipe_fd&nbsp; 
 =&nbsp;open 
 (FIFO_NAME 
 ,&nbsp;open_mode 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">printf</a> 
 ( 
 "Process %d result %d\n" 
 ,&nbsp;getpid 
 ( 
 ) 
 ,&nbsp;pipe_fd 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 if&nbsp; 
 (pipe_fd&nbsp; 
 !=&nbsp; 
 - 
 1 
 )&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 do 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; res&nbsp; 
 =&nbsp;read 
 (pipe_fd 
 ,&nbsp;buffer 
 ,&nbsp;BUFFER_SIZE 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; bytes&nbsp; 
 +=&nbsp;res 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">printf</a> 
 ( 
 "%d\n" 
 ,bytes 
 ) 
 ; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 } 
 while 
 (res&nbsp; 
 &gt;&nbsp; 
 0 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; close 
 (pipe_fd 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 }&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 else&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/exit.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">exit</a> 
 (EXIT_FAILURE 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 }&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">printf</a> 
 ( 
 "Process %d finished, %d bytes read\n" 
 ,&nbsp;getpid 
 ( 
 ) 
 ,&nbsp;bytes 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/exit.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">exit</a> 
 (EXIT_SUCCESS 
 ) 
 ;&nbsp;&nbsp; 
 
 } 
 </div> </td> 
 </tr> 
 </tbody> 
</table>

### 管道读端 pipe\_write.c ###

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 </div> </td> 
 <td style="margin:0px; padding:0px; border:0px; font-size:13px; vertical-align:top; line-height:1.4em; font-family:Monaco,'Lucida Console',monospace"> 
 <div style="margin:0px; padding:5px; border:0px; vertical-align:baseline; line-height:1.4em; color:rgb(248,248,248); background-color:rgb(12,16,33)"> 
 //pipe_write.c 
 
 
 #include &lt;stdio.h&gt; &nbsp; 
 
 #include &lt;stdlib.h&gt; &nbsp; 
 
 #include &lt;string.h&gt; &nbsp; 
 
 #include &lt;fcntl.h&gt; &nbsp; 
 
 #include &lt;limits.h&gt; &nbsp; 
 
 #include &lt;sys/types.h&gt; &nbsp; 
 
 #include &lt;sys/stat.h&gt; &nbsp; 
 &nbsp;&nbsp; 
 
 #define FIFO_NAME "/tmp/my_fifo" &nbsp; 
 
 #define BUFFER_SIZE PIPE_BUF &nbsp; 
 
 #define TEN_MEG (1024 * 100) &nbsp; 
 &nbsp;&nbsp; 
 
 int&nbsp;main 
 ( 
 )&nbsp;&nbsp; 
 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 int&nbsp;pipe_fd 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 int&nbsp;res 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 int&nbsp;open_mode&nbsp; 
 =&nbsp;O_WRONLY 
 ;&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 int&nbsp;bytes&nbsp; 
 =&nbsp; 
 0 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 char&nbsp;buffer 
 [BUFFER_SIZE&nbsp; 
 +&nbsp; 
 1 
 ] 
 ;&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 if&nbsp; 
 (access 
 (FIFO_NAME 
 ,&nbsp;F_OK 
 )&nbsp; 
 ==&nbsp; 
 - 
 1 
 )&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; res&nbsp; 
 =&nbsp;mkfifo 
 (FIFO_NAME 
 ,&nbsp; 
 0777 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 if&nbsp; 
 (res&nbsp; 
 !=&nbsp; 
 0 
 )&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/fprintf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">fprintf</a> 
 (stderr 
 ,&nbsp; 
 "Could not create fifo %s\n" 
 ,&nbsp;FIFO_NAME 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/exit.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">exit</a> 
 (EXIT_FAILURE 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 }&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 }&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">printf</a> 
 ( 
 "Process %d opening FIFO O_WRONLY\n" 
 ,&nbsp;getpid 
 ( 
 ) 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; pipe_fd&nbsp; 
 =&nbsp;open 
 (FIFO_NAME 
 ,&nbsp;open_mode 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">printf</a> 
 ( 
 "Process %d result %d\n" 
 ,&nbsp;getpid 
 ( 
 ) 
 ,&nbsp;pipe_fd 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp; 
 //sleep(20); 
 &nbsp; &nbsp;&nbsp; 
 if&nbsp; 
 (pipe_fd&nbsp; 
 !=&nbsp; 
 - 
 1 
 )&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 while&nbsp; 
 (bytes&nbsp; 
 &lt;&nbsp;TEN_MEG 
 )&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; res&nbsp; 
 =&nbsp;write 
 (pipe_fd 
 ,&nbsp;buffer 
 ,&nbsp;BUFFER_SIZE 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 if&nbsp; 
 (res&nbsp; 
 ==&nbsp; 
 - 
 1 
 )&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/fprintf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">fprintf</a> 
 (stderr 
 ,&nbsp; 
 "Write error on pipe\n" 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/exit.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">exit</a> 
 (EXIT_FAILURE 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 }&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; bytes&nbsp; 
 +=&nbsp;res 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">printf</a> 
 ( 
 "%d\n" 
 ,bytes 
 ) 
 ; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 }&nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp; close 
 (pipe_fd 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 }&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 else&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 { &nbsp;&nbsp; 
 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/exit.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">exit</a> 
 (EXIT_FAILURE 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 }&nbsp;&nbsp; 
 &nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">printf</a> 
 ( 
 "Process %d finish\n" 
 ,&nbsp;getpid 
 ( 
 ) 
 ) 
 ;&nbsp;&nbsp; 
 &nbsp; &nbsp;&nbsp; 
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/exit.html" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; color:rgb(51,51,51); outline:none; line-height:1.4em" rel="nofollow">exit</a> 
 (EXIT_SUCCESS 
 ) 
 ;&nbsp;&nbsp; 
 
 } 
 </div> </td> 
 </tr> 
 </tbody> 
</table>

[http_blog.csdn.net_firefoxbug_article_details_7358715]: http://blog.csdn.net/firefoxbug/article/details/7358715