管道的问题(写的非原子性)

天外飞客

原帖由 天外飞客 于 2007-4-25 22:38 发表

书上的一个例子:

#include <unistd.h>
#include <sys/types.h>
#include <errno.h>
#include <limits.h>
                                                           ...

```````````````````
writenum=write(pipe_fd[1],w_buf,4096);
        write(1,"completed\n",11);//在这里加上..
把4096换成4000:结果

the bytes write to pipe is 1024
completed
[root@localhost work]# child readnum is 1000
child readnum is 1000
child readnum is 1000
child readnum is 1000
child readnum is 1000
child readnum is 24
child readnum is 0
把4096 换成4100 结果:
the bytes write to pipe is 1024
child readnum is 1000
completed
[root@localhost work]# child readnum is 1000
child readnum is 1000
child readnum is 1000
child readnum is 1000
child readnum is 124
child readnum is 0

思一克

LZ到底要说明什么？

思一克

LZ看的什么书？方便说出名字吗

scutan

这是正常的啊.
向管道中写入数据时，linux将不保证写入的原子性，管道缓冲区一有空闲区域，写进程就会试图向管道写入数据。如果读进程不读走管道缓冲区中的数据，那么写操作将一直阻塞。
因为你子进程一次只读1000个字节,所以就是这样的啊.

思一克

程序哪里能说明“非原子性”？

mike_chen

1. 
   
2.    PIPE_BUF
   
3.        POSIX.1 says that write(2)s of less than PIPE_BUF bytes must be atomic:
   
4.        the  output  data  is  written  to  the  pipe as a contiguous sequence.
   
5.        Writes of more than PIPE_BUF bytes may be non-atomic:  the  kernel  may
   
6.        interleave  the  data  with  data  written by other processes.  POSIX.1
   
7.        requires PIPE_BUF to be at least 512 bytes.   (On  Linux,  PIPE_BUF  is
   
8.        4096 bytes.)  The precise semantics depend on whether the file descrip-
   
9.        tor is non-blocking (O_NONBLOCK), whether there are multiple writers to
   
10.        the pipe, and on n, the number of bytes to be written:
    
11. 
    
12.        O_NONBLOCK disabled, n <= PIPE_BUF
    
13.               All  n bytes are written atomically; write(2) may block if there
    
14.               is not room for n bytes to be written immediately
    
15. 
    
16.        O_NONBLOCK enabled, n <= PIPE_BUF
    
17.               If there is room to write n bytes to  the  pipe,  then  write(2)
    
18.               succeeds  immediately,  writing  all n bytes; otherwise write(2)
    
19.               fails, with errno set to EAGAIN.
    
20. 
    
21.        O_NONBLOCK disabled, n > PIPE_BUF
    
22.               The write is non-atomic: the  data  given  to  write(2)  may  be
    
23.               interleaved with write(2)s by other process; the write(2) blocks
    
24.               until n bytes have been written.
    
25. 
    
26.        O_NONBLOCK enabled, n > PIPE_BUF
    
27.               If the pipe is full, then write(2)  fails,  with  errno  set  to
    
28.               EAGAIN.   Otherwise,  from  1 to n bytes may be written (i.e., a
    
29.               "partial write" may occur; the caller should  check  the  return
    
30.               value  from  write(2)  to see how many bytes were actually writ-
    
31.               ten), and these bytes may be interleaved with  writes  by  other
    
32.               processes.
    

复制代码

思一克

说出来什么书。

天外飞客

这本书:
http://book.chinaunix.net/showbook.php?id=98925
第198页..

天外飞客

它上面解释错了.
我想知道为什么是这样的~~

思一克

书上自己不明白乱写的很多。

例子中的程序正好说明write pipe （< 4096 bytes)是原子的。没有地方说明不是原子的呀