sigpipe圖片_sigpipe_sigpipe信号(6)

分析一下：现在的情况是有两个子进程在对管道进行阻塞写入各68k，即每个子进程完全写入68k才返回，而父进程对管道进行阻塞读取，每次读取4k，打印每4k中的最后一个字符，如果没有数据到达就阻塞等待，如果管道剩余数据不足4k，read 很可能返回 < 4k，但因为我们写入68k是4k整数倍，故不存在这种情况。需要注意的是是边写边读，因为前面说过管道的容量只有64k，当管道被写满时子进程就阻塞等待父进程读取后再写入。由上面输出可以看出B进程先写入64k的B，然后A进程写入68k的A之后B进程接着写完最后4K的B，然后write返回。由A进程write完毕输出的提示可知此时A进程已经写完成了，但父进程还没读取A完毕，当两个子进程全部写完退出时关闭写端文件描述符，则父进程read就会返回0，退出while循环。可以得出结论：当多个进程对管道进行写入，且一次性写入数据量大于PIPE_BUF时，则不能保证写入的原子性，即可能数据是穿插着的。man 手册的解释如下：

O_NONBLOCK disabled, n > PIPE_BUF

The write is nonatomic: the data given to write(2) may be interleaved with write(2)s by other process; thewrite(2) blocks until n bytes have been written.

注意我们这里设定了size=68k，则写端不能设置成非阻塞，因为PIPE_BUF只有4k，不能一次性写入68k，如果此时管道是满的（64k)，则只能返回-1并置错误码为EAGAIN，且一个字符也不写入，若不是满的，则写入的字节数是不确定的，需要检查write的返回值，而且这些字节很可能也与其他进程写入的数据穿插着。读端也不能设置为非阻塞，如果此时尚未有数据写入（管道为空）则返回-1并置错误码为EAGAIN，如果有部分数据已经写入，则读取的数据字节数也是不确定的，需要检查read的返回值。总之测试4种不同情形下的情况也应设置不同的条件。

       O_NONBLOCK disabled, n <= PIPE_BUF
              All n bytes are written atomically; write(2) may block if there is not  room  for  n
              bytes to be written immediately

       O_NONBLOCK enabled, n <= PIPE_BUF
              If  there  is room to write n bytes to the pipe, then write(2) succeeds immediately,
              writing all n bytes; otherwise write(2) fails, with errno set to EAGAIN.

       O_NONBLOCK disabled, n > PIPE_BUF
              The write is non-atomic:  the  data  given  to  write(2)  may  be  interleaved  with
              write(2)s by other process; the write(2) blocks until n bytes have been written.

       O_NONBLOCK enabled, n > PIPE_BUF
              If the pipe is full, then write(2) fails, with errno set to EAGAIN.  Otherwise, from
              1 to n bytes may be written (i.e., a "partial write" may occur;  the  caller  should
              check  the  return value from write(2) to see how many bytes were actually written),
              and these bytes may be interleaved with writes by other processes.

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