使用 fstreamtellg 读取流的一部分直到结束

发布于 2024-08-18 06:19:30 字数 710 浏览 6 评论 0原文

我有这个简单的代码，需要获取正在写入的大型日志文件的一部分。在某些时候，它会存储从返回的当前位置流位置开始 = istream::tellg(); 方法。稍后，代码必须从流中读取缓冲区，从头到尾。代码大致如下：

streampos start = my_stream.tellg();

... // do some stuff with logging

streampos end = my_stream.tellg();
const streamsize size_to_read = (end - start);
char *buf = new char[size_to_read];

lock (m_logReadLock);
{
    my_stream.flush();
    my_stream.seekg(start);
    my_stream.read(buf, size_to_read);
    size_read = my_stream->gcount();
}
unlock (m_logReadLock);

我观察到的效果是 size_read 比 size_to_read 小，并且流设置了 eof 标志。结束指针不应该准确指定流结束的位置并且 read() 方法返回准确的数据量吗？没关系，我可以通过检查 eof 标志来解决这个问题。然而，谁能解释一下这种效应呢？

谢谢。

原文

I have this simple code that needs to get a chunk of a large log file that is being written into. At some point it stores the current location returned from
streampos start = istream::tellg();
method.
Later on the code has to read from the stream a buffer from the start till the end. The code is approximately like this:

streampos start = my_stream.tellg();

... // do some stuff with logging

streampos end = my_stream.tellg();
const streamsize size_to_read = (end - start);
char *buf = new char[size_to_read];

lock (m_logReadLock);
{
    my_stream.flush();
    my_stream.seekg(start);
    my_stream.read(buf, size_to_read);
    size_read = my_stream->gcount();
}
unlock (m_logReadLock);

The effect that I'm observing is that size_read is smaller than size_to_read and the stream has its eof flag set. Shouldn't the end pointer specify exactly where the stream ends and read() method return that exact amount of data?
It is fine, I can work round it by checking the eof flag.
However, can anyone provide the explanation for this effect?

Thanks.

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