Ifstream open（）在参数是目录时不设置错误位 [英] Ifstream open() doesn't set error bits when argument is a directory

问题描述

在C ++程序中，使用std :: ifstream，我试图打开一个用户指定的文件 - 到目前为止这么好。但是，我不小心输入了一个实际上是一个目录的文件名，我很惊讶地看到试图打开（）该目录没有生成任何错误。

这里一个最小示例：

  std :: ifstream f; 
 f.open（..）; 
 if（！f.is_open（）||！f.good（）|| f.bad（）|| f.fail（））{
 std :: cout< 打开时设置错误位< std :: endl; 
 return 1; 
} 
  

这里没有错误的迹象。如果我继续尝试getline（），getline（）设置一个错误位。

  std :: string str ; 
 getline（f，str）; 
 
 if（f.eof（））std :: cout< getline set eofbit< std :: endl; 
 else if（f.bad（））std :: cout<< getline set badbit< std :: endl; 
 else if（f.fail（））std :: cout<< getline set failbit< std :: endl; 
  

这将输出getline set badbit，这是合理的。使用>>运算符引发一个下溢异常，这也是可以的。

现在，我的问题是，如何检测用户输入的目录名称，而不是正确的文件名？有什么办法吗？从流中获取和取消读取字节似乎乏味且容易出错。

此外，为什么会这样？我意识到，从程序的角度看，这都是相同的数据，但我认为操作系统也会发送一些嘿，这是一个目录的消息。

<你不会说你的系统是什么，所以很难说，但一般来说，
filebuf :: open

/ code>将只会返回一个错误，如果你的系统级别打开
失败。我在Unix系统上工作，你可以 open（） a
目录;我甚至曾经在一些地方你可以在
打开后阅读它（至少如果它是一个本地安装的文件系统）。

至于怎么办：我可以想到的是尝试 get
第一个字符，然后将其放回。但是如果文件是
为空，这将失败，因此它也不是一个解决方案。在系统级别（和
从QoI的角度来看，如果
系统允许的话，我希望 filebuf :: open 打开目录），可以使用系统级调用
（Unix中的 stat ）来确定文件是否是目录。
（当然有一个竞争条件，当你检测到
是一个正常的文件，并且你打开的时候，另一个进程
可以删除该文件并创建一个目录。然而，它可能不是一个
频繁出现。）

In a C++ program, using std::ifstream, I'm attempting to open a user-specified file -- so far so good. However, I accidentally entered a filename that's actually a directory, and I was quite surprised to see that attempting to open() that directory didn't generate any errors.

Here's a minimal example:

std::ifstream f;
f.open("..");
if(!f.is_open() || !f.good() || f.bad() || f.fail()) {
    std::cout << "error bit set on open" << std::endl;
    return 1;
}

No sign of error here. If I go on and attempt to getline(), getline() sets an error bit all right.

std::string str;
getline(f, str);

if(f.eof()) std::cout << "getline set eofbit" << std::endl;
else if(f.bad()) std::cout << "getline set badbit" << std::endl;
else if(f.fail()) std::cout << "getline set failbit" << std::endl;

This outputs "getline set badbit", which is reasonable. Using the >> operator throws an underflow exception, which is also okay.

Now, my question is, how could I detect that the user entered a directory name instead of a proper filename? Is there any way to do that? Getting and ungetting bytes from the stream seem tedious and error-prone.

Also, why is this so? I realize that it's all just the same data from the point of view of the program, but I'd assume the OS would also send some "hey, this is a directory" kind of message.

解决方案

You don't say what your system is, so it's hard to say, but generally, filebuf::open will only return an error if your system level open fails. And I've worked on Unix systems where you could open() a directory; I've even worked on some where you could read it after the open (at least if it was a locally mounted filesystem).

As to what to do about it: about all I can think of is to try to get the first character, then put it back. But this fails if the file is empty, so it's not really a solution either. At the system level (and from a QoI point of view, I'd expect filebuf::open to do this if the system did allow opening a directory), you can use a system level call (stat in Unix) to determine whether the file is a directory or not. (There's a race condition, of course: between the moment you detect that it's a normal file, and the moment you do the open, another process could delete the file and create a directory. It's probably not a frequent occurance, however.)

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