如何在创建新文件之前检查文件是否存在 [英] How to check if a file exists before creating a new file

问题描述

我想向文件中输入一些内容，但我想先检查一下我要创建的文件名是否存在。如果是这样，即使该文件为空，我也不想创建任何文件。

I want to input some contents to a file, but I'd like to check first if a file with the name I wish to create exists. If so, I don't want to create any file, even if the file is empty.

我的尝试

bool CreateFile(char name[], char content[]){
     std::ofstream file(name);
     if(file){
         std::cout << "This account already exists" << std::endl;
        return false;
     }
     file << content;
     file.close();
     return true;
}

有什么方法可以做我想要的吗？

推荐答案

假设操作不是原子的就可以了，您可以这样做：

Assuming it is OK that the operation is not atomic, you can do:

if (std::ifstream(name))
{
     std::cout << "File already exists" << std::endl;
     return false;
}
std::ofstream file(name);
if (!file)
{
     std::cout << "File could not be created" << std::endl;
     return false;
}
... 

请注意，如果您运行多个线程尝试创建同一文件，并且肯定不会阻止第二个进程干扰文件的创建，因为您拥有 TOCTUI 问题。 [我们首先检查文件是否存在，然后创建文件-但其他人可能在检查和创建之间创建了文件-如果这很关键，则您需要做其他不能移植的操作]。

Note that this doesn't work if you run multiple threads trying to create the same file, and certainly will not prevent a second process from "interfering" with the file creation because you have TOCTUI problems. [We first check if the file exists, and then create it - but someone else could have created it in between the check and the creation - if that's critical, you will need to do something else, which isn't portable].

另一个问题是，如果您具有诸如文件不可读（因此我们无法打开它进行读取）但可写的权限，它将覆盖文件。

A further problem is if you have permissions such as the file is not readable (so we can't open it for read) but is writeable, it will overwrite the file.

在大多数情况下，这两项都不重要，因为您所关心的就是告诉某人您已经有一个类似的文件（或类似的文件）一种尽力而为的方法。

In MOST cases, neither of these things matter, because all you care about is telling someone that "you already have a file like that" (or something like that) in a "best effort" approach.

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