如何在创建文件之前测试一个文件是否存在 [英] How to test if a file exists before creating it
问题描述
您无法可靠地做到这一点。在您检查文件是否存在以及创建文件时,可能会有另一个进程创建它。
您应该继续创建文件。根据您要做的更大的事情,您可能需要这些选项中的一个,以便在文件已存在的情况下执行该操作:
open(file,O_RDWR | O_CREAT,0666)
open(file,O_WRONLY | O_CREAT | O_TRUNC,0666
)
open (file,O_WRONLY | O_CREAT | O_APPEND,0666)
open(file,O_WRONLY | O_CREAT | O_EXCL,0666)
其中大多数 iostream
界面。也许还有一种方法可以在你从 open
获得的文件描述符周围打包一个iostream,具体取决于你拥有哪个C ++库。
另外,我应该提到,如果你想自动替换文件的内容(所以没有进程看到一个不完整的文件),唯一的方法是写出新内容添加到新文件,然后使用 rename
将其移动到旧文件中。
I would like to see if a file exists. If it does not exists, then I would like to create it. I am using Linux by the way.
You can't do that reliably. Between when you check to see if the file exists and when you create it, another process may create it.
You should just go ahead and create the file. Depending on the larger thing you're trying to do, you might want one of these options for what to do if the file already exists:
- modify the previous contents in place:
open("file", O_RDWR|O_CREAT, 0666)
- erase the previous contents:
open("file", O_WRONLY|O_CREAT|O_TRUNC, 0666
) - append to the previous contents:
open("file", O_WRONLY|O_CREAT|O_APPEND, 0666)
- fail the operation:
open("file", O_WRONLY|O_CREAT|O_EXCL, 0666)
Most of these, but unfortunately not all, have equivalents at the higher level iostream
interface. There may also be a way to wrap an iostream around the file descriptor you get from open
, depending on which C++ library you have.
Also, I should mention that if you want to atomically replace the contents of a file (so no process ever sees an incomplete file) the only way to do that is to write out the new contents to a new file and then use rename
to move it over the old file.
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