如何检查是否可以创建给定长度的文件？

Question

I want to create a non-sparse file of a given length (ie 2GB), but I want to check if that is possible before actually writing stuff to disk.

In other words I want to avoid getting ENOSPC (No space left on device) while writing. I'd prefer not to create a "test file" of size 2GB or things like that just to check that there is enough space left.

Is that possible?

Answer 1

Use posix_fallocate(3) .

From the description:

The function posix_fallocate() ensures that disk space is allocated for the file referred to by the descriptor fd for the bytes in the range starting at offset and continuing for len bytes. After a successful call to posix_fallocate(), subsequent writes to bytes in the specified range are guaranteed not to fail because of lack of disk space

Answer 2

You can use the statvfs function to determine how much free bytes (and inodes) a given filesystem has.

That should be enough for a quick check, but do remember that it's not a guarantee that you'll be able to write as much (or, for that matter, that writing more than that would have failed) - other applications could also be writing to (or deleting from) the same filesystem. So do continue to check for various write errors.

fallocate or posix_fallocate can be used to allocate (and deallocate) a large chunk. Probably a better option for your use-case. (Check the man page, there's a lot of options for space management that you might find interesting.)