问题
I'm trying to understand Linux (UNIX) low-level interfaces and as an exercise want to write a code which will copy a file with holes into a new file (again with holes).
So my question is, how to read from the first file not till the first hole, but till the very end of the file?
If I'm not mistaken, read() returns 0 when reaches the first hole(EOF).
I was thinking about seeking right byte by byte and trying to read this byte, but then I have to know the number of holes in advance.
回答1:
If by holes you mean sparse files, then you have to find the holes in the input file and recreate them using lseek when writing the output file. Since Linux 3.1, you can even use lseek to jump to the beginning or end of a hole, as described in great detail in the man page.
As ThiefMaster already pointed out, normal file operations will treat holes simply as sequences of zero bytes, so you won't see the EOF you mention.
回答2:
A file is not presented as if it has any gaps. If your intention is to say that the file has sections on one area of the disk, then more on another, etc., you are not going to be able to see this through a call to open() on that file and a series of read() calls. You would instead need to open() and read() the raw disk instead, seeking to sectors on your own.
If your meaning of "holes" in a file is as @ThiefMaster says, just areas of 0 bytes -- these are only "holes" according to your application use of the data; to the file system they're just bytes in a file, no different than any other. In this case, you can copy it through a simple read of the data source and write to the data target, and you will get a full copy (along with what you're calling holes).
回答3:
For copies of sparse files, from the cp manual;
By default, sparse SOURCE files are detected by a crude heuristic and the corresponding DEST file is made sparse as well. That is the behavior selected by --sparse=auto. Specify --sparse=always to create a sparse DEST file whenever the SOURCE file contains a long enough sequence of zero bytes. Use --sparse=never to inhibit creation of sparse files.
Thus, try --sparse=always if you need to copy a sparse file 'as-is' (still seems affected by an algo)
来源：https://stackoverflow.com/questions/12299010/copy-sparse-files