How to avoid the buffer mechanism from FileSystem

Question

Take VirtualBox's virtual disk as example:if VirtualBox didn't avoid the buffer mechanism from FileSystem in host os,the FileSystem in guest os would move data from memory to meory.

In fact ,I want to write a filesystem in user space(put all directorys and files in a single big file). But if I use c api such fread and fwrite ,the FileSystem in os would buffer the data that My UserSpace-FileSystem read、write.But My UserSpace-FileSystem has implement a buffer mechanism by itself.If i didn't avoid the buffer mechanism from FileSystem in os,My UserSpace-FileSystem would move data from memory to memory.It's so bad .

Dose anyone know how to solve this problem?

Answer 1

Your question isn't very clear, but if all you want to do is use stdio without buffering, then setbuf(file, NULL); will solve your problem. A better solution might be to avoid stdio entirely and use lower-level io primitives read, write, etc. (not part of plain C but specified by POSIX, and with nearly-equivalent versions of them available on most non-POSIX systems too).

Answer 2

stdio doesn't support that.

For *NIX: man open for O_DIRECT, man fadvise and man madvise.

For Windows, check the CreateFile for FILE_FLAG_NO_BUFFERING. Probably a good idea to dig the CreateFileMapping too.