How do operating systems compute file size?

If I understand correctly, most programming language which provide a library function to 开发者_如何学Cretrieve the size of a file use a system call. But then, what does the system do under the hood? Does it depend on the file system? Is the size information stored in some kind of a file header?

Yes, this is filesystem dependent, but many filesystems do it in roughly the same way: for each file, there is a block on the hard drive that stores metadata about the file, including its size.

For many of the filesystems used in Linux/UNIX, for example, this block is called an inode. Note that the inode is not actually part of the file, so it's not really a header; it exists in a region of the disc that is reserved for storing metadata, not file data.

On NTFS, the filesystem used by Windows, file size data is stored in the master file table. This is roughly equivalent to the inode table on a Linux filesystem.

It's stored in a file's metadata, which you can retrieve with stat on POSIX systems. The metadata also includes, for example, when the file was last modified or accessed.

They're stored in a structure called an Inode: http://en.wikipedia.org/wiki/Inode

This contains all of your file's metadata; when you modify the contents of a file (or really do anything with it), your Inode gets updated.