Dumping files

This is what CRIU does to dump information about opened files.

Files, descriptors and inodes in Linux

When a task opens a file Linux kernel constructs a chain of 3 (well, 3.5) object to serve this.

Inode

This is an object which describes file as a couple of meta-data (owner, type, size) and data (the bytes themselves).

Dentry (directory entry)

A helper object that kernel uses to resolve file path into Inode object. If a file has hard-links, then one Inode will have several Dentries.

File

This one describes how a tasks works with an opened Dentry-Inode pair.

File descriptor

It's a number in task's table, that is used to reference the needed File object

So after open() (or some other, see below) system call there will be this chain in the memory

+------+
+ Task +-----+
+------+     |
             v
           +------------------------------+
           | File Descriptors Table (FDT) |
           +------------------------------+
           |   |   | 2 |      ...     |   |
           +------------------------------+
                     |
                     |
                     |      +------+    +--------+    +-------+
                     +----->| File |--->| Dentry |--->| Inode |
                            +------+    +--------+    +-------+