Memory-mapped files in Operating Systems - Step-by-Step Execution

Concept Flow - Memory-mapped files

Process requests file data

↓

OS maps file into memory

↓

Process accesses memory address

↓

If page in RAM: data returned immediately

No↓

Page fault triggers disk read

↓

Data loaded into RAM page

↓

Process continues with data

↓

Changes in memory can be synced back to file

The process asks for file data, OS maps the file into memory, process accesses memory addresses. If data is not in RAM, a page fault loads it from disk. Changes can be saved back to the file.

Execution Sample

Operating Systems

1. Open file
2. Map file to memory
3. Read data via memory
4. Modify data in memory
5. Sync changes to file

This sequence shows how a file is opened, mapped into memory, accessed and modified through memory, then changes are saved back.

Analysis Table

Step	Action	Memory State	Disk Access	Result
1	Open file	No mapping yet	No	File descriptor ready
2	Map file to memory	Mapping created, no pages loaded	No	Memory addresses linked to file
3	Read data at address	Page not in RAM	Yes	Page fault triggers disk read
4	Load page into RAM	Page loaded in RAM	Yes	Data available in memory
5	Process reads data	Page in RAM	No	Data returned immediately
6	Modify data in memory	Page modified in RAM	No	Changes in memory only
7	Sync changes to file	Page in RAM	Yes	Changes written back to disk
8	Close file	Mapping removed	No	Resources freed

💡 File closed and memory mapping removed, process ends

State Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 7	Final
File Descriptor	None	Open	Open	Open	Open	Closed
Memory Mapping	None	Created	Page Loaded	Page Modified	Page Modified	Removed
RAM Page	Empty	Empty	Loaded with data	Modified data	Modified data	Empty

Key Insights - 3 Insights

Why does accessing a memory address sometimes cause a disk read?

Because the page is not yet loaded into RAM, causing a page fault that triggers the OS to read the data from disk (see execution_table step 3).

Are changes made in memory immediately saved to the file?

No, changes are first made in RAM and only saved back to the file when syncing occurs (see execution_table step 7).

What happens when the file is closed?

The memory mapping is removed and resources like file descriptors are freed (see execution_table step 8).

Visual Quiz - 3 Questions

Look at the execution table, at which step does the page fault occur?

A Step 7

B Step 5

C Step 3

D Step 2

💡 Hint

Check the 'Disk Access' column where it says 'Page fault triggers disk read' at step 3.

According to the variable tracker, what is the state of the RAM page after step 6?

A Loaded with data

B Modified data

C Empty

D Removed

💡 Hint

Look at the 'RAM Page' row under 'After Step 6' in the variable tracker.

If the process never modifies data, which step in the execution table would be skipped?

A Step 7

B Step 4

C Step 6

D Step 8

💡 Hint

Refer to the 'Action' column where 'Sync changes to file' happens only if data was modified.

Concept Snapshot

Memory-mapped files let a process access file data via memory addresses.
OS maps file pages into RAM on demand (page faults trigger disk reads).
Process reads/writes memory directly; changes can be synced back to disk.
This improves performance by avoiding explicit read/write calls.
Mapping is removed when file is closed.

Full Transcript

Memory-mapped files allow a program to treat file contents as if they are part of its memory. When a file is opened and mapped, the operating system links the file's data to memory addresses. When the program accesses these addresses, if the data is not already in RAM, a page fault occurs, and the OS loads the needed data from disk into RAM. The program can then read or modify the data directly in memory. Changes made in memory are not immediately saved to the file; they are written back when syncing occurs. Finally, when the file is closed, the memory mapping is removed and resources are freed.