Memory-mapped files in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a memory-mapped file in Python.

Operating Systems

import mmap
with open('example.txt', 'r+b') as f:
    mm = mmap.mmap(f.fileno(), [1])
    print(mm.readline())

Drag options to blanks, or click blank then click option'

A1024

B-1

C0

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or -1 as size causes errors or unexpected behavior.

The size parameter in mmap.mmap specifies the number of bytes to map. Using 1024 maps 1 KB of the file.

2fill in blank

medium

Complete the code to open a memory-mapped file with read-only access.

Operating Systems

import mmap
with open('data.bin', 'rb') as f:
    mm = mmap.mmap(f.fileno(), 0, access=[1])
    print(mm.read(10))

Drag options to blanks, or click blank then click option'

Ammap.ACCESS_WRITE

Bmmap.ACCESS_DEFAULT

Cmmap.ACCESS_COPY

Dmmap.ACCESS_READ

Attempts:

3 left

💡 Hint

Common Mistakes

Using ACCESS_WRITE allows writing, which is not desired here.

Using mmap.ACCESS_READ opens the memory map with read-only access, preventing modifications.

3fill in blank

hard

Fix the error in the code to correctly map the entire file size.

Operating Systems

import os
import mmap
filename = 'logfile.log'
size = os.path.getsize(filename)
with open(filename, 'r+b') as f:
    mm = mmap.mmap(f.fileno(), [1])
    print(mm[:10])

Drag options to blanks, or click blank then click option'

A-1

B0

Csize

Dlen(filename)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 0 maps the whole file only on some systems, which is not portable.

The size variable holds the file size, which should be passed to mmap to map the entire file.

4fill in blank

hard

Fill both blanks to create a memory-mapped file and write data to it.

Operating Systems

import mmap
with open('file.dat', 'r+b') as f:
    mm = mmap.mmap(f.fileno(), [1], access=[2])
    mm[0:5] = b'Hello'
    mm.flush()

Drag options to blanks, or click blank then click option'

A10

Bmmap.ACCESS_READ

Cmmap.ACCESS_WRITE

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using ACCESS_READ prevents writing and causes errors.

Mapping 10 bytes with ACCESS_WRITE allows writing data and flushing changes to the file.

5fill in blank

hard

Fill all three blanks to create a memory-mapped file, read a slice, and close it properly.

Operating Systems

import mmap
with open('sample.txt', 'r+b') as f:
    mm = mmap.mmap(f.fileno(), [1], access=[2])
    data = mm[[3]:[3]+5]
    print(data)
    mm.close()

Drag options to blanks, or click blank then click option'

A20

Bmmap.ACCESS_READ

C10

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using ACCESS_WRITE when only reading is needed.

Mapping 20 bytes with read access and reading from offset 0 to 5 bytes correctly reads data from the file.

Practice

1. What is the main advantage of using memory-mapped files in an operating system?

easy

A. They allow programs to access file data as if it were in memory, improving speed.

B. They compress files automatically to save disk space.

C. They encrypt files for security purposes.

D. They convert files into executable programs.

Solution

1. Step 1: Understand what memory-mapped files do

   Memory-mapped files let programs treat file contents like part of the computer's memory.

2. Step 2: Identify the benefit of this approach

   This allows faster and easier access to file data without explicit read/write calls.

3. Final Answer:

   They allow programs to access file data as if it were in memory, improving speed. -> Option A

4. Quick Check:

   Memory-mapped files = fast file access [OK]

Hint: Memory-mapped files map files to memory for speed [OK]

Common Mistakes:

• Confusing memory-mapping with file compression
• Thinking memory-mapping encrypts files
• Assuming memory-mapping creates executables

2. Which system call is commonly used on Unix-like systems to create a memory-mapped file?

easy

A. open()

B. fork()

C. mmap()

D. exec()

Solution

1. Step 1: Recall Unix system calls related to memory mapping

   The mmap() system call is designed to map files or devices into memory.

2. Step 2: Differentiate from other system calls

   open() opens files, fork() creates processes, and exec() runs programs, none map files to memory.

3. Final Answer:

   mmap() -> Option C

4. Quick Check:

   Unix memory mapping = mmap() [OK]

Hint: Unix memory mapping uses mmap() call [OK]

Common Mistakes:

• Choosing open() instead of mmap()
• Confusing fork() with memory mapping
• Thinking exec() maps files

3. Consider this pseudocode using memory-mapped files:

map = mmap(file)
map[0] = 'A'
print(file.read(1))

What will be printed?

medium

A. Nothing, because mmap disables file reading

B. The first byte of the file before mapping

C. An error because file.read() conflicts with mmap

D. 'A', the updated first byte via memory mapping

Solution

1. Step 1: Understand memory-mapped file behavior

   Writing to the mapped memory changes the file content directly.

2. Step 2: Analyze the print statement

   Reading the first byte from the file after mapping reflects the change 'A'.

3. Final Answer:

   'A', the updated first byte via memory mapping -> Option D

4. Quick Check:

   Memory map write updates file content [OK]

Hint: Changes in mmap reflect in file reads immediately [OK]

Common Mistakes:

• Assuming file.read() shows old data
• Thinking mmap disables file reading
• Expecting an error from simultaneous access

4. Identify the error in this memory-mapped file usage snippet:

fd = open('data.txt', 'r')
map = mmap(fd, length=100)
map[0] = 'B'

medium

A. Length parameter must be zero for mmap

B. File opened in read-only mode but writing to map attempted

C. File descriptor should be closed before mmap

D. map[0] cannot be assigned a character

Solution

1. Step 1: Check file open mode

   The file is opened with 'r' (read-only), so writing to mapped memory is not allowed.

2. Step 2: Understand mmap write restrictions

   Writing to a read-only mapped area causes an error or undefined behavior.

3. Final Answer:

   File opened in read-only mode but writing to map attempted -> Option B

4. Quick Check:

   Read-only file + write mmap = error [OK]

Hint: Write to mmap needs writable file open mode [OK]

Common Mistakes:

• Ignoring file open mode when writing
• Thinking length=0 is required
• Believing file must be closed before mmap

5. You want to share data between two running processes efficiently. Which feature of memory-mapped files helps achieve this?

hard

A. Mapping the same file into both processes' memory spaces to share data

B. Compressing the file to reduce memory usage

C. Encrypting the file to secure shared data

D. Automatically syncing file changes to a remote server

Solution

1. Step 1: Understand inter-process communication needs

   Sharing data between processes requires a common memory area accessible to both.

2. Step 2: Identify memory-mapped file feature for sharing

   Mapping the same file into both processes' memory allows them to read/write shared data efficiently.

3. Final Answer:

   Mapping the same file into both processes' memory spaces to share data -> Option A

4. Quick Check:

   Memory-mapped files enable shared memory between processes [OK]

Hint: Shared mapping = shared data between processes [OK]

Common Mistakes:

• Confusing compression with sharing
• Assuming encryption shares data
• Thinking syncing to server shares memory