Inter-process communication (pipes, shared memory) in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a pipe for communication between processes.

Operating Systems

int pipefd[2];
if (pipe([1]) == -1) {
    perror("pipe");
    exit(EXIT_FAILURE);
}

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

Apipefd_array

Bfd

Cfd_pipe

Dpipefd

Attempts:

3 left

💡 Hint

Common Mistakes

Using a single integer instead of an array.

Passing an uninitialized pointer instead of an array.

The pipe system call requires an array of two integers to hold the file descriptors for the read and write ends of the pipe. Here, pipefd is the correct array name.

2fill in blank

medium

Complete the code to write data to the write end of the pipe.

Operating Systems

char buf[] = "Hello";
write([1], buf, sizeof(buf));

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

Afd[0]

Bpipefd[0]

Cpipefd[1]

Dfd[1]

Attempts:

3 left

💡 Hint

Common Mistakes

Writing to the read end of the pipe.

Confusing the indices of the pipe file descriptors.

In a pipe, pipefd[1] is the write end. To send data, you write to this descriptor.

3fill in blank

hard

Fix the error in the shared memory segment creation code.

Operating Systems

int shmid = shmget(key, [1], IPC_CREAT | 0666);
if (shmid < 0) {
    perror("shmget");
    exit(1);
}

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

A0

B1024

Csizeof(int)

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero as the size, which causes failure.

Using negative values or invalid sizes.

The size argument to shmget must be a positive integer specifying the number of bytes to allocate. 1024 bytes is a common size for a shared memory segment.

4fill in blank

hard

Fill both blanks to attach to a shared memory segment and write a value.

Operating Systems

char *shm_ptr = (char *) shmat(shmid, [1], [2]);
if (shm_ptr == (char *) -1) {
    perror("shmat");
    exit(1);
}
*shm_ptr = 'A';

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

ANULL

B0

CSHM_RDONLY

DIPC_CREAT

Attempts:

3 left

💡 Hint

Common Mistakes

Using SHM_RDONLY when intending to write.

Passing IPC_CREAT to shmat, which is invalid.

To attach to shared memory with default address and read-write access, pass NULL as the address and 0 as the flags.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps process IDs to their shared memory keys if the key is positive.

Operating Systems

shm_dict = { [1]: [2] for pid, key in processes.items() if key [3] 0 }

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

Apid

Bkey

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping keys and values in the dictionary.

Using the wrong comparison operator in the condition.

This comprehension creates a dictionary where each process ID (pid) maps to its shared memory key (key) only if the key is greater than zero.

Practice

1. Which of the following best describes a pipe in inter-process communication?

easy

A. A way to create new processes in the operating system

B. A memory area shared by multiple processes simultaneously

C. A method to encrypt data between processes

D. A channel that sends data in a stream from one process to another

Solution

1. Step 1: Understand what a pipe does

   A pipe is used to send data in a continuous stream from one process to another, allowing communication.

2. Step 2: Compare with other options

   Shared memory allows direct access to the same data, encryption is unrelated, and process creation is a different concept.

3. Final Answer:

   A channel that sends data in a stream from one process to another -> Option D

4. Quick Check:

   Pipe = Stream data channel [OK]

Hint: Pipes stream data between processes, shared memory shares data directly [OK]

Common Mistakes:

• Confusing pipes with shared memory
• Thinking pipes create processes
• Assuming pipes encrypt data

2. Which of the following is the correct syntax to create a pipe in a Unix-like operating system using C?

easy

A. pipe(int *fd);

B. pipe(fd);

C. pipe(int fd[2]);

D. pipe(fd[2]);

Solution

1. Step 1: Recall the pipe function signature

   The pipe function requires an integer array of size 2 passed by reference to store file descriptors.

2. Step 2: Match the correct syntax

   The correct syntax is pipe(fd); where fd is an integer array of size 2 declared before the call.

3. Final Answer:

   pipe(fd); -> Option B

4. Quick Check:

   pipe needs int array of size 2 [OK]

Hint: pipe() needs int array of size 2 as argument [OK]

Common Mistakes:

• Omitting the type in the argument
• Passing pointer instead of array
• Passing array without size

3. Consider the following pseudo-code using shared memory:

1. Create shared memory segment
2. Process A writes value 10 to shared memory
3. Process B reads value from shared memory
4. Process B writes value 20 to shared memory
5. Process A reads value from shared memory

What value will Process A read in step 5?

medium

A. 20

B. 10

C. 0

D. Undefined or error

Solution

1. Step 1: Track writes and reads in shared memory

   Process A writes 10, then Process B reads 10, then Process B writes 20.

2. Step 2: Determine what Process A reads after Process B's write

   Since shared memory is common, Process A will read the updated value 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   Shared memory shows last written value [OK]

Hint: Shared memory shows latest written value to all processes [OK]

Common Mistakes:

• Assuming Process A reads its own old value
• Thinking reads cause errors
• Confusing shared memory with pipes

4. A programmer tries to use a pipe for communication but notices the reading process blocks indefinitely. What is the most likely cause?

medium

A. Shared memory was used instead of a pipe

B. The pipe was created with incorrect syntax

C. The writing process has not sent any data yet

D. The pipe buffer size is too large

Solution

1. Step 1: Understand pipe blocking behavior

   A reading process blocks if no data is available to read from the pipe.

2. Step 2: Identify the cause of blocking

   If the writing process has not sent data, the reader waits indefinitely for input.

3. Final Answer:

   The writing process has not sent any data yet -> Option C

4. Quick Check:

   Reader blocks if no data sent [OK]

Hint: Reader waits until writer sends data through pipe [OK]

Common Mistakes:

• Blaming syntax errors for blocking
• Confusing pipe with shared memory
• Assuming buffer size causes blocking

5. You want two processes to share a large data structure efficiently and allow both to read and write it. Which IPC method is best and why?

hard

A. Use shared memory because it allows direct access to the same data

B. Use sockets because they work over networks

C. Use message queues because they guarantee message order

D. Use pipes because they provide fast streaming of data

Solution

1. Step 1: Analyze requirements for sharing large data structure

   Efficient sharing with read/write access means processes need direct access to the same memory.

2. Step 2: Compare IPC methods

   Pipes stream data but are unidirectional and less efficient for large shared data. Message queues and sockets add overhead and are for message passing, not direct shared access.

3. Final Answer:

   Use shared memory because it allows direct access to the same data -> Option A

4. Quick Check:

   Shared memory = direct, efficient data sharing [OK]

Hint: Shared memory is best for large, read/write shared data [OK]

Common Mistakes:

• Choosing pipes for large data sharing
• Confusing message queues with shared memory
• Thinking sockets are best for local IPC