Operating Systems: Fork and Pipes

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Questions and Answers

What happens when the fork() function is called in a C program?

The program terminates immediately without executing further statements.

A new child process is created, both parent and child execute the rest of the program. (correct)

Only the parent process continues execution.

The child process inherits all resources of the parent process.

In a C program with fork(), what information is typically printed by the child process after fork() is called?

A specific message that denotes it is a child process. (correct)

The PID of the parent process.

Only statements that belong to the parent process.

All statements including those that belong to the parent process.

Which of the following correctly describes the state of variables after a fork() in a C program?

The parent process loses access to its variables.

Variables in both processes become global and can be accessed by both.

The child process shares the same variable state as the parent process permanently.

Both processes have separate copies of variables, independently modified. (correct)

What is the purpose of the perror() function in the context of a fork operation?

To handle errors and display a relevant error message. Signup and view all the answers

What does the return value of fork() signify when it is zero?

The process created by fork() is the child process. Signup and view all the answers

What are the two descriptors returned by the pipe() call used for?

First for reading, second for writing Signup and view all the answers

Which of the following is true about unnamed pipes?

They exist only as long as the processes using them are alive. Signup and view all the answers

What happens if a process attempts to write to a pipe that has no reading process open?

A SIGPIPE signal is generated. Signup and view all the answers

Which function is used to read data from a pipe?

read() Signup and view all the answers

What is a characteristic of named pipes compared to unnamed pipes?

They have file access permissions and exist as directory entries. Signup and view all the answers

What does the function write(int fd, const void *buf, size_t count) do in the context of pipes?

Writes data to a pipe, appending it to the end. Signup and view all the answers

What condition will cause a process reading from a pipe to sleep?

When the pipe is empty and the write end is still open. Signup and view all the answers

What does the return value of read() indicate when EOF is encountered?

The number of bytes read is zero. Signup and view all the answers

In the context of pipes, what does PIPE_BUF refer to?

The maximum amount of data that can be written atomically to a pipe. Signup and view all the answers

What is the primary use of pipes in an operating system?

Facilitating inter-process communication. Signup and view all the answers

What is the role of the 'fork()' function in the provided code?

To create a new process Signup and view all the answers

Which statement correctly describes the behavior of the 'dup' function?

It duplicates an existing file descriptor. Signup and view all the answers

What happens to the variable 'i' in the child process after 'fork()' is called?

It is set to 5 before incrementing. Signup and view all the answers

What is the primary purpose of the 'pipe()' function in inter-process communication?

To enable one process to read and write data to another. Signup and view all the answers

In the context of the provided code, when is 'close(fd)' called in the child process?

Before writing data to the pipe. Signup and view all the answers

When using 'dup2()', what happens if 'newfd' is already open?

It closes 'newfd' before duplicating 'oldfd'. Signup and view all the answers

Which system call is used for executing a command specified by a filename?

execvp() Signup and view all the answers

What does the read operation in the parent process expect regarding 'readbuffer'?

It is a string buffer with a set size. Signup and view all the answers

What is a consequence of using 'close(0)' in the provided example?

It closes the stdin stream. Signup and view all the answers

What is the main difference between 'dup' and 'dup2'?

'dup' does not close the new file descriptor if it is open. Signup and view all the answers

Study Notes

Fork and Pipes

Fork is a system call that creates a new process, called a child process, which is an exact copy of the parent process.
The parent and child processes have their own copies of variables, but both processes share the same data and code.
There is no certainty as to which process will execute first.
The fork() function returns a process ID (PID) for the child process.
The PID of the parent process is 0.
The PID is -1 if there's an error.

Parent and Child Processes

The example program demonstrates the use of fork() to create a parent and child process.
The parent process uses the PID to identify itself and the child process.
The child process uses zero to identify itself.

Pipes

Pipes are a low-level communication mechanism between processes.
The pipe() system call creates a pipe with two file descriptors.
One file descriptor is used for reading and the other for writing to the pipe.
The write() system call writes data to the pipe.
The read() system call reads data from the pipe.

Pipe Synchronization

If a process tries to write to a full pipe, it's blocked until the reader process consumes some data.
If a process tries to read from an empty pipe, it's blocked until the writer produces some data.

Pipe Types

There are two types of pipes: Unnamed pipes and Named pipes.
Unnamed pipes can be used only by related processes, while named pipes can be used by unrelated processes.
Named pipes are like files with access permissions, while unnamed pipes are only available as long as the processes using them are alive.

`write()` System Call

The write() system call writes data to a file descriptor.
It has three parameters: The file descriptor, a pointer to the buffer, and the size of the buffer.
It returns the number of bytes written.
PIPE_BUF is the size of a block for an atomic write to a pipe, which is 5120 bytes.
A SIGPIPE signal is generated when a process tries to write to a pipe that is not open for reading. This signals a "broken pipe" error.

`read()` System Call

The read() system call reads from a file descriptor.
It has three parameters: The file descriptor, a pointer to the buffer, and the size of the buffer.
It returns the number of bytes read, or 0 if end-of-file is reached.
If a process reads from an empty pipe with the write end still open, it will sleep until data is available.

`pipe()` System Call

The pipe() system call creates an unnamed pipe with two file descriptors, one for reading and one for writing.
It returns an array of two file descriptors in the fildes parameter.

File Descriptor Table

The file descriptor table is a data structure used by the operating system to track open files. Each process has its own file descriptor table.
The file descriptors are integers that are used to access files.

Unnamed Pipes

Unnamed pipes are created using the pipe() system call.
They can be used by related processes such as a parent and child process.
Unnamed pipes only exist while the processes using them are alive.

`dup()` System Call

The dup() system call creates a copy of an existing file descriptor.
It returns a new file descriptor, which can be used interchangeably with the original file descriptor.
The new file descriptor has the same file access mode, file pointer and open file as the original descriptor.
The new file descriptor is set to remain open across exec functions.
The returned file descriptor will be the lowest unused file descriptor.
The library function fileno() returns the file descriptor associated with a given file stream.

`dup2()` System Call

The dup2() system call duplicates an existing file descriptor and assigns it to a new file descriptor.
The new file descriptor is closed first, if necessary.
This ensures that the new file descriptor is available to be assigned as the copy.

`execvp()` System Call

The execvp() function replaces the current process with a new process, as specified by the target file.
The current process image is replaced by the new process image, and the new process begins executing the file.
The execvp() function needs a file pathname and an array of arguments for the new process to execute.

Examples

Example programs demonstrate how to use the pipe(), dup(), and dup2() system calls.
These examples show how pipes are used for communication between processes using the read() and write() system calls.
Example code shows how to use the execvp() system to execute a new program and replace the current process.
Example code with open(), write(), close(), dup(), and dup2() system calls, demonstrates how to access and duplicate file descriptors for write operations.
Example execution steps are shown to depict the flow of execution for specific example code.

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Description

This quiz covers the fundamentals of process creation and inter-process communication in operating systems. Key concepts include the use of the fork system call, the relationship between parent and child processes, and the implementation of pipes for communication. Test your understanding of these critical components of operating systems.