Consider the following C program on a Unix like system: main() { fork(); printf("Hello World!"); } What output is printed on the terminal?

Introduction / Context:
This question tests your understanding of the fork system call in Unix like operating systems and how it interacts with standard output. The key idea is that fork creates a new child process that continues execution from the point of the call. Both the parent and the child then execute the same printf statement, which affects how many times the message appears on the screen.

Given Data / Assumptions:

• The code is running in a Unix like environment where fork creates a child process.
• The program contains no explicit flush of the output buffer and no newline character in the format string.
• Standard output is connected to a terminal, so it is line buffered or unbuffered in many implementations.

Concept / Approach:
The fork system call creates a new process. After a successful fork, two processes exist: the parent and the child. Both processes return from fork and continue executing at the next statement in the code. In this program, the next and only statement after fork is printf("Hello World!");. Since both processes execute this printf, the message is printed twice. There is no newline character, so the two strings are concatenated directly, resulting in Hello World!Hello World! on the terminal.

Step-by-Step Solution:
Step 1: Before the fork call, there is a single process.Step 2: When fork executes successfully, the operating system creates a child process that is a copy of the parent.Step 3: Both parent and child now continue execution after the fork call.Step 4: The next line is printf("Hello World!"); which runs once in the parent and once in the child.Step 5: Therefore the string Hello World! is printed twice with no newline in between, resulting in Hello World!Hello World!.

Verification / Alternative check:
If you modify the program to print the process identifier along with the message, you will see two different process identifiers, confirming that two processes execute the printf statement. Running the original code in a Unix shell also shows the message repeated twice. Any slight variation in timing does not change the fact that the message appears twice, only the exact order could vary in more complex programs.

Why Other Options Are Wrong:
Option A suggests the message prints only once, which would be true if there were no fork or if one of the processes exited before printf, but that is not the case.Option C shows the message without the exclamation mark, which does not match the format string.Option D states that no output is printed, which is incorrect because printf is executed.

Common Pitfalls:
Students sometimes forget that both parent and child execute the code after fork. Another common source of confusion is output buffering. In some examples, if output is buffered and the buffer is not flushed before fork, duplicated buffered output can occur. In this simple program, the message is printed after the fork via two independent printf calls, leading in a straightforward way to two copies of the string.

Final Answer:
The correct answer is Hello World!Hello World!.