Runtime error handling in Compiler Design - Step-by-Step Execution

Concept Flow - Runtime error handling

Program starts

↓

Execute instructions

↓

Runtime error occurs?

No→Continue execution

Yes↓

Error detected by runtime system

↓

Invoke error handler

↓

Handle error: report, recover, or terminate

↓

Program ends or resumes

The program runs instructions until a runtime error occurs, which is detected and handled by the runtime system to either recover or stop the program.

Execution Sample

Compiler Design

start program
execute instruction 1
execute instruction 2
runtime error occurs
invoke error handler
handle error
end or resume

This shows the steps from program start to error detection and handling during execution.

Analysis Table

Step	Action	Error Detected?	Handler Invoked?	Outcome
1	Start program	No	No	Program running
2	Execute instruction 1	No	No	Instruction successful
3	Execute instruction 2	No	No	Instruction successful
4	Execute instruction 3	Yes (e.g. divide by zero)	Yes	Error handler starts
5	Error handler processes error	Yes	Yes	Error reported and handled
6	Decide to recover or terminate	Yes	Yes	Program resumes or ends
7	If recovered, continue execution	No	No	Program continues
8	If terminated, stop program	No	No	Program ends

💡 Execution stops or continues based on error handler decision after runtime error detection

State Tracker

Variable	Start	After Step 3	After Step 4	After Step 6	Final
Program State	Running	Running	Error Detected	Handled (Recovered or Terminated)	Ended or Running
Error Flag	False	False	True	Handled	Cleared or Active
Instruction Pointer	1	3	Paused at 3	Moved or Stopped	Next or None

Key Insights - 3 Insights

Why does the program stop executing normal instructions after a runtime error?

Because the runtime system detects the error (see step 4 in execution_table) and immediately invokes the error handler to manage the problem before continuing.

Can the program continue running after a runtime error?

Yes, if the error handler recovers from the error (step 6 and 7), the program can resume normal execution; otherwise, it terminates.

What role does the error flag variable play during runtime error handling?

The error flag indicates whether an error has occurred (see variable_tracker). It changes from False to True when an error is detected and helps the runtime system decide to invoke the handler.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What happens when a runtime error is detected?

A The program terminates instantly

B The program continues without interruption

C The error handler is invoked immediately

D The error is ignored

💡 Hint

Check the 'Handler Invoked?' and 'Outcome' columns at step 4 in execution_table

According to variable_tracker, what is the value of 'Error Flag' after step 3?

A False

B True

C Handled

D Paused

💡 Hint

Look at the 'Error Flag' row under 'After Step 3' in variable_tracker

If the error handler decides to recover, which step in execution_table shows the program continuing execution?

A Step 5

B Step 7

C Step 8

D Step 4

💡 Hint

Refer to the 'Outcome' column for step 7 in execution_table

Concept Snapshot

Runtime error handling:
- Happens during program execution when unexpected errors occur
- Runtime system detects errors and sets an error flag
- Error handler is invoked to manage the error
- Handler can report, recover, or terminate the program
- Program may resume or stop based on handler decision

Full Transcript

Runtime error handling occurs when a program encounters an unexpected problem while running, such as dividing by zero or accessing invalid memory. The runtime system detects this error and sets an error flag. It then invokes an error handler to manage the situation. The handler can report the error, try to recover so the program continues, or terminate the program to prevent further issues. The execution flow pauses at the error, and depending on the handler's decision, the program either resumes normal execution or ends. This process ensures programs handle errors gracefully during runtime.

Practice

1. What is the main purpose of runtime error handling in a program?

easy

A. To design the program's user interface

B. To manage unexpected problems that occur while the program is running

C. To improve the program's loading speed

D. To write comments in the code

Solution

1. Step 1: Understand runtime errors

   Runtime errors happen when a program runs and encounters unexpected problems like dividing by zero or missing files.

2. Step 2: Purpose of handling these errors

   Handling runtime errors helps the program respond safely without crashing, improving user experience.

3. Final Answer:

   To manage unexpected problems that occur while the program is running -> Option B

4. Quick Check:

   Runtime error handling = manage unexpected problems [OK]

Hint: Runtime errors happen during running, handle to avoid crashes [OK]

Common Mistakes:

• Confusing runtime errors with syntax errors
• Thinking error handling improves speed
• Believing error handling is for UI design

2. Which of the following is the correct syntax structure to catch runtime errors in many programming languages?

easy

A. for (error in errors) { /* handle error */ }

B. if (error) { /* handle error */ } else { /* code */ }

C. while (error) { /* handle error */ }

D. try { /* code */ } catch (error) { /* handle error */ }

Solution

1. Step 1: Identify error handling syntax

   Most languages use try-catch blocks to run code and catch errors safely.

2. Step 2: Match syntax to options

   try { /* code */ } catch (error) { /* handle error */ } shows the correct try-catch structure; others are control flow statements, not error handling.

3. Final Answer:

   try { /* code */ } catch (error) { /* handle error */ } -> Option D

4. Quick Check:

   Try-catch syntax = try { /* code */ } catch (error) { /* handle error */ } [OK]

Hint: Try-catch blocks catch errors; others are loops or conditions [OK]

Common Mistakes:

• Confusing if or loops with error handling
• Missing catch block after try
• Using wrong keywords for error handling

3. Consider this pseudocode:
try {
result = 10 / 0;
print(result);
} catch (error) {
print("Error caught");
} finally {
print("Done");
}
What will be the output when this code runs?

medium

A. Error caughtDone

B. 10Done

C. DoneError caught

D. Runtime error and no output

Solution

1. Step 1: Analyze try block

   Dividing 10 by 0 causes a runtime error, so the print(result) is skipped.

2. Step 2: Catch and finally blocks

   The catch block prints "Error caught". The finally block always runs and prints "Done".

3. Final Answer:

   Error caught
   Done -> Option A

4. Quick Check:

   Error caught then finally runs = Error caught
   Done [OK]

Hint: Error triggers catch, finally always prints last [OK]

Common Mistakes:

• Expecting result to print despite error
• Ignoring finally block output
• Thinking error stops all output

4. Identify the error in this pseudocode for runtime error handling:
try {
print("Start");
} catch {
print("Error");
}

medium

A. Catch block must come before try block

B. Missing finally block

C. Missing error parameter in catch block

D. Try block cannot have print statements

Solution

1. Step 1: Check catch block syntax

   Most languages require catch to specify an error parameter to receive the error object.

2. Step 2: Validate other parts

   Try block can have print statements, finally is optional, and catch must follow try.

3. Final Answer:

   Missing error parameter in catch block -> Option C

4. Quick Check:

   Catch needs error parameter = Missing error parameter in catch block [OK]

Hint: Catch must name error to handle it properly [OK]

Common Mistakes:

• Thinking finally block is mandatory
• Believing print can't be in try
• Reordering try and catch blocks

5. You want to write a program that reads a list of numbers and divides 100 by each number. If a division by zero occurs, it should skip that number and continue. Which runtime error handling approach is best?

hard

A. Use try-catch inside the loop to catch division errors and continue

B. Use a single try-catch around the whole loop and stop on error

C. Do not use try-catch; just check if number is zero before dividing

D. Use finally block to skip zero values

Solution

1. Step 1: Understand the problem

   We want to handle errors for each division separately and continue processing all numbers.

2. Step 2: Choose error handling approach

   Using try-catch inside the loop catches errors per division and allows continuing. A single try-catch stops on first error. Checking zero before dividing is good but not runtime error handling. Finally block is for cleanup, not skipping values.

3. Final Answer:

   Use try-catch inside the loop to catch division errors and continue -> Option A

4. Quick Check:

   Catch errors per item to continue = Use try-catch inside the loop to catch division errors and continue [OK]

Hint: Catch errors inside loop to handle each case separately [OK]

Common Mistakes:

• Stopping loop on first error
• Not using try-catch for runtime errors
• Misusing finally block for logic control