Difference between compile time and run time

The difference between compile time and run time is known as the "phase distinction." To understand how a program behaves in each phase, it is helpful to evaluate its invariants, potential errors, postconditions, and inputs/outputs.

Compile Time

• Invariants: The program does not need to satisfy any invariants and does not even need to be well-formed initially.
  
  
• What Can Go Wrong:
  
  
  • Syntax errors
    
    
  • Typechecking errors
    
    
  • Compiler crashes (rare)
    
    
• Postconditions (If Successful):
  
  
  • The program is deemed well-formed.
    
    
  • It is ready to be executed (though it may still fail upon running).
    
    
• Inputs and Outputs:
  
  
  • Inputs: The source code, along with necessary header files, interfaces, and imported libraries.
    
    
  • Outputs: Assembly code, relocatable object code, an executable program, or error messages if compilation fails.
    
    

Run Time

• Invariants: The environment assumes nothing; invariants are solely enforced by the programmer's logic.
  
  
• What Can Go Wrong:
  
  
  • Fatal run-time errors: Division by zero, dereferencing a null pointer, or running out of memory.
    
    
  • Program-handled errors: Attempting to open a missing file or processing a malformed URL.
    
    
• Postconditions (If Successful): The program successfully finishes its tasks or continues operating without crashing.
  
  
• Inputs and Outputs: Entirely dictated by the programmer. This includes interacting with files, UI windows, network packets, or external hardware (which only occurs during run time).