Fundamental C- Side Effects, Sequence Points And Lazy Evaluation
Written by Harry Fairhead   
Monday, 11 February 2019
Article Index
Fundamental C- Side Effects, Sequence Points And Lazy Evaluation
Dangerous Expressions
Legal Expressions

Operators are one of C's strong points and it is important to know how to use them - but what are side effects and who ordered sequence points!? It all can seem confusing. This extract, from my new book on programming C in an IoT context, provides a very helpful explanation.

Fundamental C: Getting Closer To The Machine

Now available from Amazon.

  1. About C
    Extract Dependent v Independent
                 & Undefined Behavio
  2. Getting Started With C Using NetBeans
  3. Control Structures and Data
  4. Variables
    Extract Variables 
  5. Arithmetic  and Representation
  6. Operators and Expression
    Extract Side Effects, Sequence Points And Lazy Evaluation
    First Draft of Chapter: Low Down Data
  7. Functions Scope and Lifetime
  8. Arrays
  9. Strings
  10. Pointers
    Extract  Pointers, Cast & Type Punning
  11. Structs
    Extract Basic Structs **NEW
  12. Bit Manipulation
    Extract Shifts And Rotates 
  13. Files
  14. Compiling C – Preprocessor, Compiler, Linker

Also see the companion volume: Applying C




Covered in the chapter but not included in this extract.

  1. Memory Basics
  2. The Numeric Data Types
  3. Floating Point
  4. Declaring Variables
  5. Initialization
  6. Overflow – Undefined Behavior
  7. Literals
  8. Exact Size Variables
  9. Type Modifiers
  10. Variable Lifetimes
  11. lvalue and rvalue
  12. Operators and Expressions
  13. Operators in C

Side Effects

An important idea in C is that assignment is treated as a dyadic operator with the lowest precedence. The operator = stores the value on the right-hand side in the variable on the left-hand side and then its result is the value of the right-hand side.

Put more academically you can say that the result of A=B is B but the operator has a side effect of storing the value of B in A.

In general side effects are bad ideas because they spoil the purity of the picture of an operation that combines a number of values to give a result.

For example, in A=B+C the + has a higher priority so B+C is evaluated and then the result is used in A=result.

Treating assignment as an operator means that you can, if you really want to, write expressions like:


which first evaluates A=B to give B and as a side effect stores this in A and then adds C to B and assigns this to D.

Notice that == is the test for equality in C and so:


assigns C to B and tests if C is equal to A in a single expression. The brackets are necessary because == has a higher priority than =.

Taking this a little further, C also allows other operators to be combined with the assignment operator.

For example:


translates to A=A+1.

Another subtle point concerns what:


evaluates to?

Is it (A*2)+3 or is it A*(2+3)?

The answer is that it is A*(2+3) even though the * should have a higher priority than the +. The reason is of course that the *= operator has a lower priority than * or +.

Once you have introduced the idea of a side effect as part of the assignment operator you might as well carry on and introduce other operators with side effects.

For example, the unitary operator ++ when used as a postfix operator in A++ returns current value of A. That it is the same as just writing A, but as a side effect it increments A. In other words, if A is 3 then after B=A++, B is set to 3 and A is set to 4.

Notice that this is quite different from ++A, the corresponding unary prefix operator, the result of which is A+1 and which has the side effect of incrementing A. So, if A is 3 then, after B=++A, B is set to 4 and so is A. Notice that the ++ postfix operator has the highest priority and associates from left to right, whereas as a prefix operator it has a priority of 2 and associates from right to left. Also notice that A++ has the same meaning as A+=1.

Last Updated ( Sunday, 17 March 2019 )