What is side effect in C?

Question

Wikipedia says that:

In computer science, an operation, function or expression is said to have a side effect if it modifies some state variable value(s) outside its local environment, that is to say has an observable effect besides returning a value (the main effect) to the invoker of the operation.

But how can we access a variable outside its local environment, can anyone explain this situation, side effect, main effect and sequence point comprehensibly?

Answer 1

A function is (should be) a black box, in which the return value, or the value of a variable passed by reference, should be the only thing that may change depending upon the input parameters.

Any other observable change that the function produces outside these cases, is a side-effect. The most well-known example may be the printf() function which, besides returning the number of written characters, changes the contents of the standard output, which means altering some memory buffer associated with a pipe, a file, or the screen, for instance, and which doesn't belong to the local environment of the function.