Why Does `i = i ` Lead to Undefined Behaviour in C ?

The Undefined Behaviour of i = i

The C Standard defines "undefined behaviour" as behaviour that is not explicitly specified by the standard. This means that the compiler is free to generate any code it wants when encountering undefined behaviour, even if it results in unpredictable or erroneous output.

One example of undefined behaviour is the assignment of a value to a variable that is about to be modified. Consider the following code snippet:

<code class="cpp">i = 3;
i = i++;</code>

According to the standard, this code will result in undefined behaviour. However, it is often assumed that the final value of i will be 4, regardless of the order of evaluation. This is not necessarily true.

The compiler could emit code that is equivalent to any of the following:

<code class="cpp">i = 3;
int tmp = i;
++i;
i = tmp;

i = 3;
++i;
i = i - 1;

i = 3;
i = i;
++i;</code>

In the first two cases, the final value of i will be 4. However, in the third case, the final value of i will be 3.

Therefore, it is incorrect to assume that the final value of i will always be 4. The actual behaviour is undefined and compiler-dependent.

As a general rule, it is best to avoid undefined behaviour. If you encounter code that exhibits undefined behaviour, it is important to fix it as soon as possible.

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