How Can a Const Variable in C Have Two Different Values?

How Undefined Behavior Explains Two Different Values for a Variable

Although it may seem paradoxical, it is possible for a variable at the same address to produce two distinct values. This phenomenon can be attributed to undefined behavior.

In the provided code snippet, a constant integer (a1) is defined, and a constant pointer (b1) is assigned its address. Subsequently, a char pointer (c1) is cast from b1. By modifying c1, the value of a1 is effectively changed, despite being declared as a constant. Casting c1 back to an integer pointer (t) reveals this altered value.

These actions constitute undefined behavior, as attempting to modify a const variable violates the C standard. As specified in section 7.1.6.1, such behavior may lead to unpredictable results, including seemingly impossible outcomes like the one observed.

The standard clearly warns against such actions:

• Section 1.3.24 (Undefined Behavior): "... undefined behavior ranges from ignoring the situation completely with unpredictable results ... to terminating a translation or execution (with the issuance of a diagnostic message)."
• Section 7.1.6.1 The cv-qualifiers (paragraph 4): "... any attempt to modify a const object ... results in undefined behavior."

Therefore, while the observed behavior may seem curious, it is not a compiler optimization but rather a symptom of undefined behavior arising from the attempt to modify a constant variable.

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