Why Does `int x = x;` Result in Undefined Behavior in C ?

Lvalue-to-Rvalue Conversion and Undefined Behavior in int x = x;

Background:

The C code int x = x; initializes a variable x with its own value, but this value is uninitialized for an automatic variable. Hence, the value is indeterminate and undefined behavior may result.

Lvalue-to-Rvalue Conversion:

An lvalue-to-rvalue conversion transforms an lvalue (reference or object with an address) into a prvalue (value without an address). The C standard states that performing this conversion on an uninitialized value results in undefined behavior.

Does the Right-Hand x Undergo Lvalue-to-Rvalue Conversion?

Yes. Despite being on the right-hand side of the assignment, x is still an lvalue. The assignment operator requires an lvalue on the left-hand side (where the value is stored) and a prvalue on the right-hand side (the source of the value).

Conclusion:

Therefore, the right-hand x undergoes lvalue-to-rvalue conversion, and since it refers to an uninitialized value, the provided code has undefined behavior.

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