Is It Valid to Initialize a Variable with Its Own Unspecified Value in C and C ?

Behavior of anUninitialized Variable as its Own Initializer

In C and C , an uninitialized variable is not assigned a specific value. Surprisingly, using this uninitialized variable to initialize itself appears to be valid in certain scenarios with C99, C11, and C 11 standards. This raises questions about the behavior of such constructs.

Compiler Allowance

Compilers allow this behavior because the uninitialized variable has an indeterminate value when used as its initializer. This indeterminate value can be either unspecified or a trap representation.

Standard Behavior

The C/C standards state that the behavior of using an uninitialized variable in its own initialization is implementation-dependent. However, further clarification is provided:

• Undefined Behavior: If the implementation supports padding bits in integer types and the indeterminate value happens to be a trap representation, its usage results in undefined behavior.
• Unspecified Value: If the implementation does not have padding bits in integers, the indeterminate value is simply unspecified, leading to no undefined behavior.

Additional Considerations

It is important to note that undefined behavior can still occur if the uninitialized variable never has its address taken. According to the C11 standard, this results in undefined behavior when using objects with automatic storage duration that could have been declared as register storage class.

Therefore, it is generally recommended to avoid using uninitialized variables in their own initializations, as its behavior is implementation-dependent and can potentially lead to undefined results.

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