What Determines the Size of a `bool` in C ?

Implementation-Defined Size of bool in C Standard

Although the C language standard explicitly states the sizes of fundamental types like char, signed char, and unsigned char to be 1 byte each, the definition of sizeof(bool) is left to implementers' discretion.

The standard emphasizes this ambiguity in §5.3.3/1:

sizeof(char), sizeof(signed char) and sizeof(unsigned char) are 1; the result of sizeof applied to any other fundamental type is implementation-defined.

This means that the size of bool is not guaranteed to be 1 byte, and the standard includes a footnote (69) explicitly indicating:

sizeof(bool) is not required to be 1.

Therefore, the implementation may decide the size of bool based on various factors, such as architecture or platform constraints. As a result, sizeof(bool) can vary across different compilers and systems.

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