Is the Size of a Boolean Data Type in C Always 1 Byte?

Determining the Size of a Boolean Data Type in C

Every C programmer encounters the fundamental data type known as "bool," which represents logical values of true or false. One common question that arises when working with this data type is whether its size is always 1 byte.

Standard Definition of sizeof(bool)

The answer to this question, as defined by the C language standard, is that sizeof(bool) is implementation-defined. This means that the specific size of a boolean value can vary depending on the compiler and platform used.

Specifically, §5.3.3/1 of the C standard states that the sizes of char, signed char, and unsigned char are always 1, but the sizes of all other fundamental types, including bool, are implementation-defined:

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

Furthermore, Footnote 69 explicitly states that "sizeof(bool) is not required to be 1."

Implementation-Specific Sizing

As a result, the size of bool can vary across different compilers and hardware architectures. For example, some compilers may implement bool as a single bit, while others may use multiple bytes for compatibility reasons or to optimize performance on specific architectures.

Therefore, it is essential to be aware that the size of bool may not always be 1 byte and to consider this variability when developing C applications for multiple platforms or when working with interoperable libraries that may have different assumptions about the size of boolean values.

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