The C Standard, 6.7.2.1, discusses the layout of structure fields. It specifies that non-bit-field members are aligned in an implementation-defined manner and that there may be padding within or at the end of a structure. Furthermore, initializing the members of the structure does not guarantee initialization of the padding bytes. The The C Standard, 6.2.6.1, paragraph 6 [ISO/IEC 9899:2011], states
When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values.
Additionally, the storage units in which a bit-field resides may also have padding bits. For an object with automatic storage duration, these padding bits do not take on specific values and can contribute to leaking sensitive information.
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The C Standard static_assert() macro accepts a constant expression and an error message. The expression is evaluated at compile time and, if false, the compilation is terminated and the error message is output. (See DCL03-C. Use a static assertion to test the value of a constant expression for more details.) The explicit insertion of the padding bytes into the struct should ensure that no additional padding bytes are added by the compiler , and consequently both static assertions should be true. However, it is necessary to validate these assumptions to ensure that the solution is correct for a particular implementation.
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