The C Standard, 6.7.3.2.1, discusses the layout of structure fields. It 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 C Standard, 6.2.6.1, paragraph 6 [ISO/IEC 9899:20112024], 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 (e.g. structure and union assignment may or may not copy any padding bits).
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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Bibliography
| [ISO/IEC 9899:20112024] | 6.2.6.1, "General" 6.7.3.2.1, "Structure and Union Specifiers" |
| [Graff 2003] | |
| [Sun 1993] |
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