C structure allignment in 64 bit Linux

[BrianStark]

Hopefully this is an easy question. in linux 64 bit I am creating a union that contains 2 structures 16 bytes each I want to use this union to create a list in which each element of the list will occupy exactly 16 bytes, since the size of the structures are 64 bit alligned I should not have to worry about the structures being padded with extra bytes correct?

[Rubberman]

That would be correct. 16 bytes == 128 bits, or two 64-bit words. Assuming that the compiler will word-align structures, then these are naturally aligned. No filling is required.

[theNbomr]

There are two aspects to your question, and the two are related: the alignment of the structure in memory (i.e. the starting address), and the alignment of the individual elements within the structure.
The first aspect is up to the compiler, and as Rubberman points out, the compiler will tend to choose word or double-word alignment. The arrangement of elements within the structure can be influenced with the #pragma pack directive to force the style of packing that you want. Also, you may find the offsetof() macro useful for finding at runtime how the structure is aligned.
--- rod.