Fixing the unfixable autofs ABI

Posted May 3, 2012 19:10 UTC (Thu) by jzbiciak (guest, #5246)
In reply to: Fixing the unfixable autofs ABI by nybble41
Parent article: Fixing the unfixable autofs ABI

That would generally be the best idea, but as you stated, it doesn't fix this structure, because the i386 ABI doesn't require natural alignment for 64-bit types, as per the definition you gave in your other comment. The reason I asked is that i386's ABI is perverse in requiring natural alignment for some, but not all data types. So, I wasn't sure if the concept of "natural" had different meaning in the context of i386's ABI.

IMHO, any structures exposed through the ABI should be naturally aligned and marked as "packed", with explicit padding fields where necessary; alignment should not be left to the compiler where ABI compatibility is a concern.

I tend to agree, and do this myself when I care about the actual layout of a struct. Ok, I usually forego 'packed' since it isn't portable. I instead make the struct a multiple of the largest data type, and force fields onto natural boundaries for their types.

This practice is especially necessary when you have any "wire formats" or "on disk format." You can almost think of packets crossing the kernel/user-space boundary as having "wire format" semantics in this case, with the minor difference that pointers are more meaningful in user-space exchanges than they usually are over, say, a TCP link.