jextract: offset not multiple of alignment, causing ExceptionInInitializerError

[Maurizio Cimadamore]

That layout looks odd/wrong.

It seems like there might be some padding missing. Note that you have a 
64 bit field that is not 64 bit aligned - so accessing it might be 
slower. I believe in most compilers 32 bit of padding would be inserted.

Or, if this is really the way the struct is laid out, jextract should 
have relaxed the alignment requirements for "l2p_contigbytes". As it 
stands, the jextract layout seems bogus. If you have access to the code, 
I'd suggest adding:

C_LONG_LONG.withName("l2p_contigbytes").withAlignmentBits(32)

This should allow you to initialize correctly.

Maurizio

On 14/01/2021 21:34, Sebastian Stenzel wrote:
> Hey,
>
> Today I noticed a strange problem related to the MemoryLayout generated by jextract for the following struct:
>
> ```
> struct log2phys {
> 	u_int32_t	l2p_flags;	
> 	off_t		l2p_contigbytes;
> 	off_t		l2p_devoffset;
> };
> ```
>
> The generated layout looks like this:
>
> ```
> static final MemoryLayout log2phys$struct$LAYOUT_ = MemoryLayout.ofStruct(
>          C_INT.withName("l2p_flags"),
>          C_LONG_LONG.withName("l2p_contigbytes"),
>          C_LONG_LONG.withName("l2p_devoffset")
>      ).withName("log2phys");
> ```
>
> During runtime, when the class was loaded and the VarHandle for "l2p_contigbytes" was created, I got the following exception:
>
> Exception in thread "main" java.lang.ExceptionInInitializerError
> 	at com.example.fuse_h$fuse_operations.$LAYOUT(fuse_h.java:14808)
> 	at com.example.fuse_h$fuse_operations.allocate(fuse_h.java:16294)
> 	...
> Caused by: java.lang.UnsupportedOperationException: Invalid alignment requirements for layout b64(l2p_contigbytes)[abi/kind=LONG_LONG,layout/name=l2p_contigbytes]
> 	at jdk.incubator.foreign/jdk.internal.foreign.LayoutPath.checkAlignment(LayoutPath.java:273)
> 	at jdk.incubator.foreign/jdk.internal.foreign.LayoutPath.dereferenceHandle(LayoutPath.java:159)
> 	at jdk.incubator.foreign/jdk.incubator.foreign.MemoryLayout.lambda$varHandle$2(MemoryLayout.java:488)
> 	at jdk.incubator.foreign/jdk.incubator.foreign.MemoryLayout.computePathOp(MemoryLayout.java:538)
> 	at jdk.incubator.foreign/jdk.incubator.foreign.MemoryLayout.varHandle(MemoryLayout.java:488)
> 	at com.example.fuse_h_constants_1.<clinit>(fuse_h_constants_1.java:1921)
> 	... 5 more
>
> I set a breakpoint at LayoutPath.checkAlignment and can tell that "l2p_contigbytes" has an offset of 32 bit (sounds reasonable) but an alignment of 64 bit. I have no idea, what all this means, but apparently it is invalid.
>
> I were able to avoid this error in my test project, but I thought I'd better report this in case there is something wrong, as I can see nothing special about aforementioned struct.
>
> Let me know if I can provide any further information!
>
> Sebastian