George Hartzell writes: > > Quick summary, I'm trying to understand the Go structures that cgo > gives me, how they map back onto the C structures and why a Go struct > that doesn't quite match the cgo output seems to work anyway. > [...]
Ian gave me a lot of good things to chew on, but didn't answer the main question about why the different versions of the `i2c_msg` struct definition all seem to work. For those of you who've been sitting on the edges of your seats awaiting the answer... It turns out that my assumption, that the `buf` pointer's alignment requirements end up inserting padding automagically, seems to be true. I convinced myself of this using the `structlayout` tools from https://github.com/dominikh/go-tools. Starting with this input file: ```go package main // #include <linux/i2c.h> import "C" // This is what `cgo -godefs` generates type ViaGoDefs struct { addr uint16 flags uint16 len uint16 buf *uint8 } // This is what @tetsu-koba used type FromTetsu struct { addr uint16 flags uint16 len uint16 __padding uint16 buf uintptr } // This is the result of using cgo as Ian suggested type ViaCGo C.struct_i2c_msg ``` you can see that all of the structs have the same shape: ``` pi@raspberrypi:~/temper/go/structures $ structlayout -json structures ViaGoDefs | structlayout-pretty +--------+ 0 | | <- ViaGoDefs.addr uint16 (size 2, align 2) +--------+ 2 | | <- ViaGoDefs.flags uint16 (size 2, align 2) +--------+ 4 | | <- ViaGoDefs.len uint16 (size 2, align 2) +--------+ 6 | | <- padding (size 2, align 0) +--------+ 8 | | <- ViaGoDefs.buf *uint8 (size 4, align 4) +--------+ -........- +--------+ 11 | | +--------+ pi@raspberrypi:~/temper/go/structures $ structlayout -json structures FromTetsu | structlayout-pretty +--------+ 0 | | <- FromTetsu.addr uint16 (size 2, align 2) +--------+ 2 | | <- FromTetsu.flags uint16 (size 2, align 2) +--------+ 4 | | <- FromTetsu.len uint16 (size 2, align 2) +--------+ 6 | | <- FromTetsu.__padding uint16 (size 2, align 2) +--------+ 8 | | <- FromTetsu.buf uintptr (size 4, align 4) +--------+ -........- +--------+ 11 | | +--------+ pi@raspberrypi:~/temper/go/structures $ structlayout -json structures ViaCGo | structlayout-pretty +--------+ 0 | | <- ViaCGo.addr structures._Ctype_ushort (size 2, align 2) +--------+ 2 | | <- ViaCGo.flags structures._Ctype_ushort (size 2, align 2) +--------+ 4 | | <- ViaCGo.len structures._Ctype_ushort (size 2, align 2) +--------+ 6 | | <- padding (size 2, align 0) +--------+ 8 | | <- ViaCGo.buf *structures._Ctype_uchar (size 4, align 4) +--------+ -........- +--------+ 11 | | +--------+ pi@raspberrypi:~/temper/go/structures $ ``` Phew. g. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/23914.46306.691997.964218%40alice.local.
