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cmd/cgo: godefs handles gcc pragma pack inconsistently? #8267

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mikioh opened this issue Jun 23, 2014 · 2 comments

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@mikioh
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commented Jun 23, 2014

% hg id
36ffbae640bd+ tip

go tool cgo -godefs pack4.go on darwin outputs the following

// Created by cgo -godefs - DO NOT EDIT
// cgo -godefs pack4.go

package pack4

type Pack4k struct {
    X       uint32
    Pad_cgo_0   [8]byte
    Pad_cgo_1   [8]byte
}

instead of

type Pack4k struct {
    X       uint32
    Pad_cgo_0   int64
    Pad_cgo_1   int64
}

fwiw, go1.1 generates the latter.

Attachments:

  1. pack4.go (166 bytes)
@ianlancetaylor

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commented Jun 23, 2014

Comment 1:

The goal is to make a Go struct that corresponds to the C struct.  In Go, int64 is
always aligned to an 8-byte boundary.  In C it sometimes is not.  In particular, it is
not on a 32-bit x86.  Therefore the first Go struct (the one with the [8]byte fields) is
the only correct translation of the C struct in some cases.  The second Go struct (with
the int64 fields) looks right but will do the wrong thing.

Status changed to WorkingAsIntended.

@mikioh

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commented Jun 24, 2014

Comment 2:

For the record, we agreed that for now we leave all stuff that generated previous godefs
as it is. We will fix the stuff when we see serious issues caused by wrong ABI layout.
See revision 7244ce3ae9db.

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