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cmd/cgo: size of struct with 0 length trailing field has changed #11925

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ebfe opened this issue Jul 29, 2015 · 15 comments
Closed

cmd/cgo: size of struct with 0 length trailing field has changed #11925

ebfe opened this issue Jul 29, 2015 · 15 comments
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@ebfe
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ebfe commented Jul 29, 2015

package main

import (
        "fmt"
        "unsafe"
)

/*
struct st {
        int a;
        char d[0];
};
*/
import "C"

func main() {
        var s C.struct_st
        fmt.Println(unsafe.Sizeof(s))
}
$ go version
go version devel +c1ccbab Wed Jul 29 21:44:27 2015 +0000 linux/amd64
$ go run t.go
8

With go 1.4

$ go version
go version go1.4.2 linux/amd64
$ go run t.go
4

Is this expected? This potentially breaks a lot of c-bindings.

@ebfe
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ebfe commented Jul 29, 2015

This behaviour was introduced with 6f07ac2

@ianlancetaylor ianlancetaylor changed the title cgo: C.struct sizes changed in tip cmd/compile: size of struct with 0 length field has changed Jul 29, 2015
@ianlancetaylor
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Right, this doesn't have anything to do with cgo as such. It's a change in the way the Go compiler handles trailing zero length fields.

I think for clarity it would help to see an example of the kind of code that will break.

@ianlancetaylor ianlancetaylor changed the title cmd/compile: size of struct with 0 length field has changed cmd/cgo: size of struct with 0 length trailing field has changed Jul 30, 2015
@ianlancetaylor ianlancetaylor added this to the Go1.5 milestone Jul 30, 2015
@ianlancetaylor ianlancetaylor self-assigned this Jul 30, 2015
@ebfe
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ebfe commented Jul 30, 2015

I ran into something like this:

import (
        "fmt"
        "unsafe"
)

/*
struct msg_header {
        int flags;
        int len;
        char data[0];
};
*/
import "C"

func marshalMsg(payload []byte, flags int) []byte {
        var hdr C.struct_msg_header
        hdr.flags = C.int(flags)
        hdr.len = C.int(len(payload))
        msg := C.GoBytes(unsafe.Pointer(&hdr), C.int(unsafe.Sizeof(hdr)))
        return append(msg, payload...)
}
func main() {
        fmt.Printf("%x\n", marshalMsg([]byte("hello"), 0))
}

go tip: 00000000050000000000000068656c6c6f
go 1.4.2: 000000000500000068656c6c6f

@ianlancetaylor
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We can fix this by dropping the trailing zero byte field in the cgo translation, but that breaks code like that in issue #8428. You can fix your code by using C.sizeof_struct_msg_header, but I agree that it would be nice if your existing code continued to work.

I'm not seeing any good choices here.

@ebfe
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ebfe commented Jul 30, 2015

Yes, I already discovered C.sizeof_struct_.... though it doesn't seem to be documented anywhere (?).

C.sizeof_struct_a != unsafe.Sizeof(C.struct_a) is pretty surprising.

@ianlancetaylor
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I think there is definitely a bug, we just need to decide who to break.

@gopherbot
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CL https://golang.org/cl/12864 mentions this issue.

@gopherbot
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CL https://golang.org/cl/12866 mentions this issue.

ianlancetaylor added a commit that referenced this issue Jul 30, 2015
This documents the change made in https://golang.org/cl/12864 for
https://golang.org/issue/11925.

Update #11925.

Change-Id: Id09f2a489ea947a725ed12c9cf793e5daef07a06
Reviewed-on: https://go-review.googlesource.com/12866
Reviewed-by: David Crawshaw <crawshaw@golang.org>
@LK4D4
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LK4D4 commented Aug 5, 2015

@ianlancetaylor Could you help me? What is recommended way now to get those fields like char[0]?

@ianlancetaylor
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It depends on why you want to access the field.

If you want to take its address, you can write code like
(*ELEMENTTYPE)unsafe.Pointer(uintptr(unsafe.Pointer(p)) + unsafe.Sizeof(STRUCTYPE{}))

I agree that this is not ideal but I don't see how to avoid it.

@LK4D4
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LK4D4 commented Aug 6, 2015

@ianlancetaylor We had code which received such struct from C, struct looks like:

struct dm_deps {
    uint32_t count;
    uint32_t filler;
    uint64_t device[0];
};

Before in go we just had cool struct with field device, which was sorta slice, at least range worked with it, like for _, d := range deps.device. Now there is no field at all. So only way is to get pointer and cast it to slice, right?

@ianlancetaylor
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I'm not sure I understand. Given a C struct like the above, you used to get a Go struct with a zero-sized array (not a slice). Writing
for _, d := range deps.device
would give you a loop that would execute zero times. That doesn't seem useful.

You will now have to get a pointer, convert it to an array pointer, and slice the array to the desired length.

@tiborvass
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@ianlancetaylor maybe this could help

package main

/*
#include <inttypes.h>
struct dm_deps {
    uint32_t count;
    uint32_t filler;
    uint64_t device[0];
};
*/
import "C"
import "fmt"

func main() {
    deps := &C.struct_dm_deps{}
    fmt.Printf("%#v\n", deps)
    // go version go1.4.2 linux/amd64
    // &main._Ctype_struct_dm_deps{count:0x0, filler:0x0, device:[0]main._Ctype_uint64_t{}}

    // go version devel +d3ffc97 Wed Jul 29 23:50:20 2015 +0000 linux/amd64
    // &main._Ctype_struct_dm_deps{count:0x0, filler:0x0}
}

(courtesy of @vbatts)

@LK4D4
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LK4D4 commented Aug 6, 2015

@ianlancetaylor Yes, you're right I think, this code was weird. Thanks for answers!

@gopherbot
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CL https://golang.org/cl/18542 mentions this issue.

gopherbot pushed a commit that referenced this issue Jan 12, 2016
Update #9401.
Update #11925.
Update #13919.

Change-Id: I52c679353693e8165b2972d4d3974ee8bb1207ef
Reviewed-on: https://go-review.googlesource.com/18542
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
csabahenk added a commit to gluster/parsefuse that referenced this issue Sep 13, 2016
Accounting for zero size arrays fileds in structs has
changend in golang/go@6f07ac2 (go1.5beta1~2396),
which made the size of fuse_dirent (C name) / parsefuse.Dirent (Go name)
different when calculated from C (= serialized size. "real size") and Go.

Go code using cgo can get the real size through the C.sizeof_struct_...
construct (cf.
golang/go#11925 (comment)).
However, we don't use cgo, but parse C through Ruby, so this is not
available for us. Instead we take the offset of the zero size array
field, which does return the desired value.

Fixes #1
@golang golang locked and limited conversation to collaborators Jan 13, 2017
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