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cmd/cgo: doesn't support struct with bit fields #43261

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mengzhuo opened this issue Dec 18, 2020 · 2 comments
Closed

cmd/cgo: doesn't support struct with bit fields #43261

mengzhuo opened this issue Dec 18, 2020 · 2 comments
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@mengzhuo
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What version of Go are you using (go version)?

$ go version
eaa97fbf20baffac713ed1b780f864a6fee54ab6

Does this issue reproduce with the latest release?

no

What did you do?

package main

/*
struct foo {
        int bf:4;
        int bar;
};
*/
import "C"

import (
        "unsafe"
)

func main() {
        a := [24]byte{}
        c := (*C.struct_foo)(unsafe.Pointer(&a))
        c.bar = '4'
        c.bf = 1
        print(string(a[:]))
}

What did you expect to see?

go build passed

What did you see instead?

c.bf undefined (type *_Ctype_struct_foo has no field or method bf)

This is just a demo.

In https://go-review.googlesource.com/c/go/+/252378

cgo no longer try to translate
C struct bitfields into Go struct fields, even if their size can be
represented in Go. The order in which C bitfields appear in memory
is implementation dependent, so in some cases the cgo tool produced
results that were silently incorrect.

In real life, some system api like ebpf <linux/bpf.h> do has struct with bit fields.

struct bpf_insn {
        __u8    code;           /* opcode */
        __u8    dst_reg:4;      /* dest register */
        __u8    src_reg:4;      /* source register */
        __s16   off;            /* signed offset */
        __s32   imm;            /* signed immediate constant */
};

Related:
xlab/c-for-go#109

Maybe we should reconside it?
Thanks

CC @ianlancetaylor
@ianlancetaylor ianlancetaylor changed the title cgo: compiler can't gernate struct with bit fields cmd/cgo: doesn't support struct with bit fields Dec 18, 2020
@ianlancetaylor
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A C struct like

struct foo {
        int bf:4;
        int bar;
};

simply can't be represented as a Go struct. The cgo model has no way to handle C types that can't be represented in Go. The only mechanism we have is something like

/*
int getBf(struct foo *p) { return p->bf; }
void setBf(struct foo *p, int v) { p->bf = v; }
*/
import "C"

and then Go code can call C.getBf and C.setBf as needed.

I don't see what else we can do.

@mengzhuo
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A C struct like

struct foo {
        int bf:4;
        int bar;
};

simply can't be represented as a Go struct. The cgo model has no way to handle C types that can't be represented in Go. The only mechanism we have is something like

/*
int getBf(struct foo *p) { return p->bf; }
void setBf(struct foo *p, int v) { p->bf = v; }
*/
import "C"

and then Go code can call C.getBf and C.setBf as needed.

I don't see what else we can do.

OK, Thank you for the information.

@golang golang locked and limited conversation to collaborators Dec 19, 2021
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@mengzhuo @ianlancetaylor @gopherbot