On Tue, Apr 11, 2017 at 10:29 AM, Axel Wagner
<axel.wagner...@googlemail.com> wrote:
>
> This begs the question how syscalls are handled. I assume they don't use
> cgo, but often they require to pass a pointer to some memory which is
> documented as a C-struct. Is there any guidance on that (or any problems
> that might arise there)?
ObPedant: it *raises* the question, it doesn't *beg* the question.
The syscall package (and the golang.org/x/sys/unix package) defines Go
versions of a number of C structs used by system calls. Those Go
versions are generated by cgo (run in a special mode, and with the
generated files committed to the repository) and are accurate Go
representations of the C structs.
Ian
> On Tue, Apr 11, 2017 at 6:41 PM, Ian Lance Taylor <i...@golang.org> wrote:
>>
>> On Tue, Apr 11, 2017 at 2:25 AM, hui zhang <fastfad...@gmail.com> wrote:
>> >
>> > If I define the same 2 struct in c and go
>> > can they be passed directly with unsafe.Pointer
>>
>> The struct layout rules are under-defined in Go. The current rules
>> are straightforward, and it is the case that most structs with the
>> same sequence of field types will look the same in C and Go. Of
>> course you must remember that `int` in Go is often not the same size
>> as `int` in C. Also the alignment rules are not always the same, so
>> don't permit any alignment padding. And this may change in future Go
>> releases.
>>
>> > and how to export go struct from go to c ?
>>
>> You can't, not easily. But it's easy to use cgo to export a C struct
>> from C to Go, and doing that avoids all the concerns about types and
>> alignments, so you should do that if at all possible.
>>
>>
>> > /*
>> > #include <stdio.h>
>> >
>> > typedef struct {
>> > int a;
>> > int b;
>> > } Foo;
>> >
>> > void pass_struct(Foo *in) { printf("%d : %d\n", in->a, in->b); }
>> >
>> > */
>> >
>> > import "C"
>> >
>> > import (
>> > "fmt"
>> > "unsafe"
>> > )
>> >
>> > type Foo struct{ a, b int32 }
>> >
>> > C.pass_struct((*C.Foo)(unsafe.Pointer(&foo)))
>>
>> For example, how about `type Foo C.Foo`?
>>
>> Ian
>>
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