CGO: Go string -> void*/length

[Peter Mogensen]

Hi,

I was trying to find the official way to pass a Go string to CGO code as
a void* (or char*) with explicit length.
... using only one allocation/copy.

I could probably just call C.CString(string) and ignore the extra \0
added by CGO, but I'm a little puzzled that there's not a shorthand for
those with C APIs not using null-terminated strings.

Someone proposed this hack:

*((**byte)(unsafe.Pointer(&theString)))

... resulting in a *byte pointing to the data - which can then be copied
to C-managed memory.

But that relies on knowing that the string implementation starts with a
pointer to the data.

It would be great with a

func C.CStringData(string) (*C.char, C.int) // or void*

/Peter

[Pietro Gagliardi]

Just pass the null-terminated string and use C.int(len(goString)) as the length. The length of a Go string is already in bytes and does not include the terminating null (since Go has none), and I assume C.CString() produces the same byte sequence without encoding conversions.

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[Peter Mogensen]

On 2016-10-31 15:39, Pietro Gagliardi wrote:
> Just pass the null-terminated string and use C.int(len(goString)) as the length.

Yes... I mentioned that as a somewhat solution. ... it still allocates 1
byte more than needed.

/Peter

[James Bardin]

C.CBytes does essentially this, but you either need to start with a []byte, or the string conversion could be a second copy.

I don't see a need for a separate version CString without the null when you can just pass the length-1. In the rare case that the extra byte makes a difference, you could just define your own version

func CStringNoNull(s string) unsafe.Pointer {

p := C.malloc(uintptr(len(s)))

pp := (*[1<<30]byte)(p)

copy(pp[:], s)

return p

}

[Florian Weimer]

* Pietro Gagliardi:

> Just pass the null-terminated string and use C.int(len(goString)) as
> the length. The length of a Go string is already in bytes and does not
> include the terminating null (since Go has none), and I assume
> C.CString() produces the same byte sequence without encoding
> conversions.

Is C.int really the right type here?

[Pietro Gagliardi]

I was going by the example at the bottom of the OP. Use whatever type is expected by the C API, of course.