How to get a void** to be a parameter of a C library?

Question

I'm trying to call a C library from GO using cgo.

The C library has the following function:

int receive(void** data);

// I'd call it like that:
void* myptr; // myptr=null
int nbBytes = receive(&myptr);
if (nbBytes==0) { return }
// myptr has now an address to a valid buffer that contains nbBytes bytes.

// And then casting it with char* to access the data that can be anything. Example:
printf("%d", *(char*)myptr);

How can I call this receive() function from GO? Go doesn't allocate any memory, the memory address is returned via myptr and directly access from it.

• receive() is a "no-copy" and writes the actual data's address into myptr. The data is then accessed using *(char*)myptr.
• we can assume receive() allocates and frees the buffer, it's hidden from the lib's user

Ideally, I would read the data via []byte in go.

Answer 1

[Edit: you added a clarification that says this isn't just suggested. But we still don't know how the data are used afterward, from C code.]

There is not enough information in your question to answer it completely, because we don't know—the C language alone doesn't tell us—how this void ** is used. Your comments and additional code suggest (pretty strongly) that the way it's used is that receive fills in the pointer:

int receive(void **data) {
    *data = <something>;
    return <some value>;
}

where the angle-bracketed sections are unknown to us; to use this data from C code, we'd do just what you said:

void f(void) {
    void *p;
    int ret;
    ...
    ret = receive(&p);
}

What we don't know, given this much and the (justifiable) assumptions, are:

• What does the ret value indicate?
• Is p always valid afterward?
• How many bytes at *p are accessible?

For instance, would:

struct our_data dst;
memcpy(&dst, p, len);

be a valid way to get the bytes from p into the data-structure dst, and if so, where does the length len come from? Is it implied, e.g., do we know that because ret was not -1 (error) that p is valid and has sizeof(struct our_data) bytes available, and we need the memcpy just to make it properly aligned?

If we knew all of these things, we'd be able to do the job directly from Go:

var p unsafe.Pointer
var obj C.struct_our_data
ret := C.receive(&p)
C.memcpy(unsafe.Pointer(&obj), p, len) // or similar with copy()

although it might—depending on task etc—make more sense to just write a deserializer for the raw data, which we obtain as an array living in C memory; see, e.g., How to convert [1024]C.char to [1024]byte.