Calling C++ function from Go with buffer as argument

Question

We have (for all intents and purposes) something similar to the following C++ code.

calc.h

#pragma once

extern "C" {
    void doCalc(uint8_t** buffer);
}

calc.cpp

#include <cstdint>
#include "calc.h"

void doCalc(uint8_t** buffer) {
    uint8_t lb[256];

    for (int i = 0; i < 256; i++) {
        lb[i] = i;
    }

    *buffer = new uint8_t[256];

    std::memcpy(*buffer, lb, 256);
}

We need to call this method from Go, but unsure how to construct the buffer variable on the Go side to pass in (seeing as the length is unknown prior to calling - the 256 in the C++ function should be considered unknown).

C or C++? They are two very different languages, and might need different setup for the external calls tro be possible. — Some programmer dude
– Some programmer dude, Commented May 25, 2022 at 12:08
Thanks, I saw in the title I made it C, my bad. This would be C++. — Nico Huysamen
– Nico Huysamen, Commented May 25, 2022 at 12:10
There are a few problems with your C++ function: The first is that you should not use malloc in C++, not even for simple buffers. Use new[] instead; The second problem is that the argument variable buffer is a local variable within the function, and whose life-time ends when the function returns. If you want to modify the variable inside the function you either need to pass it by reference or return it; Thirdly, to even be able to call the function you need to declare it as extern "C". — Some programmer dude
– Some programmer dude, Commented May 25, 2022 at 12:15
Ah yes (it has been a few years since I have done C++). Let me update the question. — Nico Huysamen
– Nico Huysamen, Commented May 25, 2022 at 12:23

Jacob Wischnat · Accepted Answer · 2022-05-26 06:30:58Z

1

I've made a slightly different example that fits your use case:

package main

/*
#include <stdio.h>

void print_words(int count, char** words) {
    for (int i = 0; i < count; i += 1) {
        printf(words[i]);
    }
}
*/
import "C"
import "unsafe"

func main() {
    words := make([]*C.char, 2)

    words[0] = C.CString("Hello ")
    words[1] = C.CString("World\n")

    C.print_words((C.int)(len(words)), (**C.char)(unsafe.Pointer(&words[0])))
}

answered May 26, 2022 at 6:30

Jacob Wischnat

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Nico Huysamen · Accepted Answer · 2022-05-26 06:41:46Z

0

Based off @Jacob Wischnat 's answer (thanks!), I was able to get it working for my example:

func main() {
    cBuf := make([]*C.uint8_t, 1)

    C.doCalc((**C.uint8_t)(unsafe.Pointer(&cBuf[0])))
    
    gBuf := C.GoBytes(unsafe.Pointer(cBuf[0]), 256)

    // ...
}

answered May 26, 2022 at 6:41

Nico Huysamen

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