C call Go exported function - c

Question

I want to return an array to C caller, just like below, how to do it?
//export EtcdGetAllNodes
func EtcdGetAllNodes()[]uint32 {
a := []uint32{1,2,3}
return a
}
This function EtcdGetAllNodes will try to get value with a prefix for the specific key from etcd, it will return multiple values. How to return these values to C caller?

Answer 1

Command cgo
Passing pointers
A Go function called by C code may not return a Go pointer (which
implies that it may not return a string, slice, channel, and so
forth). A Go function called by C code may take C pointers as
arguments, and it may store non-pointer or C pointer data through
those pointers, but it may not store a Go pointer in memory pointed to
by a C pointer. A Go function called by C code may take a Go pointer
as an argument, but it must preserve the property that the Go memory
to which it points does not contain any Go pointers.
I want to return an array to C caller.
//export EtcdGetAllNodes
func EtcdGetAllNodes() []uint32 {
a := []uint32{1, 2, 3}
return a
}
A Go function called by C code may not return a Go pointer (which implies that it may not return a slice).
There are many possible solutions: Command cgo.
For example, here is one simple solution:
Output:
$ go build -buildmode=c-archive -o cmem.a cmem.go
$ gcc -pthread -o cmem cmem.c cmem.a
$ ./cmem
-- EtcdGetAllNodes --
nodes: 3
node 0: 1
node 1: 2
node 2: 3
$ echo $?
0
$
cmem.go:
package main
/*
#include <stdint.h>
#include <stdlib.h>
*/
import "C"
import "unsafe"
// toC: Go slice to C array
// c[0] is the number of elements,
// c[1] through c[c[0]] are the elements.
// When no longer in use, free the C array.
func toC(a []uint32) *C.uint32_t {
// C array
ca := (*C.uint32_t)(C.calloc(C.size_t(1+len(a)), C.sizeof_uint32_t))
// Go slice of C array
ga := (*[1 << 30]uint32)(unsafe.Pointer(ca))[: 1+len(a) : 1+len(a)]
// number of elements
ga[0] = uint32(len(a))
// elements
for i, e := range a {
ga[1+i] = e
}
return ca
}
//export EtcdGetAllNodes
// EtcdGetAllNodes: return all nodes as a C array.
// nodes[0] is the number of node elements.
// nodes[1] through nodes[nodes[0]] are the node elements.
// When no longer in use, free the nodes array.
func EtcdGetAllNodes() *C.uint32_t {
// TODO: code to get all etcd nodes
a := []uint32{1, 2, 3}
// nodes as a C array
return toC(a)
}
func main() {}
cmem.c:
#include "cmem.h"
#include <stdint.h>
#include <stdio.h>
int main() {
printf("-- EtcdGetAllNodes --\n");
// nodes[0] is the number of node elements.
// nodes[1] through nodes[nodes[0]] are the node elements.
// When no longer in use, free the nodes array.
uint32_t *nodes = EtcdGetAllNodes();
if (!nodes) {
return 1;
}
printf("nodes: %d\n", *nodes);
for (uint32_t i = 1; i <= *nodes; i++) {
printf("node %d: %d\n", i-1,*(nodes+i));
}
free(nodes);
return 0;
}