convert a []float32 to C *float - go

Question

I want to pass the pointer to the first element of a slice []float32 to a C variable, but somehow I can't figure out how.
C Code:
typedef struct
{ const float *data_in ;
float *data_out ;
} SRC_DATA ;
Go:
mySlice := make([]float32, 20)
foo := C.SRC_DATA{}
foo.data_in = *C.float(&mySlice[0]) // here the program breaks
As far as I have understood, it should be possible to pass directly a pointer to the first element in the slice from Go to C, without using unsafe.Pointer().
The C function will then iterate over the slice (length known).
Any hints are appreciated!

Answer 1

You need to use a slice of C.float type instead of float32:
package main
/*
#include <stdio.h>
void test(float *in) {
printf("C %f %f\n", in[0], in[1]);
}
*/
import "C"
func main() {
in := []C.float{1.23, 4.56}
C.test(&in[0]) // C 1.230000 4.560000
}
Go doesn't allow to store a Go pointer in a Go allocated structure passed to Cgo:
package main
/*
#include <stdio.h>
typedef struct {
float *in;
float *out;
} SRC_DATA;
void test(SRC_DATA *data) {
printf("C %f %f\n", data->in[0], data->in[1]);
data->out[0] = 8.8;
data->out[1] = 9.9;
}
*/
import "C"
import "fmt"
func main() {
in := []C.float{1.23, 4.56}
out := make([]C.float, 2)
data := &C.SRC_DATA{in: &in[0], out: &out[0]}
C.test(data) // panic: runtime error: cgo argument has Go pointer to Go pointer
fmt.Println("Go", out)
}
But you can create a helper function in C:
package main
/*
#include <stdio.h>
typedef struct {
float *in;
float *out;
} SRC_DATA;
void test(SRC_DATA *data) {
printf("C %f %f\n", data->in[0], data->in[1]);
data->out[0] = 8.8;
data->out[1] = 9.9;
}
void test_helper(float *in, float *out) {
SRC_DATA data;
data.in = in;
data.out = out;
test(&data);
}
*/
import "C"
import "fmt"
func main() {
in := []C.float{1.23, 4.56}
out := make([]C.float, 2)
C.test_helper(&in[0], &out[0]) // C 1.230000 4.560000
fmt.Println("Go", out) // Go [8.8 9.9]
}
Alternatively you can allocate and free SRC_DATA structure in C:
package main
/*
#include <stdio.h>
#include <stdlib.h>
typedef struct {
float *in;
float *out;
} SRC_DATA;
void test(SRC_DATA *data) {
printf("C %f %f\n", data->in[0], data->in[1]);
data->out[0] = 8.8;
data->out[1] = 9.9;
}
SRC_DATA *alloc_src_data() {
return (SRC_DATA*)malloc(sizeof(SRC_DATA));
}
void free_src_data(SRC_DATA *p) {
free(p);
}
*/
import "C"
import "fmt"
func main() {
in := []C.float{1.23, 4.56}
out := make([]C.float, 2)
data := C.alloc_src_data()
defer C.free_src_data(data)
data.in = &in[0]
data.out = &out[0]
C.test(data) // C 1.230000 4.560000
fmt.Println("Go", out) // Go [8.8 9.9]
}