Pass struct and array of structs to C function from Go
Solution 1
The problem is that Foo
and _Ctype_Foo
are different structures.
I would guess you are running on 64 bit. Note that int
is 64 bit in go, but is quite likely to be 32 bit in C.
If I change the definition of Foo
to this then it works on my machine (64 bit linux)
type Foo struct {
A int32
B int32
}
However I would say that is a recipe for trouble - make your Go and C code use the same structure with
type Foo _Ctype_Foo
Solution 2
I know this is a fairly old topic but I stumbled across it. Here's a modified (correct) version with some additional manipulation from C land on the Go structs.
package main
/*
#include <stdio.h>
typedef struct {
int a;
int b;
} Foo;
void pass_struct(Foo *in) { printf("%d : %d\n", in->a, in->b); }
void pass_array(Foo **in, int len) {
for(int i = 0; i < len; i++) {
pass_struct(in[i]);
in[i]->a += 1;
in[i]->b += 1;
}
}
*/
import "C"
import (
"fmt"
"unsafe"
)
type Foo struct{ a, b int32 }
func main() {
foo := Foo{10, 20}
foos := []*Foo{&Foo{1, 2}, &Foo{3, 4}}
fmt.Println("from C land")
C.pass_struct((*C.Foo)(unsafe.Pointer(&foo)))
C.pass_array((**C.Foo)(unsafe.Pointer(&foos[0])), C.int(len(foos)))
fmt.Println("a & b should have incremented with 1")
fmt.Println("from Go land")
for _, foo := range foos {
fmt.Printf("%d : %d\n", foo.a, foo.b)
}
}
Output:
from C land
10 : 20
1 : 2
3 : 4
a & b should have incremented with 1
from Go land
2 : 3
4 : 5
Solution 3
Neither of these answers work now (at least under Go 1.12). I wrote two another solutions:
package main
/*
#include <stdio.h>
#include <stdlib.h>
typedef struct {
int a;
int b;
} Foo;
int pass_array(Foo **in) {
int i;
int r = 0;
for(i = 0; i < 2; i++) {
r += in[i]->a;
r *= in[i]->b;
}
return r;
}
*/
import "C"
import (
"fmt"
"unsafe"
)
type Foo struct {
A int32
B int32
}
func a() {
foos := []Foo{{1, 2}, {3, 4}}
l := len(foos)
values := (*[1 << 28]*C.Foo)(C.malloc(C.size_t(C.sizeof_Foo * l)))
for i, f := range foos {
foo := (*C.Foo)(C.malloc(C.size_t(C.sizeof_Foo)))
(*foo).a = C.int(f.A)
(*foo).b = C.int(f.B)
values[i] = foo
}
val := C.pass_array(&values[0])
for i := 0; i < l; i++ {
C.free(unsafe.Pointer(values[i]))
}
C.free(unsafe.Pointer(values))
fmt.Println("A finished", val)
}
func b() {
foos := []Foo{{5, 6}, {7, 8}}
values := make([]*C.Foo, len(foos))
for i, f := range foos {
p := (*C.Foo)(C.malloc(C.size_t(C.sizeof_Foo)))
values[i] = p
(*p).a = C.int(f.A)
(*p).b = C.int(f.B)
}
val := C.pass_array(&values[0])
for _, v := range values {
C.free(unsafe.Pointer(v))
}
fmt.Println("B finished", val)
}
func main() {
a()
b()
}
![tacobot](https://i.stack.imgur.com/Q52NB.jpg?s=256&g=1)
tacobot
Updated on June 19, 2022Comments
-
tacobot about 2 years
Stuck with this problem. Able to get only the first member of passed structure... What I do wrong? And what is the right way to pass the structure from Go to C?
This is my example of how it doesn't work:
package main /* #include <stdio.h> typedef struct { int a; int b; } Foo; void pass_array(Foo **in) { int i; for(i = 0; i < 2; i++) { fprintf(stderr, "[%d, %d]", in[i]->a, in[i]->b); } fprintf(stderr, "\n"); } void pass_struct(Foo *in) { fprintf(stderr, "[%d, %d]\n", in->a, in->b); } */ import "C" import ( "unsafe" ) type Foo struct { A int B int } func main() { foo := Foo{25, 26} foos := []Foo{{25, 26}, {50, 51}} // wrong result = [25, 0] C.pass_struct((*_Ctype_Foo)(unsafe.Pointer(&foo))) // doesn't work at all, SIGSEGV // C.pass_array((**_Ctype_Foo)(unsafe.Pointer(&foos[0]))) // wrong result = [25, 0], [50, 0] out := make([]*_Ctype_Foo, len(foos)) out[0] = (*_Ctype_Foo)(unsafe.Pointer(&foos[0])) out[1] = (*_Ctype_Foo)(unsafe.Pointer(&foos[1])) C.pass_array((**_Ctype_Foo)(unsafe.Pointer(&out[0]))) }
-
Jonno over 9 yearsAgree with your conclusion about using the C type explicitly. Just a note, golang's
int
is not necessarily 64bit; it's at least 32bit: golang.org/pkg/builtin/#int -
river over 5 yearsI get this error info
panic: runtime error: cgo argument has Go pointer to Go pointer