Pass struct and array of structs to C function from Go

Question

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])))
}

Nick Craig-Wood · Accepted Answer · 2013-11-11 16:38:12Z

9

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

answered Nov 11, 2013 at 16:38

Nick Craig-Wood

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1 Comment

Jonno Over a year ago

Agree 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

Alexey Kamenskiy · Accepted Answer · 2018-03-08 06:45:21Z

7

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

edited Mar 8, 2018 at 6:45

Alexey Kamenskiy

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answered Dec 3, 2014 at 12:06

Jeffrey W.

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1 Comment

river Over a year ago

I get this error info panic: runtime error: cgo argument has Go pointer to Go pointer

Guy Fawkes · Accepted Answer · 2019-05-15 11:13:31Z

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()
}

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