Golang Cgo: panic: runtime error: cgo argument has Go pointer to Go pointer

Question

I am working with a C library that, unlike below, I do not control. I need to pass to a C function a pointer to an array that also contains pointers.

package main

/*
#include <stdio.h>

typedef int* pInt;

void foo(pInt p[]) {
  printf("foo()\n");
}
*/
import "C"
import "unsafe"

func main() {
    var i C.int
    var p1 C.pInt = (*C.int)(unsafe.Pointer(&i))
    var p2 C.pInt = (*C.int)(unsafe.Pointer(&i))
    var ps []C.pInt = []C.pInt{p1, p2}
    C.foo(unsafe.Pointer(&ps[0]))
}

This code results in the error panic: runtime error: cgo argument has Go pointer to Go pointer. I am wondering how I can rewrite the Go portion of this code so that it satisfies Cgo's pointer rules. My hope is that I can do this without having to write C helper code.

May I suggest you to post your question to the go-nuts mailing list? Cgo-related questions have a good chance to be answered there exactly by the people who tightened cgo rules in 1.6 (for good!) :-) — kostix
– kostix, Commented Mar 10, 2016 at 19:32
If you need to pass an array of pointers, you need to allocate the pointers in C, not Go. — Mr_Pink
– Mr_Pink, Commented Mar 10, 2016 at 19:48

OneOfOne · Accepted Answer · 2016-03-10 22:08:04Z

7

Bad news, you have to define your helpers in C. Good news, well it's only 2 lines.

package main

/*
#include <stdlib.h> // for malloc/free
#include <stdio.h>

typedef int* pInt;

void foo(pInt p[]) { // you probably wanna pass a len to the function.
    *p[0] = 100;
    printf("foo()\n");
}

pInt * allocArray(size_t ln) { return (pInt*) malloc(ln * sizeof(pInt)); }
void freeArr(pInt * p) { free(p); }

*/
import "C"
import "unsafe"

func main() {
    var (
        i, sz  = 0, 2
        arr    = C.allocArray(C.size_t(sz))
        ps     = (*[100000]C.pInt)(unsafe.Pointer(arr))[:sz:sz]
        p1, p2 = (C.pInt)(unsafe.Pointer(&i)), (C.pInt)(unsafe.Pointer(&i))
    )
    ps[0], ps[1] = p1, p2
    C.foo(arr)
    C.freeArr(arr)
    println("i", i)
}

edited Mar 10, 2016 at 22:08

answered Mar 10, 2016 at 21:50

OneOfOne

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WeakPointer · Accepted Answer · 2016-03-25 02:03:42Z

5

Modifying OneOfOne's solution without the C helper functions, although it is nice to see how easy C helper functions are to write in the comments of the go file.

package main

/*
#include <stdio.h>

typedef int* pInt;

void foo(pInt p[]) { // you probably wanna pass a len to the function.
    *p[0] = 100;
    printf("foo()\n");
}

*/
import "C"
import "unsafe"

func main() {
    var (
        i, sz  = 0, 2
        arr    = (*C.pInt)(C.malloc(C.size_t(sz)))
        ps     = (*[100000]C.pInt)(unsafe.Pointer(arr))[:sz:sz]
        p1, p2 = (C.pInt)(unsafe.Pointer(&i)), (C.pInt)(unsafe.Pointer(&i))
    )
    ps[0], ps[1] = p1, p2
    C.foo(arr)
    C.free(unsafe.Pointer(arr))
    println("i", i)
}

answered Mar 25, 2016 at 2:03

WeakPointer

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