On Tue, Jan 28, 2020 at 4:44 AM mark mellar <radiatorf...@gmail.com> wrote:
>
> Hi Folks, I'm having some trouble instantiating a C struct from a 3rd party
> API.
>
> I'm not sure if this is more a C question or a Go question, details in the
> code below, any help would be much appreciated!
>
> M
>
> // This non-compiling example demonstrates an issue I'm having integrating a
> 3rd party C api with my Go code
> // The comment preceding 'import "C"' constitutes C code which is interpreted
> as a header when compiling.
> // 'api_struct' represents the 3rd party struct I'm attempting to instantiate
> and populate in my Go Code
> // we cannot make changes to this struct
> // other c functions in the comment are helper functions I've written to
> instantiate certain C types
>
> // The go code defines an array of strings and an array of ints. For each of
> these arrays a helper
> // function is called to create a C array of the C equivalent type before
> populating that array.
> // Once C arrays have been populated they are assigned to fields in the a new
> api_struct.
>
> // This method works fine for the string array, but compile fails when we get
> to the int array
> // This is because makeIntArray returns *_Ctype_int but api_struct.rLimits
> expects [12]_Ctype_int. Here's the error...
> // cannot use cInts (type *_Ctype_int) as type [12]_Ctype_int in field value
>
> // I'd like to modify makeIntArray to return a [12]_Ctype_int but I can't
> seem to find the right syntax to achieve this
> package main
>
> /*
> #define ARRLEN 12
> #include <stdlib.h>
>
> struct api_struct {
> char **askedHosts;
> int rLimits[ARRLEN];
> };
>
> static char**makeCharArray(int size) {
> return calloc(sizeof(char*), size);
> }
>
> static void setArrayString(char **a, char *s, int n) {
> a[n] = s;
> }
>
> static void freeCharArray(char **a, int size) {
> int i;
> for (i = 0; i < size; i++)
> free(a[i]);
> free(a);
> }
>
> static int*makeIntArray() {
> int* p = malloc(sizeof(p) * ARRLEN);
> return p;
> }
>
> static void setArrayInt(int *a, int s, int n) {
> a[n] = s;
> }
>
> static void freeIntArray(int *a, int size) {
> free(a);
> }
>
>
> */
> import "C"
> import "fmt"
>
> func main() {
> goHosts := []string{"host1", "host2", "host3"}
> cHosts := C.makeCharArray(C.int(len(goHosts)))
> for i, s := range goHosts {
> C.setArrayString(cHosts, C.CString(s), C.int(i))
> }
>
> goLimits := []int{1,2,3,4,5,6,7,8,9,10,11,12}
> cInts := C.makeIntArray()
> for i, s := range goLimits {
> C.setArrayInt(cInts, C.int(s), C.int(i))
> }
>
> s := C.struct_api_struct{
> askedHosts: cHosts,
> rLimits: cInts,
> }
> fmt.Printf("%+v\n", s)
> }
The easy, and likely more efficient, way is to just set the rLimits
field element by element.
There is no safe way to convert from a *C.int to a [12]C.int in Go.
You can do it easily enough using unsafe.
a := *(*[12]C.int)(unsafe.Pointer(p))
This is of course only safe if p does in fact point to 12 C.int values.
Ian
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