C pointer casting to and from Go - c

Question

I'm writing an app for the windows platform using FFmpeg and it's golang wrapper goav, but I'm having trouble understanding how to pass the C pointers between C and Go.
I've stripped out all the relevant parts of the C code, the wrapper and my code, shown below:
C code - libavutil/frame.h
#include <stdint.h>
typedef struct AVFrame {
#define AV_NUM_DATA_POINTERS 8
uint8_t *data[AV_NUM_DATA_POINTERS];
}
Go goav wrapper
package avutil
/*
#cgo pkg-config: libavutil
#include <libavutil/frame.h>
#include <stdlib.h>
// C code I added:
#include <stdio.h>
void SaveFrame(const char* location, uint8_t *data, int width, int height) {
FILE *pFile;
int y;
// Open file
pFile=fopen(location, "wb");
if(pFile==NULL)
return;
// Write header
fprintf(pFile, "P6\n%d %d\n255\n", width, height);
// Write pixel data
for(y=0; y<height; y++)
fwrite(data+y*width, 1, width*3, pFile);
// Close file
fclose(pFile);
}
*/
import "C"
import (
"unsafe"
)
type Frame C.struct_AVFrame
func Data(f *Frame) *uint8 {
// i think this is the function thats not working?
return (*uint8)(unsafe.Pointer((*C.uint8_t)(unsafe.Pointer(&f.data))))
}
func SaveFrame(location string, data *uint8, width int, height int) {
C.SaveFrame(C.CString(location), unsafe.Pointer(data), C.int(width), C.int(height))
}
My Go code
package main
import "github.com/giorgisio/goav/avutil"
func main() {
var frame *avutil.Frame
var data *uint8
//... initialize frame
data = avutil.Data(frame)
avutil.SaveFrame("frame0.ppm", data, 1920, 1080)
}
When I try to save the frame, the resulting image is garbled because the pointer is wrong, how do i fix this?

Answer 1

The data field of the AVFrame struct is an array of 8 pointers.
That is, it's a contiguous chunk of memory containing 8 slots
adjacent to each other—each holding a pointer (to a value of type uint8_t).
IOW, the declaration
uint8_t *data[AV_NUM_DATA_POINTERS];
is better interpreted as
uint8_t* data[AV_NUM_DATA_POINTERS];
and in Go, you'd declare it something like
var data [AV_NUM_DATA_POINTERS]*uint8_t
Now your SaveFrame function declares its data arguments to
be of type uint8_t*. That is okay to pass the address of the
data array of an AVFrame struct as that argument, but more idiomatic
would be to pass the address of its zeroth element—something like
func Data(frame *Frame) *uint8 {
return (*uint8)(unsafe.Pointer(&frame.data[0]))
}