How to access a C pointer array from Golang

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 use the C pointers to gain access to an array.

I'm trying to get the streams stored in the AVFormatContext class to use in go, and eventually add frames to a texture in OpenGl to make a video player with cool transitions.

I think understanding how to cast and access the C data will make coding this a lot easier.

I've stripped out all the relevant parts of the C code, the wrapper and my code, shown below:

C code - libavformat/avformat.h

typedef struct AVFormatContext { 
    unsigned int nb_streams; 
    AVStream **streams; 
}

Golang goav wrapper

package avutil

//#cgo pkg-config: libavformat
//#include <libavformat/avformat.h>
import "C"
import (
    "unsafe"
)

type Context C.struct_AVFormatContext; 

func (ctxt *Context) StreamsGet(i uintptr) *Stream {
    streams := (**Stream)(unsafe.Pointer(ctxt.streams));
    // I think this is where it's going wrong, I'm brand new to this stuff 
    return (*Stream)(unsafe.Pointer(uintptr(unsafe.Pointer(streams)) + i*unsafe.Sizeof(*streams)));
}

My Golang code

package main

import "github.com/giorgisio/goav/avformat"

func main() {
    ctx := &avformat.Context{} // the actual function to initiate this does an mallocz for the streams

    stream := ctx.StreamsGet(0)

    //do stuff with stream...
}

In C it looks like I just have to do just streams[i], but that wont work in go, so I added a function to the wrapper using the technique from my question here. However I'm not getting the data; It looks like I'm getting a pointer to somewhere random in memory. So, how can I access these elements form golang? Any resources would be helpful too; I'm going to be investing a fair bit of time into this.

putu · Accepted Answer · 2018-04-24T13:12:00.720

As you noticed, the problem is in the following code:

func (ctxt *Context) StreamsGet(i uintptr) *Stream {
    streams := (**Stream)(unsafe.Pointer(ctxt.streams));
    // I think this is where it's going wrong, I'm brand new to this stuff 
    return (*Stream)(unsafe.Pointer(uintptr(unsafe.Pointer(streams)) + i*unsafe.Sizeof(*streams)));
}

In the code, variable streams is double pointer, thus the result of adding offset to streams is also a double pointer (i.e. the type is **Stream). But, in your snippets, it is casted to *Stream which is incorrect. The correct code is:

func (ctxt *Context) StreamsGet(i uintptr) *Stream {
    streams := (**Stream)(unsafe.Pointer(ctxt.streams))
    // Add offset i then cast it to **Stream
    ptrPtr := (**Stream)(unsafe.Pointer(uintptr(unsafe.Pointer(streams)) + i*unsafe.Sizeof(*streams)))
    return *ptrPtr
}

Additional note:
If you would like to avoid pointer arithmetic in Go side, you can define a helper function for accessing the element of the pointer (i.e. streams) in C side as follows:

/*
void * ptr_at(void **ptr, int idx) {
    return ptr[idx];
}

struct AVStream * stream_at(struct AVFormatContext *c, int idx) {
    if (i >= 0 && i < c->nb_streams)
        return c->streams[idx];
    return NULL;
}
*/
import "C"
import (
    "unsafe"
)

type Context C.struct_AVFormatContext
type Stream C.struct_AVStream

func (ctx *Context) StreamAt(i int) *Stream {
    p := (*unsafe.Pointer)(unsafe.Pointer(ctx.streams))
    ret := C.ptr_at(p, C.int(i))

    return (*Stream)(ret)
}

func (ctx *Context) StreamAt2(i int) *Stream {
    ret := C.stream_at((*C.struct_AVFormatContext)(ctx), C.int(i))

    return (*Stream)(ret)
}

You may choose either ptr_at function which accepts generic (any) double pointer as its argument, or more specific stream_at function which only accepts pointer to AVFormatContext as its argument. The former approach can be used to access element from any double pointer such as: AVProgram **, AVChapter **, etc. The later approach is preferable if we need to implement additional processing such as boundary checking.

Cool, the ptr_at and stream_at functions look like the ones. How does stream_at work? In the example you're passing it an AVFormatContext (ctx) rather than AVStream (ctx.streams), does it search for streams in that class or something? — nevernew, Apr 24 '18 at 11:43
@nevernew Sorry, it was my mistake. You must pass in a pointer to `AVFormatContext` to function `stream_at`. I updated the explanation. — putu, Apr 24 '18 at 13:14
question continued: https://stackoverflow.com/questions/50010555/cgo-how-do-i-write-to-a-file-in-golang-using-a-pointer-to-the-c-data — nevernew, Apr 24 '18 at 20:54

How to access a C pointer array from Golang

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