2018-04-19 22:19
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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 the data array they point to.

I'm trying to get the data stored in the AVFrame class and use Go to write it to a file, and eventually 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 - libavutil/frame.h

#include <stdint.h>

typedef struct AVFrame {
    uint8_t *data[AV_NUM_DATA_POINTERS];

Golang goav wrapper - I don't really know whats going on here with the unsafe.Pointers and casting but it gives me access to the underlying C code

package avutil

    #cgo pkg-config: libavutil
    #include <libavutil/frame.h>
    #include <stdlib.h>
import "C"
import (

type Frame C.struct_AVFrame

func AvFrameAlloc() *Frame {
    return (*Frame)(unsafe.Pointer(C.av_frame_alloc()))

func Data(f *Frame) *uint8 {
    return (*uint8)(unsafe.Pointer((*C.uint8_t)(unsafe.Pointer(&

My Golang code

package main

import ""

func main() {
    videoFrame := avutil.AvFrameAlloc()

    data := avutil.Data(videoFrame)

    fmt.Println(data) // here i want the values from data[0] to data[7], but how?
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1条回答 默认 最新

  • dpepbjp126917 2018-04-21 23:59

    Since the library author did not construct a slice header for you to work with you will instead need to cast the return value you get to an unsafe.Pointer and then to a uintptr this will allow you to perform pointer arithmetic on it to get elements later in memory.

    Here's some example code that should run as-is on the go playground.

    package main
    import (
    func main() {
        nums := []uint8{1, 2, 3, 4, 5, 6, 7, 8}
        val := &nums[0] // val is the equivalent of the *uint8 the Data function returns
        ptr := unsafe.Pointer(val)
        sixthVal := (*uint8)(unsafe.Pointer(uintptr(ptr) + 5*unsafe.Sizeof(*val)))
        fmt.Println("Sixth element:", *sixthVal)

    Of course, you will need to be very certain you know how many elements there are so that you do not access invalid memory.

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