如何在Go中将函数指针传递给C代码

Question

I want to write a small go application for processing videos and decided to use this library goav which is a FFmpeg binding for go. However, it does not support reading the video straightaway from memory. As a workaround, I decided to call the C function directly.

The signature of the function I want to call is as follows.

AVIOContext *avio_alloc_context(
                  unsigned char *buffer,
                  int buffer_size,
                  int write_flag,
                  void *opaque,
                  int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),
                  int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
                  int64_t (*seek)(void *opaque, int64_t offset, int whence));

The go code I wrote to call this -

package video

// #include<libavformat/avio.h>
import "C"
import (
  "bytes"
  "github.com/giorgisio/goav/avformat"
  "github.com/giorgisio/goav/avutil"
  "unsafe"
)

func DecodeStream(data *bytes.Buffer) {
  bufferSize := 8192
  formatContext := avformat.AvformatAllocContext()
  buffer := (*C.uchar)(avutil.AvMalloc(uintptr(bufferSize)))
  ioContext := C.avio_alloc_context(
    buffer,
    C.int(bufferSize),
    C.int(0),
    unsafe.Pointer(data),
    &[0]byte{},
    &[0]byte{},
    &[0]byte{}, 
  )                                       // <- error in this line
  formatContext.SetPb((*avformat.AvIOContext)(unsafe.Pointer(ioContext)))
  if formatContext.AvformatFindStreamInfo(nil) < 0 {
    panic("Couldn't find stream info")
  }
  println("DURATION: ", formatContext.Duration())
}

However, I am getting this error runtime error: cgo argument has Go pointer to Go pointer.

I tried replacing &[0]byte{} with nil too. Can't seem to get away!

Answer 1

The runtime error is actually complaining about the unsafe.Pointer(data) argument. The bytes.Buffer type has internal (Go) pointers, and cgo doesn't allow passing pointers to C if those pointers point to Go objects that contain Go pointers themselves.

These rules are detailed in the cgo documentation here: https://golang.org/cmd/cgo/#hdr-Passing_pointers

The short summary is that the Go garbage collector needs to be aware when Go pointers are modified. The Go compilers make sure that compiled code appropriately informs the garbage collector, but C compilers don't know to do this. So to prevent GC problems, cgo simply doesn't allow passing pointers like this.