How to return []byte from internal void * in C function by CGO?

Question

I am wrapping an C function with the following definition:

int parser_shift(parser* parser, void* buffer, int length);

It removes up to length bytes from internal buffer of unparsed bytes, storing them in the given buffer.

Now I wish to wrap it into a Go function with the following definition:

func (p *Parser) Shift() []byte {
    var buffer []byte
    // TODO:

    return buffer
}

What is the right way of writing to finish the TODO in above with CGO?

I tried the following way, however it crashed with error: Error in "/path/to/my/program': free(): invalid next size (fast): 0x00007f8fe0000aa0:

var buffer []byte
bufStr := C.CString(string(buffer))
defer C.free(unsafe.Pointer(bufStr))
C.parser_shift(p.Cparser, unsafe.Pointer(bufStr), C.int(8192))
buffer = []byte(C.GoString(bufStr))
return buffer

Answer 1

Assuming that your parser_shift function returns the number of bytes actually stored in the buffer, you can do something like this:

func (p *Parser) Shift() []byte {
    var buffer [8192]byte
    parsed := int(C.parser_shift(p.Cparser, unsafe.Pointer(&buffer[0]), C.int(len(buffer))))
    return buffer[:parsed]
}

There is no need to convert to or from string, just pass it the memory that you want it to write to.

Answer 2

try this function

// C pointer, length to Go []byte
func C.GoBytes(unsafe.Pointer, C.int) []byte