Extract Exif info from an image

exif.go

package main

import (
	"bufio"
	"bytes"
	"encoding/binary"
	"errors"
	"io"
)

const (
	// JPEG APP1 marker
	jpegAPP1 = 0xE1
)

var (
	errNoExifHeader = errors.New("exif: exif header not found")

	exifHeader = append([]byte("Exif"), 0x00, 0x00)
)

func extractExif(r io.Reader) ([]byte, error) {
	br := bufio.NewReader(r)
	for {
		// Find APP1 marker
		if _, err := br.ReadBytes(0xFF); err != nil {
			return nil, err
		}
		marker, err := br.ReadByte()
		if err != nil {
			return nil, err
		}
		if marker != jpegAPP1 {
			continue
		}

		// Extract section length
		dataLenBytes := make([]byte, 2)
		io.ReadFull(br, dataLenBytes)
		dataLen := int64(binary.BigEndian.Uint16(dataLenBytes)) - 2
		if dataLen < int64(len(exifHeader)) {
			return nil, errNoExifHeader
		}

		// Reconstruct APP1 section
		var exif bytes.Buffer
		exif.WriteByte(0xFF)
		exif.WriteByte(jpegAPP1)
		exif.Write(dataLenBytes)
		_, err = io.CopyN(&exif, br, dataLen)
		if err != nil {
			return nil, err
		}

		// Check whether APP1 contains Exif or not
		data := exif.Bytes()[4:] // 0xFF + marker + 2 bytes for length
		headerBytes := data[:len(exifHeader)]
		if !bytes.Equal(headerBytes, exifHeader) {
			return nil, errNoExifHeader
		}

		return exif.Bytes(), nil
	}
}