PngImagePlugin.py 文件源码

def crc(self, cid, data):
        "Read and verify checksum"

        # Skip CRC checks for ancillary chunks if allowed to load truncated images
        # 5th byte of first char is 1 [specs, section 5.4]
        if ImageFile.LOAD_TRUNCATED_IMAGES and (i8(cid[0]) >> 5 & 1):
            self.crc_skip(cid, data)
            return

        try:
            crc1 = Image.core.crc32(data, Image.core.crc32(cid))
            crc2 = i16(self.fp.read(2)), i16(self.fp.read(2))
            if crc1 != crc2:
                raise SyntaxError("broken PNG file (bad header checksum in %r)"
                                  % cid)
        except struct.error:
            raise SyntaxError("broken PNG file (incomplete checksum in %r)"
                              % cid)