api guide

This guide intends to give you a quick start to the Xdelta3 programming interface. This is not a complete reference, the comments inside source file xdelta3.h and the command-line application, xdelta3-main.h offer more complete information.

Have you read the command-line interface?

stream interface

To begin with, there are three external structures, only two of which are discussed here. The xd3_stream struct plays the main role, one of these contains the state necessary to encode or decode one stream of data. An xd3_source struct maintains state about the (optional) source file, against which differences are computed. The third structure, xd3_config deals with configuring various encoder parameters.

At a glance, the interface resembles Zlib. The program puts data in, which the xd3_stream consumes. After computing over the data, the xd3_stream in turn generates output for the application to consume, or it requests more input. The xd3_stream also issues requests to the application to read a block of source data. The request to read a source block may be handled in one of two ways, according to application preference. If a xd3_getblk callback function is provided, the application handler will be called from within the library, suspending computation until the request completes. If no callback function is provided the library returns a special code (XD3_GETSRCBLK), allowing the application to issue the request and resume computation whenever it likes. In both cases, the xd3_source struct contains the requested block number and a place to store the result.


The code to declare and initialize the xd3_stream:

int ret;
xd3_stream stream;
xd3_config config;

xd3_init_config (&config, 0 /* flags */);
config.winsize = 32768;
ret = xd3_config_stream (&stream, &config);

if (ret != 0) { /* error */ }

xd3_init_config() initializes the xd3_config struct with default values. Many settings remain undocumented in the beta release. The most relevant setting, xd3_config.winsize, sets the encoder window size. The encoder allocates a buffer of this size if the program supplies input in smaller units (unless the XD3_FLUSH flag is set). xd3_config_stream() initializes the xd3_stream object with the supplied configuration.

setting the source

The stream is ready for input at this point, though for encoding the source data must be supplied now. To declare an initialize the xd3_source:

xd3_source source;
void *IO_handle = ...;

source.name = "...";
source.size = file_size;
source.ioh= IO_handle;
source.blksize= 32768;
source.curblkno = (xoff_t) -1;
source.curblk = NULL;

ret = xd3_set_source (&stream, &source);

if (ret != 0) { /* error */ }

The decoder sets source data in the same manner, but it may delay this step until the application header has been received (XD3_GOTHEADER). The application can also check whether source data is required for decoding with the xd3_decoder_needs_source().

xd3_source.blksize determines the block size used for requesting source blocks. If the first source block (or the entire source) is already in memory, set curblkno to 0 and curblk to that block of data.

input/output loop

The stream is now ready for input, which the application provides by calling xd3_avail_input(). The application initiates encoding or decoding at this point by calling one of two functions:

int xd3_encode_input (xd3_stream *stream)
int xd3_decode_input (xd3_stream *stream)

Unless there is an error, these routines return one of six result codes which the application must handle. In many cases, all or most of the handler code is shared between encoding and decoding. The codes are:

  • XD3_INPUT: The stream is ready for (or requires) more input. The application should call xd3_avail_input when (if) more data is available.
  • XD3_OUTPUT: The stream has pending output. The application should write or otherwise consume the block of data found in the xd3_stream fields next_out and avail_out, then call xd3_consume_output.
  • XD3_GETSRCBLK: The stream is requesting a source block be read, as described above. This is only ever returned if the xd3_getblk callback was not provided.
  • XD3_GOTHEADER: This decoder-specific code indicates that the first VCDIFF window header has been received. This gives the application a chance to inspect the application header before encoding the first window.
  • XD3_WINSTART: This is returned by both encoder and decoder prior to processing a window. For encoding, this code is returned once there is enough available input. For decoding, this is returned following each window header (except the first, when XD3_GOTHEADER is returned instead).
  • XD3_WINFINISH: This is called when the output from a single window has been fully consumed.

An application could be structured something like this:

do {
  read (&indata, &insize);
  if (reached_EOF) {
    xd3_set_flags (&stream, XD3_FLUSH);
  xd3_avail_input (&stream, indata, insize);
  ret = xd3_xxcode_input (&stream);
  switch (ret) {
  case XD3_INPUT:
  case XD3_OUTPUT:
    /* write data */
    goto process;
    /* set source block */
    goto process;
  case XD3_WINSTART:
    /* no action necessary */
    goto process;
    /* error */
} while (! reached_EOF);

All that remains is to close the stream and free its resources. The xd3_close_stream() checks several error conditions but otherwise involves no input or output. The xd3_free_stream() routine frees all memory allocated by the stream.


There are two convenience functions for encoding to and decoding from in-memory buffers. See the xd3_encode_completely and xd3_decode_completely interfaces.

There are two routines to get and set the application header. When encoding, sthe application header must be set before the first XD3_WINSTART. When decoding, the application header is available after after the first XD3_GOTHEADER.