I wasn’t entirely satisfied with my previous attempt at HTTP/POST gzip-compression, so I wrote a more elegant solution (no XMLRPC libraries this time).

A function to perform gzip-compression on a string:

// see zlib's compress.c:compress() function for the original of this
// modified function
//
// dest      : destination buffer, malloc'd already
// destLen   : size of the malloc'd buffer
// source    : the uncompressed text
// sourceLen : the size of the uncompressed text
//
// this function returns an error code from the zlib library.
// upon return, dest contains the compressed output, and destLen
// contains the size of dest.
int string_gzip (char *dest, unsigned long *destLen, char *source, unsigned long sourceLen)
{
  char *header;

  sprintf(dest, "%c%c%c%c%c%c%c%c%c%c", gz_magic[0], gz_magic[1],
          Z_DEFLATED, 0       , 0,0,0,0      , 0        , OS_CODE);
        //          , flags   , time         , xflags   ,        );

  header = dest;
  dest = &dest[10]; // skip ahead of the header
  *destLen -= 10; // update our available length

  z_stream stream;
  int err;

  stream.next_in = source;
  stream.avail_in = sourceLen;
  #ifdef MAXSEG_64K
    /* Check for source > 64K on 16-bit machine: */
    if (stream.avail_in != sourceLen)
      return Z_BUF_ERROR;
  #endif
  stream.next_out = dest;
  stream.avail_out = *destLen;
  if (stream.avail_out != *destLen)
    return Z_BUF_ERROR;

  stream.zalloc = Z_NULL;
  stream.zfree = Z_NULL;
  stream.opaque = Z_NULL;

  // instructs zlib not to write a zlib header
  err = deflateInit2( &stream, Z_DEFAULT_COMPRESSION, Z_DEFLATED,
                      -MAX_WBITS, DEF_MEM_LEVEL, Z_DEFAULT_STRATEGY );
  if (err != Z_OK)
    return err;

  err = deflate(&stream, Z_FINISH); // Z_FINISH or Z_NO_FLUSH if we have
                                    // more input still (we don't)
  if (err != Z_STREAM_END) {
      deflateEnd(&stream);
      return err == Z_OK ? Z_BUF_ERROR : err;
  }
  *destLen = stream.total_out;

  err = deflateEnd(&stream);

  dest = header; // put the header back on
  *destLen += 10; // update length of our data block

  return err;
}

Using this function to make a gzip-compressed HTTP/POST using libcURL:

   char *data = readXMLInput();
   unsigned long dataLength = strlen(data);
   unsigned long compressedDataLength = dataLength*1.1 + 22; // see zlib's compress.c
   char *compressedData = calloc(compressedDataLength,1);
   string_gzip(compressedData, &compressedDataLength, (char*)data, dataLength);

    curl =  curl_easy_init();
    struct curl_slist *header_list=NULL;

    curl_easy_setopt(curl, CURLOPT_URL, "www.example.com/xmlrpc.php");
    // set the "Accept-Encoding: " header
    curl_easy_setopt(curl, CURLOPT_ENCODING, "gzip");
    // set the "Content-Encoding: ", "Content-Length: ", "Content-Type: " headers
    header_list = curl_slist_append(header_list, "Content-Encoding: gzip");
    header_list = curl_slist_append(header_list, "Content-Type: text/xml");
    sprintf(contentLengthBuf, "Content-Length: %d", compressedDataLength);
    header_list = curl_slist_append(header_list, contentLengthBuf);
    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, header_list);

    // Now specify the POST data
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, compressedData);
    curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, compressedDataLength);

    // Perform request
    res = curl_easy_perform(curl);
    curl_slist_free_all(header_list);
    curl_easy_cleanup(curl);
    free(compressedData);

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