libjpeg 라이브러리 질문 입니다.

jpeg_CreateDecompress (j_decompress_ptr cinfo, int version, size_t structsize)
{
  int i;
 
  /* Guard against version mismatches between library and caller. */
  cinfo->mem = NULL;		/* so jpeg_destroy knows mem mgr not called */
 
  /*
  if (version != JPEG_LIB_VERSION)
    ERREXIT2(cinfo, JERR_BAD_LIB_VERSION, JPEG_LIB_VERSION, version);
  if (structsize != SIZEOF(struct jpeg_decompress_struct))
    ERREXIT2(cinfo, JERR_BAD_STRUCT_SIZE, 
	     (int) SIZEOF(struct jpeg_decompress_struct), (int) structsize);
 
 
  */
  /* For debugging purposes, we zero the whole master structure.
   * But the application has already set the err pointer, and may have set
   * client_data, so we have to save and restore those fields.
   * Note: if application hasn't set client_data, tools like Purify may
   * complain here.
   */
  {
    struct jpeg_error_mgr * err = cinfo->err;
    void * client_data = cinfo->client_data; /* ignore Purify complaint here */
    MEMZERO(cinfo, SIZEOF(struct jpeg_decompress_struct));
    cinfo->err = err;
    cinfo->client_data = client_data;
  }
  cinfo->is_decompressor = TRUE;
 
  /* Initialize a memory manager instance for this object */
  jinit_memory_mgr((j_common_ptr) cinfo);
 
  /* Zero out pointers to permanent structures. */
  cinfo->progress = NULL;
  cinfo->src = NULL;
 
  for (i = 0; i < NUM_QUANT_TBLS; i++)
    cinfo->quant_tbl_ptrs[i] = NULL;
 
  for (i = 0; i < NUM_HUFF_TBLS; i++) {
    cinfo->dc_huff_tbl_ptrs[i] = NULL;
    cinfo->ac_huff_tbl_ptrs[i] = NULL;
  }
 
  /* Initialize marker processor so application can override methods
   * for COM, APPn markers before calling jpeg_read_header.
   */
  cinfo->marker_list = NULL;
  jinit_marker_reader(cinfo);
 
  /* And initialize the overall input controller. */
  jinit_input_controller(cinfo);
 
  /* OK, I'm ready */
  cinfo->global_state = DSTATE_START;
}