text: ////////////////////////////////////////////////////////////////////////
  //
  //	Note : this file is included as part of the Smaller Animals Software
  //	JpegFile package. Though this file has not been modified from it's 
  //	original IJG 6a form, it is not the responsibility on the Independent
  //	JPEG Group to answer questions regarding this code.
  //	
  //	Any questions you have about this code should be addressed to :
  //
  //	CHRISDL@PAGESZ.NET	- the distributor of this package.
  //
  //	Remember, by including this code in the JpegFile package, Smaller 
  //	Animals Software assumes all responsibilities for answering questions
  //	about it. If we (SA Software) can't answer your questions ourselves, we 
  //	will direct you to people who can.
  //
  //	Thanks, CDL.
  //
  ////////////////////////////////////////////////////////////////////////
  
  /*
   * jdatasrc.c
   *
   * Copyright (C) 1994-1996, Thomas G. Lane.
   * This file is part of the Independent JPEG Group's software.
   * For conditions of distribution and use, see the accompanying README file.
   *
   * This file contains decompression data source routines for the case of
   * reading JPEG data from a file (or any stdio stream).  While these routines
   * are sufficient for most applications, some will want to use a different
   * source manager.
   * IMPORTANT: we assume that fread() will correctly transcribe an array of
   * JOCTETs from 8-bit-wide elements on external storage.  If char is wider
   * than 8 bits on your machine, you may need to do some tweaking.
   */
  
  /* this is not a core library module, so it doesn't define JPEG_INTERNALS */
  #include "jinclude.h"
  #include "jpeglib.h"
  #include "jerror.h"
  
  
  /* Expanded data source object for stdio input */
  
  typedef struct {
    struct jpeg_source_mgr pub;	/* public fields */
  
    FILE * infile;		/* source stream */
    JOCTET * buffer;		/* start of buffer */
    boolean_def start_of_file;	/* have we gotten any data yet? */
  } my_source_mgr;
  
  typedef my_source_mgr * my_src_ptr;
  
  #define INPUT_BUF_SIZE  4096	/* choose an efficiently fread'able size */
  
  
  /*
   * Initialize source --- called by jpeg_read_header
   * before any data is actually read.
   */
  
  METHODDEF(void)
  init_source (j_decompress_ptr cinfo)
  {
    my_src_ptr src = (my_src_ptr) cinfo->src;
  
    /* We reset the empty-input-file flag for each image,
     * but we don't clear the input buffer.
     * This is correct behavior for reading a series of images from one source.
     */
    src->start_of_file = TRUE;
  }
  
  
  /*
   * Fill the input buffer --- called whenever buffer is emptied.
   *
   * In typical applications, this should read fresh data into the buffer
   * (ignoring the current state of next_input_byte & bytes_in_buffer),
   * reset the pointer & count to the start of the buffer, and return TRUE
   * indicating that the buffer has been reloaded.  It is not necessary to
   * fill the buffer entirely, only to obtain at least one more byte.
   *
   * There is no such thing as an EOF return.  If the end of the file has been
   * reached, the routine has a choice of ERREXIT() or inserting fake data into
   * the buffer.  In most cases, generating a warning message and inserting a
   * fake EOI marker is the best course of action --- this will allow the
   * decompressor to output however much of the image is there.  However,
   * the resulting error message is misleading if the real problem is an empty
   * input file, so we handle that case specially.
   *
   * In applications that need to be able to suspend compression due to input
   * not being available yet, a FALSE return indicates that no more data can be
   * obtained right now, but more may be forthcoming later.  In this situation,
   * the decompressor will return to its caller (with an indication of the
   * number of scanlines it has read, if any).  The application should resume
   * decompression after it has loaded more data into the input buffer.  Note
   * that there are substantial restrictions on the use of suspension --- see
   * the documentation.
   *
   * When suspending, the decompressor will back up to a convenient restart point
   * (typically the start of the current MCU). next_input_byte & bytes_in_buffer
   * indicate where the restart point will be if the current call returns FALSE.
   * Data beyond this point must be rescanned after resumption, so move it to
   * the front of the buffer rather than discarding it.
   */
  
  METHODDEF(boolean_def)
  fill_input_buffer (j_decompress_ptr cinfo)
  {
    my_src_ptr src = (my_src_ptr) cinfo->src;
    size_t nbytes;
  
    nbytes = JFREAD(src->infile, src->buffer, INPUT_BUF_SIZE);
  
    if (nbytes <= 0) {
      if (src->start_of_file)	/* Treat empty input file as fatal error */
        ERREXIT(cinfo, JERR_INPUT_EMPTY);
      WARNMS(cinfo, JWRN_JPEG_EOF);
      /* Insert a fake EOI marker */
      src->buffer[0] = (JOCTET) 0xFF;
      src->buffer[1] = (JOCTET) JPEG_EOI;
      nbytes = 2;
    }
  
    src->pub.next_input_byte = src->buffer;
    src->pub.bytes_in_buffer = nbytes;
    src->start_of_file = FALSE;
  
    return TRUE;
  }
  
  
  /*
   * Skip data --- used to skip over a potentially large amount of
   * uninteresting data (such as an APPn marker).
   *
   * Writers of suspendable-input applications must note that skip_input_data
   * is not granted the right to give a suspension return.  If the skip extends
   * beyond the data currently in the buffer, the buffer can be marked empty so
   * that the next read will cause a fill_input_buffer call that can suspend.
   * Arranging for additional bytes to be discarded before reloading the input
   * buffer is the application writer's problem.
   */
  
  METHODDEF(void)
  skip_input_data (j_decompress_ptr cinfo, long num_bytes)
  {
    my_src_ptr src = (my_src_ptr) cinfo->src;
  
    /* Just a dumb implementation for now.  Could use fseek() except
     * it doesn't work on pipes.  Not clear that being smart is worth
     * any trouble anyway --- large skips are infrequent.
     */
    if (num_bytes > 0) {
      while (num_bytes > (long) src->pub.bytes_in_buffer) {
        num_bytes -= (long) src->pub.bytes_in_buffer;
        (void) fill_input_buffer(cinfo);
        /* note we assume that fill_input_buffer will never return FALSE,
         * so suspension need not be handled.
         */
      }
      src->pub.next_input_byte += (size_t) num_bytes;
      src->pub.bytes_in_buffer -= (size_t) num_bytes;
    }
  }
  
  
  /*
   * An additional method that can be provided by data source modules is the
   * resync_to_restart method for error recovery in the presence of RST markers.
   * For the moment, this source module just uses the default resync method
   * provided by the JPEG library.  That method assumes that no backtracking
   * is possible.
   */
  
  
  /*
   * Terminate source --- called by jpeg_finish_decompress
   * after all data has been read.  Often a no-op.
   *
   * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding
   * application must deal with any cleanup that should happen even
   * for error exit.
   */
  
  METHODDEF(void)
  term_source (j_decompress_ptr cinfo)
  {
    /* no work necessary here */
  }
  
  
  /*
   * Prepare for input from a stdio stream.
   * The caller must have already opened the stream, and is responsible
   * for closing it after finishing decompression.
   */
  
  GLOBAL(void)
  jpeg_stdio_src (j_decompress_ptr cinfo, FILE * infile)
  {
    my_src_ptr src;
  
    /* The source object and input buffer are made permanent so that a series
     * of JPEG images can be read from the same file by calling jpeg_stdio_src
     * only before the first one.  (If we discarded the buffer at the end of
     * one image, we'd likely lose the start of the next one.)
     * This makes it unsafe to use this manager and a different source
     * manager serially with the same JPEG object.  Caveat programmer.
     */
    if (cinfo->src == NULL) {	/* first time for this JPEG object? */
      cinfo->src = (struct jpeg_source_mgr *)
        (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
  				  SIZEOF(my_source_mgr));
      src = (my_src_ptr) cinfo->src;
      src->buffer = (JOCTET *)
        (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
  				  INPUT_BUF_SIZE * SIZEOF(JOCTET));
    }
  
    src = (my_src_ptr) cinfo->src;
    src->pub.init_source = init_source;
    src->pub.fill_input_buffer = fill_input_buffer;
    src->pub.skip_input_data = skip_input_data;
    src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
    src->pub.term_source = term_source;
    src->infile = infile;
    src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
    src->pub.next_input_byte = NULL; /* until buffer loaded */
  }
  