compiler/libjpeg/main/jdatasrc.c

   1 /*
   2     $Id$
   3 */
   4
   5 /*
   6  * jdatasrc.c
   7  *
   8  * Copyright (C) 1994-1996, Thomas G. Lane.
   9  * This file is part of the Independent JPEG Group's software.
  10  * For conditions of distribution and use, see the accompanying README file.
  11  *
  12  * This file contains decompression data source routines for the case of
  13  * reading JPEG data from a file (or any stdio stream).  While these routines
  14  * are sufficient for most applications, some will want to use a different
  15  * source manager.
  16  * IMPORTANT: we assume that fread() will correctly transcribe an array of
  17  * JOCTETs from 8-bit-wide elements on external storage.  If char is wider
  18  * than 8 bits on your machine, you may need to do some tweaking.
  19  */
  20
  21 /* this is not a core library module, so it doesn't define JPEG_INTERNALS */
  22 #include "jinclude.h"
  23 #include "jpeglib.h"
  24 #include "jerror.h"
  25
  26
  27 /* Expanded data source object for stdio input */
  28
  29 typedef struct {
  30   struct jpeg_source_mgr pub;   /* public fields */
  31
  32   FILE * infile;                /* source stream */
  33   JOCTET * buffer;              /* start of buffer */
  34   boolean start_of_file;        /* have we gotten any data yet? */
  35 } my_source_mgr;
  36
  37 typedef my_source_mgr * my_src_ptr;
  38
  39 #define INPUT_BUF_SIZE  4096    /* choose an efficiently fread'able size */
  40
  41
  42 /*
  43  * Initialize source --- called by jpeg_read_header
  44  * before any data is actually read.
  45  */
  46
  47 METHODDEF(void)
  48 init_source (j_decompress_ptr cinfo)
  49 {
  50   my_src_ptr src = (my_src_ptr) cinfo->src;
  51
  52   /* We reset the empty-input-file flag for each image,
  53    * but we don't clear the input buffer.
  54    * This is correct behavior for reading a series of images from one source.
  55    */
  56   src->start_of_file = TRUE;
  57 }
  58
  59
  60 /*
  61  * Fill the input buffer --- called whenever buffer is emptied.
  62  *
  63  * In typical applications, this should read fresh data into the buffer
  64  * (ignoring the current state of next_input_byte & bytes_in_buffer),
  65  * reset the pointer & count to the start of the buffer, and return TRUE
  66  * indicating that the buffer has been reloaded.  It is not necessary to
  67  * fill the buffer entirely, only to obtain at least one more byte.
  68  *
  69  * There is no such thing as an EOF return.  If the end of the file has been
  70  * reached, the routine has a choice of ERREXIT() or inserting fake data into
  71  * the buffer.  In most cases, generating a warning message and inserting a
  72  * fake EOI marker is the best course of action --- this will allow the
  73  * decompressor to output however much of the image is there.  However,
  74  * the resulting error message is misleading if the real problem is an empty
  75  * input file, so we handle that case specially.
  76  *
  77  * In applications that need to be able to suspend compression due to input
  78  * not being available yet, a FALSE return indicates that no more data can be
  79  * obtained right now, but more may be forthcoming later.  In this situation,
  80  * the decompressor will return to its caller (with an indication of the
  81  * number of scanlines it has read, if any).  The application should resume
  82  * decompression after it has loaded more data into the input buffer.  Note
  83  * that there are substantial restrictions on the use of suspension --- see
  84  * the documentation.
  85  *
  86  * When suspending, the decompressor will back up to a convenient restart point
  87  * (typically the start of the current MCU). next_input_byte & bytes_in_buffer
  88  * indicate where the restart point will be if the current call returns FALSE.
  89  * Data beyond this point must be rescanned after resumption, so move it to
  90  * the front of the buffer rather than discarding it.
  91  */
  92
  93 METHODDEF(boolean)
  94 fill_input_buffer (j_decompress_ptr cinfo)
  95 {
  96   my_src_ptr src = (my_src_ptr) cinfo->src;
  97   size_t nbytes;
  98
  99   nbytes = JFREAD(src->infile, src->buffer, INPUT_BUF_SIZE);
 100
 101   if (nbytes <= 0) {
 102     if (src->start_of_file)     /* Treat empty input file as fatal error */
 103       ERREXIT(cinfo, JERR_INPUT_EMPTY);
 104     WARNMS(cinfo, JWRN_JPEG_EOF);
 105     /* Insert a fake EOI marker */
 106     src->buffer[0] = (JOCTET) 0xFF;
 107     src->buffer[1] = (JOCTET) JPEG_EOI;
 108     nbytes = 2;
 109   }
 110
 111   src->pub.next_input_byte = src->buffer;
 112   src->pub.bytes_in_buffer = nbytes;
 113   src->start_of_file = FALSE;
 114
 115   return TRUE;
 116 }
 117
 118
 119 /*
 120  * Skip data --- used to skip over a potentially large amount of
 121  * uninteresting data (such as an APPn marker).
 122  *
 123  * Writers of suspendable-input applications must note that skip_input_data
 124  * is not granted the right to give a suspension return.  If the skip extends
 125  * beyond the data currently in the buffer, the buffer can be marked empty so
 126  * that the next read will cause a fill_input_buffer call that can suspend.
 127  * Arranging for additional bytes to be discarded before reloading the input
 128  * buffer is the application writer's problem.
 129  */
 130
 131 METHODDEF(void)
 132 skip_input_data (j_decompress_ptr cinfo, long num_bytes)
 133 {
 134   my_src_ptr src = (my_src_ptr) cinfo->src;
 135
 136   /* Just a dumb implementation for now.  Could use fseek() except
 137    * it doesn't work on pipes.  Not clear that being smart is worth
 138    * any trouble anyway --- large skips are infrequent.
 139    */
 140   if (num_bytes > 0) {
 141     while (num_bytes > (long) src->pub.bytes_in_buffer) {
 142       num_bytes -= (long) src->pub.bytes_in_buffer;
 143       (void) src->pub.fill_input_buffer(cinfo);
 144       /* note we assume that fill_input_buffer will never return FALSE,
 145        * so suspension need not be handled.
 146        */
 147     }
 148     src->pub.next_input_byte += (size_t) num_bytes;
 149     src->pub.bytes_in_buffer -= (size_t) num_bytes;
 150   }
 151 }
 152
 153
 154 /*
 155  * An additional method that can be provided by data source modules is the
 156  * resync_to_restart method for error recovery in the presence of RST markers.
 157  * For the moment, this source module just uses the default resync method
 158  * provided by the JPEG library.  That method assumes that no backtracking
 159  * is possible.
 160  */
 161
 162
 163 /*
 164  * Terminate source --- called by jpeg_finish_decompress
 165  * after all data has been read.  Often a no-op.
 166  *
 167  * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding
 168  * application must deal with any cleanup that should happen even
 169  * for error exit.
 170  */
 171
 172 METHODDEF(void)
 173 term_source (j_decompress_ptr cinfo)
 174 {
 175   /* no work necessary here */
 176 }
 177
 178
 179 /*
 180  * Prepare for input from a stdio stream.
 181  * The caller must have already opened the stream, and is responsible
 182  * for closing it after finishing decompression.
 183  */
 184
 185 JGLOBAL(void)
 186 jpeg_stdio_src (j_decompress_ptr cinfo, FILE * infile)
 187 {
 188   my_src_ptr src;
 189
 190   /* The source object and input buffer are made permanent so that a series
 191    * of JPEG images can be read from the same file by calling jpeg_stdio_src
 192    * only before the first one.  (If we discarded the buffer at the end of
 193    * one image, we'd likely lose the start of the next one.)
 194    * This makes it unsafe to use this manager and a different source
 195    * manager serially with the same JPEG object.  Caveat programmer.
 196    */
 197   if (cinfo->src == NULL) {     /* first time for this JPEG object? */
 198     cinfo->src = (struct jpeg_source_mgr *)
 199       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
 200                                   SIZEOF(my_source_mgr));
 201     src = (my_src_ptr) cinfo->src;
 202     src->buffer = (JOCTET *)
 203       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
 204                                   INPUT_BUF_SIZE * SIZEOF(JOCTET));
 205   }
 206
 207   src = (my_src_ptr) cinfo->src;
 208   src->pub.init_source = init_source;
 209   src->pub.fill_input_buffer = fill_input_buffer;
 210   src->pub.skip_input_data = skip_input_data;
 211   src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
 212   src->pub.term_source = term_source;
 213   src->infile = infile;
 214   src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
 215   src->pub.next_input_byte = NULL; /* until buffer loaded */
 216 }