jpeg/jdatasrc.c

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