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      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 }
    213