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plt-scheme  4.2.1
jdatasrc.c
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00001 /*
00002  * jdatasrc.c
00003  *
00004  * Copyright (C) 1994-1996, Thomas G. Lane.
00005  * This file is part of the Independent JPEG Group's software.
00006  * For conditions of distribution and use, see the accompanying README file.
00007  *
00008  * This file contains decompression data source routines for the case of
00009  * reading JPEG data from a file (or any stdio stream).  While these routines
00010  * are sufficient for most applications, some will want to use a different
00011  * source manager.
00012  * IMPORTANT: we assume that fread() will correctly transcribe an array of
00013  * JOCTETs from 8-bit-wide elements on external storage.  If char is wider
00014  * than 8 bits on your machine, you may need to do some tweaking.
00015  */
00016 
00017 /* this is not a core library module, so it doesn't define JPEG_INTERNALS */
00018 #include "jinclude.h"
00019 #include "jpeglib.h"
00020 #include "jerror.h"
00021 
00022 
00023 /* Expanded data source object for stdio input */
00024 
00025 typedef struct {
00026   struct jpeg_source_mgr pub;      /* public fields */
00027 
00028   FILE * infile;            /* source stream */
00029   JOCTET * buffer;          /* start of buffer */
00030   boolean start_of_file;    /* have we gotten any data yet? */
00031 } my_source_mgr;
00032 
00033 typedef my_source_mgr * my_src_ptr;
00034 
00035 #define INPUT_BUF_SIZE  4096       /* choose an efficiently fread'able size */
00036 
00037 
00038 /*
00039  * Initialize source --- called by jpeg_read_header
00040  * before any data is actually read.
00041  */
00042 
00043 METHODDEF(void)
00044 init_source (j_decompress_ptr cinfo)
00045 {
00046   my_src_ptr src = (my_src_ptr) cinfo->src;
00047 
00048   /* We reset the empty-input-file flag for each image,
00049    * but we don't clear the input buffer.
00050    * This is correct behavior for reading a series of images from one source.
00051    */
00052   src->start_of_file = TRUE;
00053 }
00054 
00055 
00056 /*
00057  * Fill the input buffer --- called whenever buffer is emptied.
00058  *
00059  * In typical applications, this should read fresh data into the buffer
00060  * (ignoring the current state of next_input_byte & bytes_in_buffer),
00061  * reset the pointer & count to the start of the buffer, and return TRUE
00062  * indicating that the buffer has been reloaded.  It is not necessary to
00063  * fill the buffer entirely, only to obtain at least one more byte.
00064  *
00065  * There is no such thing as an EOF return.  If the end of the file has been
00066  * reached, the routine has a choice of ERREXIT() or inserting fake data into
00067  * the buffer.  In most cases, generating a warning message and inserting a
00068  * fake EOI marker is the best course of action --- this will allow the
00069  * decompressor to output however much of the image is there.  However,
00070  * the resulting error message is misleading if the real problem is an empty
00071  * input file, so we handle that case specially.
00072  *
00073  * In applications that need to be able to suspend compression due to input
00074  * not being available yet, a FALSE return indicates that no more data can be
00075  * obtained right now, but more may be forthcoming later.  In this situation,
00076  * the decompressor will return to its caller (with an indication of the
00077  * number of scanlines it has read, if any).  The application should resume
00078  * decompression after it has loaded more data into the input buffer.  Note
00079  * that there are substantial restrictions on the use of suspension --- see
00080  * the documentation.
00081  *
00082  * When suspending, the decompressor will back up to a convenient restart point
00083  * (typically the start of the current MCU). next_input_byte & bytes_in_buffer
00084  * indicate where the restart point will be if the current call returns FALSE.
00085  * Data beyond this point must be rescanned after resumption, so move it to
00086  * the front of the buffer rather than discarding it.
00087  */
00088 
00089 METHODDEF(boolean)
00090 fill_input_buffer (j_decompress_ptr cinfo)
00091 {
00092   my_src_ptr src = (my_src_ptr) cinfo->src;
00093   size_t nbytes;
00094 
00095   nbytes = JFREAD(src->infile, src->buffer, INPUT_BUF_SIZE);
00096 
00097   if (nbytes <= 0) {
00098     if (src->start_of_file) /* Treat empty input file as fatal error */
00099       ERREXIT(cinfo, JERR_INPUT_EMPTY);
00100     WARNMS(cinfo, JWRN_JPEG_EOF);
00101     /* Insert a fake EOI marker */
00102     src->buffer[0] = (JOCTET) 0xFF;
00103     src->buffer[1] = (JOCTET) JPEG_EOI;
00104     nbytes = 2;
00105   }
00106 
00107   src->pub.next_input_byte = src->buffer;
00108   src->pub.bytes_in_buffer = nbytes;
00109   src->start_of_file = FALSE;
00110 
00111   return TRUE;
00112 }
00113 
00114 
00115 /*
00116  * Skip data --- used to skip over a potentially large amount of
00117  * uninteresting data (such as an APPn marker).
00118  *
00119  * Writers of suspendable-input applications must note that skip_input_data
00120  * is not granted the right to give a suspension return.  If the skip extends
00121  * beyond the data currently in the buffer, the buffer can be marked empty so
00122  * that the next read will cause a fill_input_buffer call that can suspend.
00123  * Arranging for additional bytes to be discarded before reloading the input
00124  * buffer is the application writer's problem.
00125  */
00126 
00127 METHODDEF(void)
00128 skip_input_data (j_decompress_ptr cinfo, long num_bytes)
00129 {
00130   my_src_ptr src = (my_src_ptr) cinfo->src;
00131 
00132   /* Just a dumb implementation for now.  Could use fseek() except
00133    * it doesn't work on pipes.  Not clear that being smart is worth
00134    * any trouble anyway --- large skips are infrequent.
00135    */
00136   if (num_bytes > 0) {
00137     while (num_bytes > (long) src->pub.bytes_in_buffer) {
00138       num_bytes -= (long) src->pub.bytes_in_buffer;
00139       (void) fill_input_buffer(cinfo);
00140       /* note we assume that fill_input_buffer will never return FALSE,
00141        * so suspension need not be handled.
00142        */
00143     }
00144     src->pub.next_input_byte += (size_t) num_bytes;
00145     src->pub.bytes_in_buffer -= (size_t) num_bytes;
00146   }
00147 }
00148 
00149 
00150 /*
00151  * An additional method that can be provided by data source modules is the
00152  * resync_to_restart method for error recovery in the presence of RST markers.
00153  * For the moment, this source module just uses the default resync method
00154  * provided by the JPEG library.  That method assumes that no backtracking
00155  * is possible.
00156  */
00157 
00158 
00159 /*
00160  * Terminate source --- called by jpeg_finish_decompress
00161  * after all data has been read.  Often a no-op.
00162  *
00163  * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding
00164  * application must deal with any cleanup that should happen even
00165  * for error exit.
00166  */
00167 
00168 METHODDEF(void)
00169 term_source (j_decompress_ptr cinfo)
00170 {
00171   /* no work necessary here */
00172 }
00173 
00174 
00175 /*
00176  * Prepare for input from a stdio stream.
00177  * The caller must have already opened the stream, and is responsible
00178  * for closing it after finishing decompression.
00179  */
00180 
00181 GLOBAL(void)
00182 jpeg_stdio_src (j_decompress_ptr cinfo, FILE * infile)
00183 {
00184   my_src_ptr src;
00185 
00186   /* The source object and input buffer are made permanent so that a series
00187    * of JPEG images can be read from the same file by calling jpeg_stdio_src
00188    * only before the first one.  (If we discarded the buffer at the end of
00189    * one image, we'd likely lose the start of the next one.)
00190    * This makes it unsafe to use this manager and a different source
00191    * manager serially with the same JPEG object.  Caveat programmer.
00192    */
00193   if (cinfo->src == NULL) { /* first time for this JPEG object? */
00194     cinfo->src = (struct jpeg_source_mgr *)
00195       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
00196                               SIZEOF(my_source_mgr));
00197     src = (my_src_ptr) cinfo->src;
00198     src->buffer = (JOCTET *)
00199       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
00200                               INPUT_BUF_SIZE * SIZEOF(JOCTET));
00201   }
00202 
00203   src = (my_src_ptr) cinfo->src;
00204   src->pub.init_source = init_source;
00205   src->pub.fill_input_buffer = fill_input_buffer;
00206   src->pub.skip_input_data = skip_input_data;
00207   src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
00208   src->pub.term_source = term_source;
00209   src->infile = infile;
00210   src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
00211   src->pub.next_input_byte = NULL; /* until buffer loaded */
00212 }