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
25typedef 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
33typedef 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
43METHODDEF(void)
44init_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
89METHODDEF(boolean)
90fill_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
127METHODDEF(void)
128skip_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
168METHODDEF(void)
169term_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
181GLOBAL(void)
182jpeg_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