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.
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
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.
17 /* this is not a core library module, so it doesn't define JPEG_INTERNALS */
23 /* Expanded data source object for stdio input */
26 struct jpeg_source_mgr pub; /* public fields */
28 FILE * infile; /* source stream */
29 JOCTET * buffer; /* start of buffer */
30 boolean start_of_file; /* have we gotten any data yet? */
33 typedef my_source_mgr * my_src_ptr;
35 #define INPUT_BUF_SIZE 4096 /* choose an efficiently fread'able size */
39 * Initialize source --- called by jpeg_read_header
40 * before any data is actually read.
44 init_source (j_decompress_ptr cinfo)
46 my_src_ptr src = (my_src_ptr) cinfo->src;
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.
52 src->start_of_file = TRUE;
57 * Fill the input buffer --- called whenever buffer is emptied.
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.
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.
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
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.
90 fill_input_buffer (j_decompress_ptr cinfo)
92 my_src_ptr src = (my_src_ptr) cinfo->src;
95 nbytes = JFREAD(src->infile, src->buffer, INPUT_BUF_SIZE);
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;
107 src->pub.next_input_byte = src->buffer;
108 src->pub.bytes_in_buffer = nbytes;
109 src->start_of_file = FALSE;
116 * Skip data --- used to skip over a potentially large amount of
117 * uninteresting data (such as an APPn marker).
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.
128 skip_input_data (j_decompress_ptr cinfo, long num_bytes)
130 my_src_ptr src = (my_src_ptr) cinfo->src;
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.
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.
144 src->pub.next_input_byte += (size_t) num_bytes;
145 src->pub.bytes_in_buffer -= (size_t) num_bytes;
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
160 * Terminate source --- called by jpeg_finish_decompress
161 * after all data has been read. Often a no-op.
163 * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding
164 * application must deal with any cleanup that should happen even
169 term_source (j_decompress_ptr cinfo)
171 /* no work necessary here */
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.
182 jpeg_stdio_src (j_decompress_ptr cinfo, FILE * infile)
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.
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));
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 */