jdatasrc.c (7330B)
1 /* 2 * jdatasrc.c 3 * 4 * Copyright (C) 1994, 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 18 /* this is not a core library module, so it doesn't define JPEG_INTERNALS */ 19 #include "jinclude.h" 20 #include "jpeglib.h" 21 #include "jerror.h" 22 23 24 /* Expanded data source object for stdio input */ 25 26 typedef struct { 27 struct jpeg_source_mgr pub; /* public fields */ 28 29 unsigned char *infile; /* source stream */ 30 JOCTET * buffer; /* start of buffer */ 31 boolean start_of_file; /* have we gotten any data yet? */ 32 } my_source_mgr; 33 34 typedef my_source_mgr * my_src_ptr; 35 36 #define INPUT_BUF_SIZE 4096 /* choose an efficiently fread'able size */ 37 38 39 /* 40 * Initialize source --- called by jpeg_read_header 41 * before any data is actually read. 42 */ 43 44 METHODDEF void 45 init_source (j_decompress_ptr cinfo) 46 { 47 my_src_ptr src = (my_src_ptr) cinfo->src; 48 49 /* We reset the empty-input-file flag for each image, 50 * but we don't clear the input buffer. 51 * This is correct behavior for reading a series of images from one source. 52 */ 53 src->start_of_file = TRUE; 54 } 55 56 57 /* 58 * Fill the input buffer --- called whenever buffer is emptied. 59 * 60 * In typical applications, this should read fresh data into the buffer 61 * (ignoring the current state of next_input_byte & bytes_in_buffer), 62 * reset the pointer & count to the start of the buffer, and return TRUE 63 * indicating that the buffer has been reloaded. It is not necessary to 64 * fill the buffer entirely, only to obtain at least one more byte. 65 * 66 * There is no such thing as an EOF return. If the end of the file has been 67 * reached, the routine has a choice of ERREXIT() or inserting fake data into 68 * the buffer. In most cases, generating a warning message and inserting a 69 * fake EOI marker is the best course of action --- this will allow the 70 * decompressor to output however much of the image is there. However, 71 * the resulting error message is misleading if the real problem is an empty 72 * input file, so we handle that case specially. 73 * 74 * In applications that need to be able to suspend compression due to input 75 * not being available yet, a FALSE return indicates that no more data can be 76 * obtained right now, but more may be forthcoming later. In this situation, 77 * the decompressor will return to its caller (with an indication of the 78 * number of scanlines it has read, if any). The application should resume 79 * decompression after it has loaded more data into the input buffer. Note 80 * that there are substantial restrictions on the use of suspension --- see 81 * the documentation. 82 * 83 * When suspending, the decompressor will back up to a convenient restart point 84 * (typically the start of the current MCU). next_input_byte & bytes_in_buffer 85 * indicate where the restart point will be if the current call returns FALSE. 86 * Data beyond this point must be rescanned after resumption, so move it to 87 * the front of the buffer rather than discarding it. 88 */ 89 90 METHODDEF boolean 91 fill_input_buffer (j_decompress_ptr cinfo) 92 { 93 my_src_ptr src = (my_src_ptr) cinfo->src; 94 95 memcpy( src->buffer, src->infile, INPUT_BUF_SIZE ); 96 97 src->infile += INPUT_BUF_SIZE; 98 99 src->pub.next_input_byte = src->buffer; 100 src->pub.bytes_in_buffer = INPUT_BUF_SIZE; 101 src->start_of_file = FALSE; 102 103 return TRUE; 104 } 105 106 107 /* 108 * Skip data --- used to skip over a potentially large amount of 109 * uninteresting data (such as an APPn marker). 110 * 111 * Writers of suspendable-input applications must note that skip_input_data 112 * is not granted the right to give a suspension return. If the skip extends 113 * beyond the data currently in the buffer, the buffer can be marked empty so 114 * that the next read will cause a fill_input_buffer call that can suspend. 115 * Arranging for additional bytes to be discarded before reloading the input 116 * buffer is the application writer's problem. 117 */ 118 119 METHODDEF void 120 skip_input_data (j_decompress_ptr cinfo, long num_bytes) 121 { 122 my_src_ptr src = (my_src_ptr) cinfo->src; 123 124 /* Just a dumb implementation for now. Could use fseek() except 125 * it doesn't work on pipes. Not clear that being smart is worth 126 * any trouble anyway --- large skips are infrequent. 127 */ 128 if (num_bytes > 0) { 129 while (num_bytes > (long) src->pub.bytes_in_buffer) { 130 num_bytes -= (long) src->pub.bytes_in_buffer; 131 (void) fill_input_buffer(cinfo); 132 /* note we assume that fill_input_buffer will never return FALSE, 133 * so suspension need not be handled. 134 */ 135 } 136 src->pub.next_input_byte += (size_t) num_bytes; 137 src->pub.bytes_in_buffer -= (size_t) num_bytes; 138 } 139 } 140 141 142 /* 143 * An additional method that can be provided by data source modules is the 144 * resync_to_restart method for error recovery in the presence of RST markers. 145 * For the moment, this source module just uses the default resync method 146 * provided by the JPEG library. That method assumes that no backtracking 147 * is possible. 148 */ 149 150 151 /* 152 * Terminate source --- called by jpeg_finish_decompress 153 * after all data has been read. Often a no-op. 154 * 155 * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding 156 * application must deal with any cleanup that should happen even 157 * for error exit. 158 */ 159 160 METHODDEF void 161 term_source (j_decompress_ptr cinfo) 162 { 163 /* no work necessary here */ 164 } 165 166 167 /* 168 * Prepare for input from a stdio stream. 169 * The caller must have already opened the stream, and is responsible 170 * for closing it after finishing decompression. 171 */ 172 173 GLOBAL void 174 jpeg_stdio_src (j_decompress_ptr cinfo, unsigned char *infile) 175 { 176 my_src_ptr src; 177 178 /* The source object and input buffer are made permanent so that a series 179 * of JPEG images can be read from the same file by calling jpeg_stdio_src 180 * only before the first one. (If we discarded the buffer at the end of 181 * one image, we'd likely lose the start of the next one.) 182 * This makes it unsafe to use this manager and a different source 183 * manager serially with the same JPEG object. Caveat programmer. 184 */ 185 if (cinfo->src == NULL) { /* first time for this JPEG object? */ 186 cinfo->src = (struct jpeg_source_mgr *) 187 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT, 188 SIZEOF(my_source_mgr)); 189 src = (my_src_ptr) cinfo->src; 190 src->buffer = (JOCTET *) 191 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT, 192 INPUT_BUF_SIZE * SIZEOF(JOCTET)); 193 } 194 195 src = (my_src_ptr) cinfo->src; 196 src->pub.init_source = init_source; 197 src->pub.fill_input_buffer = fill_input_buffer; 198 src->pub.skip_input_data = skip_input_data; 199 src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */ 200 src->pub.term_source = term_source; 201 src->infile = infile; 202 src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */ 203 src->pub.next_input_byte = NULL; /* until buffer loaded */ 204 }