jdatasrc.cpp (7639B)
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 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 my_src_ptr src = (my_src_ptr) cinfo->src; 92 93 memcpy( src->buffer, src->infile, INPUT_BUF_SIZE ); 94 95 src->infile += INPUT_BUF_SIZE; 96 97 src->pub.next_input_byte = src->buffer; 98 src->pub.bytes_in_buffer = INPUT_BUF_SIZE; 99 src->start_of_file = FALSE; 100 101 return TRUE; 102 } 103 104 105 /* 106 * Skip data --- used to skip over a potentially large amount of 107 * uninteresting data (such as an APPn marker). 108 * 109 * Writers of suspendable-input applications must note that skip_input_data 110 * is not granted the right to give a suspension return. If the skip extends 111 * beyond the data currently in the buffer, the buffer can be marked empty so 112 * that the next read will cause a fill_input_buffer call that can suspend. 113 * Arranging for additional bytes to be discarded before reloading the input 114 * buffer is the application writer's problem. 115 */ 116 117 METHODDEF void 118 skip_input_data( j_decompress_ptr cinfo, long num_bytes ) { 119 my_src_ptr src = (my_src_ptr) cinfo->src; 120 121 /* Just a dumb implementation for now. Could use fseek() except 122 * it doesn't work on pipes. Not clear that being smart is worth 123 * any trouble anyway --- large skips are infrequent. 124 */ 125 if ( num_bytes > 0 ) { 126 while ( num_bytes > (long) src->pub.bytes_in_buffer ) { 127 num_bytes -= (long) src->pub.bytes_in_buffer; 128 (void) fill_input_buffer( cinfo ); 129 /* note we assume that fill_input_buffer will never return FALSE, 130 * so suspension need not be handled. 131 */ 132 } 133 src->pub.next_input_byte += (size_t) num_bytes; 134 src->pub.bytes_in_buffer -= (size_t) num_bytes; 135 } 136 } 137 138 139 /* 140 * An additional method that can be provided by data source modules is the 141 * resync_to_restart method for error recovery in the presence of RST markers. 142 * For the moment, this source module just uses the default resync method 143 * provided by the JPEG library. That method assumes that no backtracking 144 * is possible. 145 */ 146 147 148 /* 149 * Terminate source --- called by jpeg_finish_decompress 150 * after all data has been read. Often a no-op. 151 * 152 * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding 153 * application must deal with any cleanup that should happen even 154 * for error exit. 155 */ 156 157 METHODDEF void 158 term_source( j_decompress_ptr cinfo ) { 159 /* no work necessary here */ 160 } 161 162 163 /* 164 * Prepare for input from a stdio stream. 165 * The caller must have already opened the stream, and is responsible 166 * for closing it after finishing decompression. 167 */ 168 169 GLOBAL void 170 jpeg_stdio_src( j_decompress_ptr cinfo, unsigned char * infile ) { 171 my_src_ptr src; 172 173 /* The source object and input buffer are made permanent so that a series 174 * of JPEG images can be read from the same file by calling jpeg_stdio_src 175 * only before the first one. (If we discarded the buffer at the end of 176 * one image, we'd likely lose the start of the next one.) 177 * This makes it unsafe to use this manager and a different source 178 * manager serially with the same JPEG object. Caveat programmer. 179 */ 180 if ( cinfo->src == NULL ) {/* first time for this JPEG object? */ 181 cinfo->src = (struct jpeg_source_mgr *) 182 ( * cinfo->mem->alloc_small )( (j_common_ptr) cinfo, JPOOL_PERMANENT, 183 SIZEOF( my_source_mgr ) ); 184 src = (my_src_ptr) cinfo->src; 185 src->buffer = (JOCTET *) 186 ( *cinfo->mem->alloc_small )( (j_common_ptr) cinfo, JPOOL_PERMANENT, 187 INPUT_BUF_SIZE * SIZEOF( JOCTET ) ); 188 } 189 190 src = (my_src_ptr) cinfo->src; 191 src->pub.init_source = init_source; 192 src->pub.fill_input_buffer = fill_input_buffer; 193 src->pub.skip_input_data = skip_input_data; 194 src->pub.resync_to_restart = jpeg_resync_to_restart;/* use default method */ 195 src->pub.term_source = term_source; 196 src->infile = infile; 197 src->pub.bytes_in_buffer = 0;/* forces fill_input_buffer on first read */ 198 src->pub.next_input_byte = NULL;/* until buffer loaded */ 199 }