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 }