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1 /*
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2 * jdatasrc.c
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3 *
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4 * Copyright (C) 1994-1996, Thomas G. Lane.
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5 * This file is part of the Independent JPEG Group's software.
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6 * For conditions of distribution and use, see the accompanying README file.
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7 *
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8 * This file contains decompression data source routines for the case of
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9 * reading JPEG data from a file (or any stdio stream). While these routines
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10 * are sufficient for most applications, some will want to use a different
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11 * source manager.
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12 * IMPORTANT: we assume that fread() will correctly transcribe an array of
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13 * JOCTETs from 8-bit-wide elements on external storage. If char is wider
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14 * than 8 bits on your machine, you may need to do some tweaking.
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15 */
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16
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17 /* this is not a core library module, so it doesn't define JPEG_INTERNALS */
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18 #include "jinclude.h"
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19 #include "jpeglib.h"
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20 #include "jerror.h"
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21
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22
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23 /* Expanded data source object for stdio input */
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24
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25 typedef struct {
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26 struct jpeg_source_mgr pub; /* public fields */
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27
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28 FILE * infile; /* source stream */
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29 JOCTET * buffer; /* start of buffer */
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30 boolean start_of_file; /* have we gotten any data yet? */
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31 } my_source_mgr;
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32
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33 typedef my_source_mgr * my_src_ptr;
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34
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35 #define INPUT_BUF_SIZE 4096 /* choose an efficiently fread'able size */
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36
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37
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38 /*
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39 * Initialize source --- called by jpeg_read_header
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40 * before any data is actually read.
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41 */
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42
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43 METHODDEF(void)
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44 init_source (j_decompress_ptr cinfo)
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45 {
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46 my_src_ptr src = (my_src_ptr) cinfo->src;
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47
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48 /* We reset the empty-input-file flag for each image,
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49 * but we don't clear the input buffer.
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50 * This is correct behavior for reading a series of images from one source.
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51 */
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52 src->start_of_file = TRUE;
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53 }
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54
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55
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56 /*
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57 * Fill the input buffer --- called whenever buffer is emptied.
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58 *
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59 * In typical applications, this should read fresh data into the buffer
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60 * (ignoring the current state of next_input_byte & bytes_in_buffer),
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61 * reset the pointer & count to the start of the buffer, and return TRUE
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62 * indicating that the buffer has been reloaded. It is not necessary to
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63 * fill the buffer entirely, only to obtain at least one more byte.
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64 *
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65 * There is no such thing as an EOF return. If the end of the file has been
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66 * reached, the routine has a choice of ERREXIT() or inserting fake data into
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67 * the buffer. In most cases, generating a warning message and inserting a
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68 * fake EOI marker is the best course of action --- this will allow the
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69 * decompressor to output however much of the image is there. However,
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70 * the resulting error message is misleading if the real problem is an empty
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71 * input file, so we handle that case specially.
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72 *
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73 * In applications that need to be able to suspend compression due to input
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74 * not being available yet, a FALSE return indicates that no more data can be
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75 * obtained right now, but more may be forthcoming later. In this situation,
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76 * the decompressor will return to its caller (with an indication of the
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77 * number of scanlines it has read, if any). The application should resume
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78 * decompression after it has loaded more data into the input buffer. Note
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79 * that there are substantial restrictions on the use of suspension --- see
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80 * the documentation.
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81 *
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82 * When suspending, the decompressor will back up to a convenient restart point
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83 * (typically the start of the current MCU). next_input_byte & bytes_in_buffer
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84 * indicate where the restart point will be if the current call returns FALSE.
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85 * Data beyond this point must be rescanned after resumption, so move it to
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86 * the front of the buffer rather than discarding it.
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87 */
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88
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89 METHODDEF(boolean)
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90 fill_input_buffer (j_decompress_ptr cinfo)
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91 {
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92 my_src_ptr src = (my_src_ptr) cinfo->src;
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93 size_t nbytes;
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94
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95 nbytes = JFREAD(src->infile, src->buffer, INPUT_BUF_SIZE);
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96
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97 if (nbytes <= 0) {
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98 if (src->start_of_file) /* Treat empty input file as fatal error */
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99 ERREXIT(cinfo, JERR_INPUT_EMPTY);
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100 WARNMS(cinfo, JWRN_JPEG_EOF);
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101 /* Insert a fake EOI marker */
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102 src->buffer[0] = (JOCTET) 0xFF;
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103 src->buffer[1] = (JOCTET) JPEG_EOI;
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104 nbytes = 2;
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105 }
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106
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107 src->pub.next_input_byte = src->buffer;
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108 src->pub.bytes_in_buffer = nbytes;
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109 src->start_of_file = FALSE;
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110
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111 return TRUE;
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112 }
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113
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114
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115 /*
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116 * Skip data --- used to skip over a potentially large amount of
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117 * uninteresting data (such as an APPn marker).
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118 *
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119 * Writers of suspendable-input applications must note that skip_input_data
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120 * is not granted the right to give a suspension return. If the skip extends
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121 * beyond the data currently in the buffer, the buffer can be marked empty so
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122 * that the next read will cause a fill_input_buffer call that can suspend.
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123 * Arranging for additional bytes to be discarded before reloading the input
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124 * buffer is the application writer's problem.
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125 */
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126
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127 METHODDEF(void)
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128 skip_input_data (j_decompress_ptr cinfo, long num_bytes)
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129 {
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130 my_src_ptr src = (my_src_ptr) cinfo->src;
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131
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132 /* Just a dumb implementation for now. Could use fseek() except
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133 * it doesn't work on pipes. Not clear that being smart is worth
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134 * any trouble anyway --- large skips are infrequent.
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135 */
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136 if (num_bytes > 0) {
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137 while (num_bytes > (long) src->pub.bytes_in_buffer) {
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138 num_bytes -= (long) src->pub.bytes_in_buffer;
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139 (void) fill_input_buffer(cinfo);
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140 /* note we assume that fill_input_buffer will never return FALSE,
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141 * so suspension need not be handled.
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142 */
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143 }
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144 src->pub.next_input_byte += (size_t) num_bytes;
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145 src->pub.bytes_in_buffer -= (size_t) num_bytes;
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146 }
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147 }
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148
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149
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150 /*
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151 * An additional method that can be provided by data source modules is the
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152 * resync_to_restart method for error recovery in the presence of RST markers.
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153 * For the moment, this source module just uses the default resync method
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154 * provided by the JPEG library. That method assumes that no backtracking
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155 * is possible.
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156 */
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157
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158
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159 /*
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160 * Terminate source --- called by jpeg_finish_decompress
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161 * after all data has been read. Often a no-op.
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162 *
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163 * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding
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164 * application must deal with any cleanup that should happen even
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165 * for error exit.
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166 */
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167
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168 METHODDEF(void)
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169 term_source (j_decompress_ptr cinfo)
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170 {
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171 /* no work necessary here */
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172 }
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173
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174
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175 /*
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176 * Prepare for input from a stdio stream.
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177 * The caller must have already opened the stream, and is responsible
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178 * for closing it after finishing decompression.
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179 */
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180
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181 GLOBAL(void)
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182 jpeg_stdio_src (j_decompress_ptr cinfo, FILE * infile)
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183 {
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184 my_src_ptr src;
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185
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186 /* The source object and input buffer are made permanent so that a series
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187 * of JPEG images can be read from the same file by calling jpeg_stdio_src
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188 * only before the first one. (If we discarded the buffer at the end of
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189 * one image, we'd likely lose the start of the next one.)
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190 * This makes it unsafe to use this manager and a different source
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191 * manager serially with the same JPEG object. Caveat programmer.
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192 */
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193 if (cinfo->src == NULL) { /* first time for this JPEG object? */
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194 cinfo->src = (struct jpeg_source_mgr *)
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195 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
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196 SIZEOF(my_source_mgr));
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197 src = (my_src_ptr) cinfo->src;
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198 src->buffer = (JOCTET *)
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199 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
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200 INPUT_BUF_SIZE * SIZEOF(JOCTET));
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201 }
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202
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203 src = (my_src_ptr) cinfo->src;
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204 src->pub.init_source = init_source;
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205 src->pub.fill_input_buffer = fill_input_buffer;
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206 src->pub.skip_input_data = skip_input_data;
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207 src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
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208 src->pub.term_source = term_source;
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209 src->infile = infile;
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210 src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
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211 src->pub.next_input_byte = NULL; /* until buffer loaded */
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212 }
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