保存JPEG文件時出現分段錯誤（來自RGB數據數組）

Question

我在網上找到以下代碼，用於使用libjpeg庫讀取和寫入JPEG文件。

我將函數oid write_JPEG_file (char * filename, int quality)更改為以下內容：

void write_JPEG_vetor (JSAMPLE * image_data, int height, int width, int quality)
{
  printf("%s\n","write_JPEG_vetor");
  /* This struct contains the JPEG compression parameters and pointers to
   * working space (which is allocated as needed by the JPEG library).
   * It is possible to have several such structures, representing multiple
   * compression/decompression processes, in existence at once.  We refer
   * to any one struct (and its associated working data) as a "JPEG object".
   */
  struct jpeg_compress_struct cinfo;
  /* This struct represents a JPEG error handler.  It is declared separately
   * because applications often want to supply a specialized error handler
   * (see the second half of this file for an example).  But here we just
   * take the easy way out and use the standard error handler, which will
   * print a message on stderr and call exit() if compression fails.
   * Note that this struct must live as long as the main JPEG parameter
   * struct, to avoid dangling-pointer problems.
   */
  struct jpeg_error_mgr jerr;
  /* More stuff */
  FILE * outfile;       /* target file */
  JSAMPROW row_pointer[1];  /* pointer to JSAMPLE row[s] */
  int row_stride;       /* physical row width in image buffer */

  printf ("%s\n","Step 1: allocate and initialize JPEG compression object */");

  /* We have to set up the error handler first, in case the initialization
   * step fails.  (Unlikely, but it could happen if you are out of memory.)
   * This routine fills in the contents of struct jerr, and returns jerr's
   * address which we place into the link field in cinfo.
   */
  cinfo.err = jpeg_std_error(&jerr);
  /* Now we can initialize the JPEG compression object. */
  jpeg_create_compress(&cinfo);

  printf ("%s\n","/* Step 2: specify data destination (eg, a file) */");
  /* Note: steps 2 and 3 can be done in either order. */

  /* Here we use the library-supplied code to send compressed data to a
   * stdio stream.  You can also write your own code to do something else.
   * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
   * requires it in order to write binary files.
   */

  char * filename = {"novo_arquivo.jpeg"};

  if ((outfile = fopen(filename, "wb")) == NULL) {
    fprintf(stderr, "can't open %s\n", filename);
    exit(1);
  }
  jpeg_stdio_dest(&cinfo, outfile);

  printf ("%s\n","/* Step 3: set parameters for compression */");

  /* First we supply a description of the input image.
   * Four fields of the cinfo struct must be filled in:
   */
  cinfo.image_width = width;    /* image width and height, in pixels */
  cinfo.image_height = height;
  cinfo.input_components = 3;       /* # of color components per pixel */
  cinfo.in_color_space = JCS_RGB;   /* colorspace of input image */
  /* Now use the library's routine to set default compression parameters.
   * (You must set at least cinfo.in_color_space before calling this,
   * since the defaults depend on the source color space.)
   */
  jpeg_set_defaults(&cinfo);
  /* Now you can set any non-default parameters you wish to.
   * Here we just illustrate the use of quality (quantization table) scaling:
   */
  jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);

  printf ("%s\n","/* Step 4: Start compressor */");

  /* TRUE ensures that we will write a complete interchange-JPEG file.
   * Pass TRUE unless you are very sure of what you're doing.
   */
  jpeg_start_compress(&cinfo, TRUE);

  printf ("%s\n","/* Step 5: while (scan lines remain to be written) */");
  /*           jpeg_write_scanlines(...); */

  /* Here we use the library's state variable cinfo.next_scanline as the
   * loop counter, so that we don't have to keep track ourselves.
   * To keep things simple, we pass one scanline per call; you can pass
   * more if you wish, though.
   */
  row_stride = width * 3;   /* JSAMPLEs per row in image_buffer */

  while (cinfo.next_scanline < cinfo.image_height) {
    printf ("%s\n","Loop WHILE");
    /* jpeg_write_scanlines expects an array of pointers to scanlines.
     * Here the array is only one element long, but you could pass
     * more than one scanline at a time if that's more convenient.
     */

    row_pointer[0] = &image_data[cinfo.next_scanline * row_stride];
    (void) jpeg_write_scanlines(&cinfo, row_pointer, row_stride);
  }

  printf ("%s\n","/* Step 6: Finish compression */");

  jpeg_finish_compress(&cinfo);
  /* After finish_compress, we can close the output file. */
  fclose(outfile);

  printf ("%s\n","/* Step 7: release JPEG compression object */");

  /* This is an important step since it will release a good deal of memory. */
  jpeg_destroy_compress(&cinfo);

  /* And we're done! */
}

現在，當我在Linux環境中運行該程序時，出現了錯誤Segmentation Fault。 有人可以說出為什么會這樣嗎？ 我主要的懷疑是代碼：

while (cinfo.next_scanline < cinfo.image_height) {
    printf ("%s\n","Loop WHILE");
    /* jpeg_write_scanlines expects an array of pointers to scanlines.
     * Here the array is only one element long, but you could pass
     * more than one scanline at a time if that's more convenient.
     */

    row_pointer[0] = &image_data[cinfo.next_scanline * row_stride];
    (void) jpeg_write_scanlines(&cinfo, row_pointer, row_stride);
  }

但是我不確定，盡管花了很多時間，也找不到解決方案。

===更新===我在代碼的這一部分中包含以下調試代碼：

while (cinfo.next_scanline < cinfo.image_height) {
    printf ("%s\n","Loop WHILE");
    /* jpeg_write_scanlines expects an array of pointers to scanlines.
     * Here the array is only one element long, but you could pass
     * more than one scanline at a time if that's more convenient.
     */

    printf ("%s\n","parte 1.1");
    row_pointer[0] = &image_data[cinfo.next_scanline * row_stride];
    printf ("%s\n","parte 1.2");
    printf ("%s\n","parte 2.1");
    (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
    printf ("%s\n","parte 2.2");
  }

這樣，運行programa時的輸出為：

Loop WHILE
parte 1.1
parte 1.2
parte 2.1

===更新2 ===記錄一下，在我的程序中，此函數正在接收此函數的返回值：

JSAMPLE * inverte_imagem()
{
  int tamanho = image_height*image_width*image_colors;
  int i;
  JSAMPLE * vetor = malloc(sizeof(JSAMPLE)*(image_height*image_width*image_colors));
  for( i=0; i<tamanho; i++)
    vetor [i] = image_buffer [tamanho - (i+1)];
}

Answer 1

這看起來是錯誤的：

(void) jpeg_write_scanlines(&cinfo, row_pointer, row_stride);

最后一個參數是要寫入的行數，而不是行長。 您可能想要：

(void) jpeg_write_scanlines(&cinfo, row_pointer, 1);

Answer 2

好的，我解決了將對write_JPEG_vector的調用放在函數inverte_imagem（）中的問題。 我不知道為什么，但是當我從主函數調用此函數時，發生了內存問題（Linux上的錯誤分段錯誤）。