在“VS2013編譯libjpeg庫”這篇文章中本人介紹瞭如何在VS中編譯libjpeg庫並提供了一個應用的範例，而這篇文章將在此基礎上，介紹如何用libjpeg庫來解碼記憶體中的jpeg資料。

    其實這個需求已經不新鮮了，網上也能找到別人提供的一些解決方法，之所以要再次寫不過是因為本人覺得那些方法或多或少都有些不對的地方，或者說因為版本的迭代，本來是對的，現在有點問題。當然，本人並沒有很費心思的自己去一行一行的看原始碼，實現的過程基本是參考這篇文章來做的，所以這裡只是提供怎麼改，而為什麼這麼改則可參考上文。

    OK,言歸正傳，下面正式進入主題。

    要讓libjpeg庫能直接解碼記憶體中的jpeg資料，首先想到的自然是修改其原始碼，讓其提供一個這樣的函式介面給我們呼叫，本人也正是這麼幹的。在“

    找到jdatasrc.c這個檔案（從命名可以看出其跟輸入資料相關），在my_source_mgr這個結構體的上方新增一個結構體(jpeg記憶體塊描述相關)如下：

typedef struct{
	UINT8* img_buffer;
	long buffer_size;
	long pos;
}BUFF_JPG;

typedef struct {
  struct jpeg_source_mgr pub;	/* public fields */
  union{
	  BUFF_JPG jpg;		/* jpeg image buffer */
	  FILE * infile;		/* source stream */
  };
  JOCTET * buffer;		/* start of buffer */
  boolean start_of_file;	/* have we gotten any data yet? */
} my_source_mgr;

/*
* This function will read the jpeg memery block to fill the library buffer.
*/
METHODDEF(boolean)
jpg_fill_input_buffer (j_decompress_ptr cinfo)
{
  my_src_ptr src = (my_src_ptr) cinfo->src;
  size_t nbytes;

  if(src->jpg.img_buffer == NULL || src->jpg.pos >= src->jpg.buffer_size){
    nbytes = -1;
  }
  else {
    nbytes = (src->jpg.pos + INPUT_BUF_SIZE > src->jpg.buffer_size ? 	\
   	src->jpg.buffer_size - src->jpg.pos : INPUT_BUF_SIZE);
    memcpy(src->buffer, src->jpg.img_buffer + src->jpg.pos, nbytes);
    src->jpg.pos += nbytes;
  }

  if (nbytes <= 0) {
    if (src->start_of_file)	/* Treat empty input file as fatal error */
      ERREXIT(cinfo, JERR_INPUT_EMPTY);
    WARNMS(cinfo, JWRN_JPEG_EOF);
    /* Insert a fake EOI marker */
    src->buffer[0] = (JOCTET) 0xFF;
    src->buffer[1] = (JOCTET) JPEG_EOI;
    nbytes = 2;
  }

  src->pub.next_input_byte = src->buffer;
  src->pub.bytes_in_buffer = nbytes;
  src->start_of_file = FALSE;

  return TRUE;
}

/*
* This function improve the library can use the jpeg memory block as source.
*/
GLOBAL(void)
jpeg_stdio_buffer_src (j_decompress_ptr cinfo, UINT8 * buffer, long size)
{
  my_src_ptr src;

  if (cinfo->src == NULL) {	/* first time for this JPEG object? */
    cinfo->src = (struct jpeg_source_mgr *)
      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
				  SIZEOF(my_source_mgr));
    src = (my_src_ptr) cinfo->src;
    src->buffer = (JOCTET *)
      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
				  INPUT_BUF_SIZE * SIZEOF(JOCTET));
  }

  src = (my_src_ptr) cinfo->src;
  src->pub.init_source = init_source;
  src->pub.fill_input_buffer = jpg_fill_input_buffer;
  src->pub.skip_input_data = skip_input_data;
  src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
  src->pub.term_source = term_source;
  //src->infile = infile;
  src->jpg.img_buffer = buffer;
  src->jpg.buffer_size = size;
  src->jpg.pos = 0;
  src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
  src->pub.next_input_byte = NULL; /* until buffer loaded */
}

EXTERN(void) jpeg_stdio_buffer_src JPP((j_decompress_ptr cinfo, UINT8 * buffer, long size));

    下面本人還是提供一個範例，如果懶得去改的話也可以直接用這個範例裡面本人編好的“lib”和修改過的“.h”標頭檔案。

    範例地址