modified logic judgment for checking JPEG format.JFIF and Exif is mor…

…st popular file format of JPEG encoded image,but not all,for MJPEG format that used by camera,the flag is 'AVI1' OR not any flag ,starting FFC0 after FFD8.

so the old logic is not work for other JPEG format.see  my blog for detail : https://blog.csdn.net/10km/article/details/82155967

src/mybmp/jpeg.c

@@ -258,7 +258,6 @@ void* __mg_init_jpg (MG_RWops *fp, MYBITMAP* mybmp, RGB* pal)
 {
     int i;
     unsigned char magic[5];
-    Uint16 magic_db;
 
     /* This struct contains the JPEG decompression parameters
      * and pointers to working space 
@@ -273,17 +272,8 @@ void* __mg_init_jpg (MG_RWops *fp, MYBITMAP* mybmp, RGB* pal)
     if (magic[0] != 0xFF || magic[1] != 0xD8)
         goto err;        /* not JPEG image*/
 
-    magic_db = MGUI_ReadLE16 (fp);
-    MGUI_RWread (fp, magic, 2, 1);
 
-    MGUI_RWread (fp, magic, 4, 1);
-    magic [4] = '\0';
-    if (magic_db != 0xDBFF 
-                    && strncmp((char*)magic, "JFIF", 4) != 0 
-                    && strncmp((char*)magic, "Exif", 4) != 0)
-        goto err;        /* not JPEG image*/
-
-    MGUI_RWseek (fp, -10, SEEK_CUR);
+    MGUI_RWseek (fp, 0, SEEK_SET);
 
     /* Step 1: allocate and initialize JPEG decompression object */
     cinfo = calloc (1, sizeof(struct jpeg_decompress_struct));
@@ -318,7 +308,10 @@ void* __mg_init_jpg (MG_RWops *fp, MYBITMAP* mybmp, RGB* pal)
     my_jpeg_data_src (cinfo, fp);
 
     /* Step 3: read file parameters with jpeg_read_header() */
-    jpeg_read_header (cinfo, TRUE);
+    if(JPEG_HEADER_OK != jpeg_read_header (cinfo, TRUE))
+    {
+    	longjmp (jerr->setjmp_buffer, 1);
+    }
 
     /* Step 4: set parameters for decompression */
     cinfo->out_color_space = (mybmp->flags & MYBMP_LOAD_GRAYSCALE) ? JCS_GRAYSCALE: JCS_RGB;
@@ -456,24 +449,65 @@ int __mg_load_jpg (MG_RWops* fp, void* init_info, MYBITMAP *my_bmp,
 BOOL __mg_check_jpg (MG_RWops* fp)
 {
     unsigned char magic [5];
-    Uint16 magic_db;
 
     if (!MGUI_RWread (fp, magic, 2, 1))
         return FALSE;        /* not JPEG image*/
     if (magic[0] != 0xFF || magic[1] != 0xD8)
         return FALSE;        /* not JPEG image*/
 
-    magic_db = MGUI_ReadLE16 (fp);
-    MGUI_RWread (fp, magic, 2, 1);
+    MGUI_RWseek (fp, 0, SEEK_SET);
 
-    MGUI_RWread (fp, magic, 4, 1);
-    magic [4] = '\0';
-    if (magic_db != 0xDBFF 
-                    && strncmp((char*)magic, "JFIF", 4) != 0 
-                    && strncmp((char*)magic, "Exif", 4) != 0)
-        return FALSE;        /* not JPEG image*/
+    /* This struct contains the JPEG decompression parameters
+     * and pointers to working space
+     * (which is allocated as needed by the JPEG library).
+     */
+    struct jpeg_decompress_struct *cinfo;
+    struct my_error_mgr *jerr;
+    jpeg_init_info_t* init_info;
 
-    return TRUE;
+    /* Step 1: allocate and initialize JPEG decompression object */
+    cinfo = calloc (1, sizeof(struct jpeg_decompress_struct));
+    jerr = calloc (1, sizeof(struct my_error_mgr));
+    init_info = calloc (1, sizeof(jpeg_init_info_t));
+
+    if (cinfo == NULL || jerr == NULL || init_info == NULL)
+        return FALSE;
+
+    /* We set up the normal JPEG error routines first. */
+    cinfo->err = jpeg_std_error (&jerr->pub);
+    jerr->pub.error_exit = my_error_exit;
+
+
+    /* return value*/
+    BOOL ret = TRUE;
+
+    /* Establish the setjmp return context for my_error_exit to use. */
+    if (setjmp (jerr->setjmp_buffer))
+    {
+        /* If we get here, the JPEG code has signaled an error */
+        ret = FALSE;
+        goto end;
+    }
+
+    /* Now we can initialize the JPEG decompression object. */
+    jpeg_create_decompress (cinfo);
+    init_info->dec_started = 0;
+
+    /* Step 2: specify data source */
+    my_jpeg_data_src (cinfo, fp);
+
+    /* Step 3: read file parameters with jpeg_read_header() */
+    if(JPEG_HEADER_OK != jpeg_read_header (cinfo, TRUE))
+    {
+    	longjmp (jerr->setjmp_buffer, 1);
+    }
+
+end:
+    jpeg_destroy_decompress (cinfo);
+    free (init_info);
+    free (jerr);
+    free (cinfo);
+    return ret;
 }
 
 #endif /* _MGIMAGE_JPG */