Unexpected result when Comparing sample image with database of images - Opencv c++ - c++

Question

I am working on Image matching program, it captures image from IR camera stores it in one directory. A database of images exist in a different directory. I get an unexpected result while executing. I somehow realize that it's because I'm messing with different directories and something basic. Any help in getting around the mistake? Here's a snippet of the code.
VideoCapture cap(0);
Mat frame;
Mat src, dst, tmp,img_3,img_4;
filename3 = (char *)malloc(sizeof(char));
printf("\n-------------------------------------------------\n");
printf("\nOne to Many Image matching with SURF Algorithm.\n");
printf("\n-------------------------------------------------\n");
//Get the Vein image from Webcam that needs to be compared with the database.
// The below function gets frame and saves it in /home/srikbaba/opencv/veins
veincnt = captureVein(veincnt, cap);
if(veincnt == -1)
cout << "A problem has occured" << endl;
printf("\nPlease enter the filename of saved image with the extension.\n");
scanf("%s",filename3);
//Scan the directory for images.
DIR *dir;
struct dirent *ent;
clock_t tstart1 = clock(); //start clock function, do something!!!!
if ((dir = opendir ("/home/srikbaba/images")) != NULL)
{
/* print all the files and directories within directory */
while ((ent = readdir (dir)) != NULL)
{
if(ent->d_type!= DT_DIR)
{
//Print the images
printf ("%s\n", ent->d_name);
//Store the Filename
img_db = ent->d_name;
//Open each file name as Mat identifier and loop the process
img_3 = imread (filename3, IMREAD_GRAYSCALE);
//Filename3 is saved in /home/srikbaba/opencv/veins --> different directory
// I feel this is the problem
img_4 = imread (img_db, IMREAD_GRAYSCALE);
if( !img_3.data)
{
std::cout<< " --(!) Invalid filename or file not found! " << std::endl;
return -1;
}
Because of this I always get --(!) Invalid filename or file not found message. Is there a way I can compare one image from a directory and another image from a different directory? I hope what I asked is not confusing. Kindly help.
Working on Opencv - C++ on Ubuntu 12.04 LTS

Answer 1

You are getting undefined behavior because you are allocating space for only 1 character for filename3 in the following line:
filename3 = (char *)malloc(sizeof(char));
So in the following line...
scanf("%s",filename3);
Any file name greater than 1 character will invoke undefined behavior.
In my opinion, there is no need of malloc. You can just allocate large enough filename3 of fixed size like this:
char filename3[256];