Either segmentation fault 11 or (-215) N >= K - c++

Question

I am trying to create a Bag of visual Words program, but I am running into an issue. Every time I run the program, I either get a segmentation fault: 11 error, or if I change the dictSize variable I get a error: (-215) N >= K in function kmeans. I have tried resizing the images, using different ones but nothing seems to help. Here is what I have up to now:
#include <opencv2/core/core.hpp>
#include "opencv2/highgui/highgui.hpp"
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/features2d/features2d.hpp>
#include <opencv2/xfeatures2d.hpp>
#include <iostream>
#include <stdio.h>
#include <dirent.h>
#include <string.h>
using namespace std;
using namespace cv;
int main(int argc, const char** argv) {
//=================================== LEARN ===================================
struct dirent *de = NULL;
DIR *d = NULL;
d = opendir(argv[1]);
if(d == NULL)
{
perror("Couldn't open directory");
return(2);
}
Mat input;
vector<KeyPoint> keypoints;
Mat descriptor;
Mat featuresUnclustered;
Ptr<DescriptorExtractor> detector = xfeatures2d::SIFT::create();
while((de = readdir(d))){
if ((strcmp(de->d_name,".") != 0) && (strcmp(de->d_name,"..") != 0) && (strcmp(de->d_name,".DS_Store") != 0)) {
char fullPath[] = "./";
strcat(fullPath, argv[1]);
strcat(fullPath, de->d_name);
printf("Current File: %s\n",fullPath);
input = imread(fullPath,CV_LOAD_IMAGE_GRAYSCALE);
cout << "Img size => x: " << input.size().width << ", y: " << input.size().height << endl;
// If the incoming frame is too big, resize it
if (input.size().width > 3000) {
double ratio = (3000.0)/(double)input.size().width;
resize(input, input, cvSize(0, 0), ratio, ratio);
cout << "New size => x: " << input.size().width << ", y: " << input.size().height << endl;
}
detector->detect(input, keypoints);
detector->compute(input, keypoints, descriptor);
featuresUnclustered.push_back(descriptor);
}
}
closedir(d);
int dictSize = 200;
TermCriteria tc(CV_TERMCRIT_ITER,100,0.001);
int retries = 1;
int flags = KMEANS_PP_CENTERS;
BOWKMeansTrainer bowTrainer(dictSize,tc,retries,flags);
Mat dictionary = bowTrainer.cluster(featuresUnclustered);
FileStorage fs("dict.yml",FileStorage::WRITE);
fs << "vocabulary" << dictionary;
fs.release();
return 0;
}

Answer 1

char fullPath[] = "./";
strcat(fullPath, argv[1]);
strcat(fullPath, de->d_name);
That part of your code is a serious bug (undefined behavior, but seg fault most likely).
strcat does not allocate any extra space for the concatenation. It only overwrites whatever follows the terminating null in the first string.
Your fullPath is allocated with just enough space for the initial 2 characters plus terminating null. Whatever follows that terminating null may be memory belonging to some other part of your program.
If you know the maximum file path length for your OS, you can use the crude correction of putting that max plus 2 as a number (or named constant) between the [] in the declaration of fullPath.
The less crude correction is to compute the required lenth of the string you want to build and malloc that much space (be sure to count the terminating null) and combine the three strings there.