Question

I am getting access violation error when I run the following code on Visual studio. Perhaps I am trying to read some pointer location that I have not allocated or something, But I cannot seem to find where the problem exactly is. Need some help with the same.

#include <iostream>

class fileReader
{
public:
    FILE *fp;
    char** lines;
    fileReader()
    {
        fp = NULL;
    }
    fileReader(char* path)
    {
        int j=0;
        fp = fopen(path,"r");
        if (fp == NULL) 
            return;
        else 
        {
            lines = (char**) malloc(sizeof(char *)*56000);
            for (int i=0; i<56000; i++)
                lines[i] = (char*)malloc(sizeof(char)*1440);
            while ( fgets(lines[j], 1440, fp) )
                j++;
            fclose(fp);
        }
    }
};

int main(int argv, char** argc)
{
    char* path = "D:\\testfile.txt";
    fileReader *p = new fileReader(path);
    for (int i=0; i<2; i++)
        std::cout<<p->lines[i];
    return 0;
}

Answer 1

There is no protection on j exceeding number of elements in lines:

while ( fgets(lines[j], 1440, fp) )
    j++; 

If file contains more that 56000 lines, then this will access beyond the bounds of the array.

As this is C++, you should consider using ifstream, std::getline() and std::vector<std::string> to read the file. std::vector<std::string> will manage the memory allocation for you:

std::vector<std::string> lines;
std::ifstream in("D:\\testfile.txt");

if (in.is_open())
{
    std::string line;
    while (std::getline(in, line))
    {
        lines.push_back(line);
    }
    in.close();
}