Realloc "random" crash

Question

I have this function to read a triangular 2d array, but sometimes it crashes on realloc. Always on the 6th realloc (current_row = 7). Sometimes it runs fine. Cannot reproduce the error in gdb (works every time). What's wrong?

TRIANGLE *read_triangle(char *file_name)
{
std::ifstream fin(file_name);
int current_row = 0, current_column = 0, buffer;
TRIANGLE *triangle = new TRIANGLE();
triangle->triangle_values[0] = new int[1];

while (fin >> buffer)
{
    if (current_column == current_row+1)
    {
        current_column = 0;
        triangle->triangle_values = (int**)realloc(&((void*)triangle->triangle_values), (++current_row+1)*sizeof(int*));
        triangle->triangle_values[current_row] = new int[current_row];
    }
    triangle->triangle_values[current_row][current_column++] = buffer;
}
triangle->rows = current_row-1;
return triangle;
}

The TRIANGLE Definition

struct TRIANGLE 
{ 
    int **triangle_values; 
    int rows; 
    TRIANGLE(): triangle_values(NULL) 
    {
        triangle_values = new int*[1];
    } 
};

Input file example:

75
95 64
17 47 82
18 35 87 10
20 04 82 47 65
19 01 23 75 03 34
88 02 77 73 07 63 67
99 65 04 28 06 16 70 92
41 41 26 56 83 40 80 70 33
41 48 72 33 47 32 37 16 94 29
53 71 44 65 25 43 91 52 97 51 14
70 11 33 28 77 73 17 78 39 68 17 57
91 71 52 38 17 14 91 43 58 50 27 29 48
63 66 04 68 89 53 67 30 73 16 69 87 40 31
04 62 98 27 23 09 70 98 73 93 38 53 60 04 23

Answer 1

You're mixing the old style (realloc) and new style (new) memory allocation functions which is not guaranteed to work. By that, I mean they'll work provided you keep them separate but allocating memory with new and then trying to expand that same memory with realloc is a definite no-no.

From C++11 20.6.13, when talking about how the old style functions control the reachability of their blocks:

It also allows malloc() to be implemented with a separate allocation arena.

Hence, there's no guarantee that the memory arenas for old and new style are related to each other at all.

C++ provides all sort of wonderful collection classes with far better resizing methods than malloc/realloc. You should fully embrace the language by using them (such as vector).

Generally, C++ programmers don't use the legacy C stuff unless they're writing things that need to be usable in both C and C++, in which case they're probably better referred to as C programmers, at least temporarily :-)