C program memory violation dependant on std::cout (?)

Question

I've written an extension in C++ for Python, and I'm currently debugging it. The extension takes 3 numpy matrices and produces 2 as a result. To the inner C++ function that does the actualy calculation I pass 3 float C arrays (just flattened and converted from input numpy arrays), and return a C float array of arrays. Everything works as intended but ONLY if I print this output array of arrays before returning it.

What the hell is going on in here?

float** gradient(float* inputs, float* kernels, float* grads, npy_intp* input_dims, npy_intp* kernels_dims, npy_intp* output_dims){


float* g_inputs = new float[batch*h*w*ch_in];
for (int i = 0; i < batch*h*w*ch_in; i++) g_inputs[i] = 0;
float* g_kernels = new float[size*ch_out];
for (int i = 0; i < size*ch_out; i++) g_kernels[i] = 0;


float* ret[2] = {{g_inputs}, {g_kernels}};
std::cout<<ret<<std::endl; //<---without this it doesn't work
return ret;
}

I've omitted irrelevant code for clarity.

Answer 1

You are returning a pointer to an object with automatic lifetime. In other words, your function returns a dangling pointer, which is Undefined Behaviour.

Although aerostatic lizards are an uncommon result of UB, anything can happen and the symptom you observe, unlike the lizards, is common.