Why does malloc() fail when there is enough memory?

Question

I\'m using a server with 128GB memory to do some computation. I need to malloc() a 2D float array of size 56120 * 56120. An example code is as follows:

Answer 1

int num = 56120,i,j;
ls = (float *)malloc((num * num)*sizeof(float));

num * num is 56120*56120 which is 3149454400 which overflows a signed int which causes undefined behavoir.

The reason 40000 works is that 40000*40000 is representable as an int.

Change the type of num to long long (or even unsigned int)