使用“calloc”进行内存分配失败

Question

I am trying to allocate memory using calloc function but when the array size (type double ) is 48,000 x 48,000 it returns NULL pointer. However, it works perfectly for another case which is 23,000 x 23,000. Technically there should be no problem because I have more than enough RAM on this computer. I am also using the 64bit libraries.

OS: Linux 64bit

Installed memory: 376GB

Compiler: Intel MKL 2019

Compiler link line:

icc Main.c -qopenmp -DMKL_ILP64 -m64 -I$MKLROOT/include \
    -L$MKLROOT/lib/intel64 -lmkl_intel_ilp64 -lmkl_intel_thread \
    -lmkl_core -liomp5 -lpthread -lm -ldl -O2 -o main.out

And here the piece of code that I use:

int Jsize = 48000;
double *J = NULL;
J = (double *)calloc(Jsize*Jsize, sizeof(double));
if (J == NULL) printf("Null\n");

Answer 1

You have an int object named Jsize , initialized to 48000. You then compute Jsize*Jsize . The mathematical value of that expression is 2304000000 , which exceeds 2 ³¹ -1.

Type int on your system is almost certainly 32 bits, which means the multiplication will overflow. The behavior is undefined, but most likely you'll get a negative result.

Remember that the type of an expression in C is determined by the expression itself, not by the context in which it's evaluated. Multiplying an int by an int always yields an int result, even if that result is assigned to something bigger.

That value is then passed as the first argument to calloc() , which expects an argument of type size_t . If size_t is 64 bits (which it probably is), then you're passing a huge positive value, probably 18446744071718584320 if my calculations are correct.

On my 64-bit Ubuntu system, when I add a couple of printf calls to your code, I get:

Jsize*Jsize = -1990967296
(size_t)(Jsize*Jsize) = 18446744071718584320
Null

You don't have that much memory.

If you define Jsize with type size_t , you can at least try to allocate the 17+ GiB that you want. (On my system, calloc fails with ENOMEM , but I don't have as much RAM as you do.)

Note, however, that the OS might not let you allocate that much memory for one process.