Use size_t, which is better for arrays of very large sizes.

src/list.c

@@ -19,7 +19,7 @@ void initList(List *restrict l) {
 }
 
 void deInitList(List *restrict l) {
-	for(ulli i = 0; i < l->size; ++i) {
+	for(size_t i = 0; i < l->size; ++i) {
 		mpz_clear(l->list[i]);
 	}
 	free(l->list);

src/list.h

@@ -2,15 +2,13 @@
 #include <gmp.h>
 #include <stdbool.h>
 
-#include "types.h"
-
 /**
  * @brief An infinitely expanding list type.
  */
 typedef struct {
 	mpz_t *list; //!< The list of elements
-	ulli size; //!< How many elements are in the list
-	ulli end; //!< The last element of the list (in use)
+	size_t size; //!< How many elements are in the list
+	size_t end; //!< The last element of the list (in use)
 } List;
 
 /**

src/main.c

@@ -5,7 +5,6 @@
 #include <gmp.h>
 
 #include "list.h"
-#include "types.h"
 
 static bool run;
 void leave();
@@ -43,7 +42,7 @@ int main(void) {
 		// Calculate half of `num'
 		mpz_fdiv_q_ui(halfNum, num, 2);
 		// Loop through found primes
-		for(ulli i = 0; i < primes.end; ++i) {
+		for(size_t i = 0; i < primes.end; ++i) {
 			if(mpz_cmp(primes.list[i], halfNum) > 0) break;
 			// If `num' is divisible by a prime then go to the next number
 			if(mpz_divisible_p(num, primes.list[i]) != 0)

src/types.h

@@ -1,2 +0,0 @@
-#pragma once
-typedef unsigned long long int ulli;