Data read by fread is longer than what I wrote with fwrite - c

Question

I made a program that can look for prime numbers. It works fine until it hits a point where the file is suddenly longer than what I wrote with fwrite. Here is the code. I am sorry, it is a bit long, but it is complete and executable.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
long long *loadPrimeNums(char *fileName);
void putPrimeNumsInList(long long *primeNums);
void addIntNode(void);
void initialize(void);
void DestroyIntNodes(void);
void writeInFile(void);
void run(void);
int main ()
{
initialize();
putPrimeNumsInList(loadPrimeNums("primeNums"));
run();
writeInFile();
DestroyIntNodes();
_fcloseall();
return 0;
}
Second file:
#include <stdio.h>
#include <stdlib.h>
struct longLongNode
{
long long value;
struct longLongNode *next;
};
long long *loadPrimeNums(char *fileName);
void putPrimeNumsInList(long long *primeNums);
void addLongLongNode(void);
void initialize(void);
void DestroyIntNodes(void);
void writeInFile(void);
void run(void);
FILE *primeNums_fp;
long long currentOffset;
struct longLongNode *head, *tail, *lastOldPrimeNum;
void run()
{
struct longLongNode *current;
current = head;
int isPrime = 1;
for (int i = 0; i < 100; i++)
{
while (current != NULL)
{
if (currentOffset % current->value == 0)
{
isPrime = 0;
break;
}
current = current->next;
}
if (isPrime == 0)
{
isPrime = 1;
}
else
{
addLongLongNode();
}
currentOffset++;
current = head;
}
}
void writeInFile()
{
int numberOfNewPrimes = 0;
long long *newPrimes;
struct longLongNode *current;
current = lastOldPrimeNum;
while (current->next != NULL)
{
numberOfNewPrimes++;
current = current->next;
}
if (numberOfNewPrimes > 0)
{
current = lastOldPrimeNum->next;
newPrimes = (long long *)malloc(sizeof *newPrimes * numberOfNewPrimes);
for (int i = 0; i < numberOfNewPrimes; i++)
{
newPrimes[i] = current->value;
current = current->next;
}
fseek(primeNums_fp, 0L, 2);
fwrite(newPrimes, sizeof *newPrimes, numberOfNewPrimes, primeNums_fp);
printf("Freeing newPrimes: The array that is used to write the new prime numbers to file.\n");
free(newPrimes);
}
}
void initialize()
{
head = (struct longLongNode *)malloc(sizeof *head);
head->value = 2;
head->next = NULL;
lastOldPrimeNum = head;
}
void addLongLongNode()
{
struct longLongNode *temp;
temp = (struct longLongNode *)malloc(sizeof *temp);
temp->value = currentOffset;
temp->next = NULL;
tail->next = temp;
tail = temp;
}
long long *loadPrimeNums(char *fileName)
{
int num;
long long *ret;
primeNums_fp = fopen("primeNums", "a+");
fseek(primeNums_fp, 0L, 2);
if (ftell(primeNums_fp) != 0)
{
if (ftell(primeNums_fp) % sizeof *ret == 0)
{
num = ftell(primeNums_fp) / sizeof *ret;
ret = (long long *)malloc(sizeof *ret * (num + 1));
rewind(primeNums_fp);
fread(ret, sizeof *ret, num, primeNums_fp);
ret[num] = -1;
return ret;
}
else if (ftell(primeNums_fp) != -1)
{
printf("ERROR!!! ftell(fp): %d", ftell(primeNums_fp));
_fcloseall();
exit(0);
}
}
return NULL;
}
void putPrimeNumsInList(long long *primeNums)
{
struct longLongNode *current, *temp;
current = head;
tail = head;
if (primeNums != NULL)
{
for (int i = 0; primeNums[i] != -1; i++)
{
temp = (struct longLongNode *)malloc(sizeof *temp);
temp->value = primeNums[i];
temp->next = NULL;
current->next = temp;
current = temp;
}
tail = current;
lastOldPrimeNum = current;
}
currentOffset = current->value + 1;
printf("Freeing primeNums: The array that contains the initially loaded prime numbers of the file.\n");
free(primeNums);
}
void DestroyIntNodes()
{
struct longLongNode *current, *next;
current = head;
for (int i = 1; current != NULL; i++)
{
next = current->next;
printf("%d: Freeing %d\n", i, current->value);
free(current);
current = next;
}
}
What I get after finding 384 prime numbers is an error message that I made myself which shows how big the file is. The file should be divisible by 8, because the prime numbers are stored as long long integers, but at some point I got a file size of 3073.
Edit: You need to run the program multiple times for the error to occur. I did not know how to multithread, so I made it so it stops after a 100 iterations. Then the prime numbers it found will be stored in a file which should be divisible by 8, but isn't.
Edit2: At the start, the file does not exist. And I am using Windows 10. I am trying now the suggestion of setting fopen mode "ab+".
Edit3: Solved thanks to #user3121023!!