Finding prime number from StandardInput

Makefile

CC=gcc
CFLAGS=-O3

all: prime pi

pi:
	$(CC) $(CFLAGS) -o $@ pi.c

prime: 
	$(CC) $(CFLAGS) -o $@ prime.c -lm

clean:
	$(RM) prime pi

test: prime
	./pi | ./prime

pi.c

#include <stdio.h>

int main(){
    int a=10000;
    int b;
    int c = 2800;
    int d;
    int e;
    int f[2801];
    int g;
    for(;b-c;)
        f[b++]=a/5;
    for(;d=0,g=c*2;c-=14,printf("%.4d",e+d/a),e=d%a)
        for(b=c;d+=f[b]*a,f[b]=d%--g,d/=g--,--b;d*=b);
    return 0;
}

prime.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <stdbool.h>

inline bool isPrime(int num)
{
    if (num < 2) return false;
    if (num == 2) return true;
    if (num > 2 && num % 2 == 0) return false;
    int end = ceil(sqrt(num));
    for (int i = 3;i <= end;i += 2) {
        if (num % i == 0) {
            return false;
        }
    }
    return true;
}

int main(int argc, char *argv[])
{
    char inputBuf[2048];
    scanf("%s", inputBuf);
    // printf("len=%lu\n", strlen(inputBuf));
    char *numbers = (char*)malloc(sizeof(char) * strlen(inputBuf));
    size_t limit = strlen(inputBuf) - 2;
    for (size_t i = 2;i < limit;i++) {
        numbers[i - 2] = inputBuf[i] - '0';
    }
    // for (size_t i = 0;i < limit;i++) {
    //     printf("%i", numbers[i]);
    //     if ((i + 1) % 50 == 0) {
    //         puts("");
    //     }
    // }

    short digit = 4;
    if (argc > 1) {
        digit = atoi(argv[1]);
    }

    for (size_t i = digit - 1;i < limit;i++) {
        char c = numbers[i];
        int num = 0;
        for (int j = digit - 1;j >= 0;j--) {
            num *= 10;
            num += numbers[i - j];
        }
        if (isPrime(num)) {
            printf("@%lu prime => %d\n", (i + 1), num);
        }
    }

    free(numbers);
    return 0;
}