full source code with sample input: https://github.com/trhgquan/CPP/blob/master/sth_else/MISC/NEARPRIME.c

NEARPRIME.c

/*
 * https://github.com/trhgquan
 */
#include <stdio.h>

int PRIME[2000004];

void generatePrimes() {
	for (int i = 2; i <= 2000004; ++i)
		PRIME[i] = 1;
	for (int i = 2; i <= 2000004; ++i)
		for (int j = 2; j * i <= 2000004; ++j)
			PRIME[i * j] = 0;
}

int nearestPrime(int n) {
	// Special case:
	if (n == 1) return 2;

    generatePrimes();
    
    int a, b;
    for (int i = n; i >= 2; --i)
		if (PRIME[i]) { a = i; break; }
	for (int i = n; i <= 2000004; ++i)
		if (PRIME[i]) { b = i; break; }

	if (n - a > b - n) return b;
	return a;
}


int main() {
	int n;
	scanf("%d", &n);
	printf("%d\n", nearestPrime(n));
	return 0;
}