What is the ungetc function?

main.c

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

void f() {
    char c = getchar();
    printf("Again the same result: %c\n", c);
}

int main(int argc, char* argv[]) {
    char c = getchar();
    printf("Result: %c\n", c);
    int res = ungetc('f', stdin);
    if (res == EOF) {
        printf("Error\n");
    }
    f();
    return 0;
}