How do I translate function from Java to C?

We've been asked to build a library of random letters in Java. I did the homework. We are asked to write in C now. I did it with nanotime in Java. I did a little research, and I couldn't find nanotime in C.

Already tried in C: (It didn't work)

Random SayiOlustur(){
Random this;
this = (Random) malloc(sizeof(struct RANDOM));
char randomly;
int i;
struct timeval before , after;
gettimeofday(&before , NULL);
for (i=1 ; i <= 100 ; i++){
   printf("%d %d %d ",i, i*i, i*i*i);}
gettimeofday(&after , NULL);
printf("%d",Nano(before,after)); 
int random;
double test;
while(true){
    test = Nano(before,after);
    random = (int)test % (int)123.0;
    if((random >= 65 && random <= 90) || (random >= 97 && random <= 122)){
    randomly = (char) random;
    break;
    }}}
int Nano(struct timeval x,struct timeval y){
    double x_ms , y_ms , diff;
    x_ms = (double)x.tv_sec*1000000 + (double)x.tv_usec;
    y_ms = (double)y.tv_sec*1000000 + (double)y.tv_usec;
    diff = (double)y_ms - (double)x_ms;
    return diff;
}

Java Code: (working)

 private long Now(){
        long now = System.nanoTime();
        return now;
 }
public char Random(){
        char rastgele;
        while(true){
            random = (int)((Now())%123);
            // ASCI
            if((random >= 65 && random <= 90) || (random >= 97 && random <= 122))
                break;
        }
        rastgele = (char) random;
        return rastgele;
    }
public static void main(String[] args) {
        RastgeleKarakter rastgele = new RastgeleKarakter();
        System.out.println("Rastgele Karakter: " + rastgele.Random());
}

Out:

run:
Rastgele Karakter: m \\ Random

Out:

run
Rastgele Karakter: y \\ Random