C/string.h/memset
memset: copies ch into the first count characters of *buf
//Declaration: void *memset(void *buf, int ch, size_t count);
//Return: returns buf.
#include <string.h>
#include <stdio.h>
int main(void){
char buf[100];
memset(buf, "\0", 100);
memset(buf, "X", 10);
printf(buf);
}
/*
XXXXXXXXXX
*/