clearing data in an array



how would i clear all data from an array? here is what i'm doing:

code:
while (1) {
        if ((numbytes = recv(sock2, buf, 100, 0)) == -1) {
                perror("recv");
        }
        printf("%s : %s", inet_ntoa(their_addr.sin_addr), buf);
}buf[] still has extra data in it, so when i print it, things from previous data is put in there.

of course, that is only the one part of the program, but it was unnecessary to put the entire program in the post

1. is 'buf' one hundred bytes long?

2. if so what happens when you receive one hundred bytes?