消息UDP中的文件名

Question

I have simple program in C (client,server) UDP. Client send message with filename or source to text file and server open this file and shows first line. how to format buf so that the file can be opened correctly?

CLIENT

bzero(buf,BUF_SIZE);
fgets(buf, BUF_SIZE-1, stdin);
n = sendto(sfd, buf, strlen(buf), 0, rp->ai_addr, rp->ai_addrlen);

SERVER

peer_addr_len = sizeof(struct sockaddr_storage);
n = recvfrom(sfd, buf, BUF_SIZE, 0,
            (struct sockaddr *) &peer_addr, &peer_addr_len);
FILE *fp;
char buff[255];
fp = fopen(buf,"r");

fgets(buff, 255, (FILE *)fp);
printf("First line: %s \n", buff);

fclose(fp);

Answer 1

Function fopen needs a nul-terminated string, so you have to make sure the file name from the received buffer is nul-terminated before passing it to fopen . With the implementation in your code snippets the file name may be followed by random data if you don't fill buf with '\\0' before calling recvfrom .

There are different options fulfill this requirement.

Send the file name including the '\\0' :

/* use strlen(buf)+1 to include the terminating '\0' */
n = sendto(sfd, buf, strlen(buf)+1, 0, rp->ai_addr, rp->ai_addrlen);

In this case the receiver will get a buffer including the '\\0' . But it might be good to check that the received string is actually terminated with '\\0' .

Or append a '\\0' after receiving the string:

/* use BUF_SIZE-1 to reserve at least 1 byte for '\0' */
n = recvfrom(sfd, buf, BUF_SIZE-1, 0,
        (struct sockaddr *) &peer_addr, &peer_addr_len);
if(n >= 0) {
    buf[n] = '\0';
}

Adding both modifications will make the program more robust.

In real code you should add error handling for all functions that may indicate an error with their return value.