C - How to send char array of bytes over UDP that contain multiple 0 bytes?

Question

I've been trying to send a custom frame over UDP using the sendto() commands to another PC. Works fine but as soon as there is a 0 byte in the array it will (ofcourse) recognize it as a \0 value and stop at that byte. How do can I bypass this to then send a 0-Byte (0x00) over the network.

char buffer[26] = {0x06, 0x10, 0x02,
0x05, 0x00, 0x1a, 0x08, 0x01, 0xc0,
0xa8, 0x7e, 0x80, 0x0e, 0x58, 0x08,
0x01, 0xc0, 0xa8, 0x7e, 0x80, 0x0e,
0x58, 0x04, 0x04, 0x02, 0x00};

printf("Enter port # to listen to: \n");
int PORT;
scanf("%d", &PORT);
printf("Enter IP Address to send to: \n");
char SERVER[20];
scanf("%s", SERVER);

struct sockaddr_in si_other;
int s, slen=sizeof(si_other);
char buf[26];
char message[26];
if ((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
    die ("socket()");
}

memset((char *) &si_other, 0, sizeof(si_other));
si_other.sin_family = AF_INET;
si_other.sin_port = htons(PORT);

if (inet_aton(SERVER, &si_other.sin_addr) == 0) {
    fprintf(stderr, "inet_aton() failed\n");
    exit(1);
}

while(1) {
    printf("Enter message: ");
    gets(message);
    memcpy(message, buffer, 26);

    int te = sendto(s, message, strlen(message), 0,     (struct sockaddr *) & si_other, slen);
    //Send message
    if ( te == -1) {
        die("sendto()");
    }

    //Receive reply and print
    memset(buf,'\0', BUFLEN);

    //Receive Data, blocking
    if(recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) & si_other, & slen) == -1) {
        die("receive()");
    }
    puts(buf);
}

close(s);
return 0;

As you see in the above defined array I have a 0x00 byte at place 5. Sendto will only send the first 4 bytes.

Community · Accepted Answer · 2017-05-23 12:12:54Z

1

Don't use strlen() if your string contains valid '\0' characters. I suggest you change:

int te = sendto(s, message, strlen(message), 0, (struct sockaddr *) & si_other, slen);

to:

int te = sendto(s, message, sizeof(message), 0, (struct sockaddr *) & si_other, slen);

Also note that you should not be using gets(), as it is unsafe - use fgets() instead. Change:

gets(message);

to:

fgets(message, sizeof(message), stdin);

edited May 23, 2017 at 12:12

CommunityBot

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answered Mar 11, 2015 at 11:40

Paul R

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