How can I prepend a string safely in C++ without changing the format?

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I am making a simple UDP server and client pair and I need to prepend a number between 0 and 255 to a c-string, and in order to do that I need to convert the number into a c-string.

However, I am not sure how this can be safely done as if I get a null char between the number and the c-string, it will cause numerous side effects when I send the entire c-string through the network and I read it on the other side.

If I get the number 0 and I convert it to a c-string will I get 1 and NULL inside the char nb[3] array, which I will prepend to char mess[256]? If it is the case, can you suggest a safe way to do it?

The format is as follows: the first 3 bytes must contain a number, the rest will contain a string followed by a null char.

C++

int main ()
{
    char number[3]; //if I put 1 in there I will have 1 null and null
    char message[256];
    char packet[259]; //first 3 bytes contain a number
    ...
    strcat(number, message);
    sendto(s, packet, 259, 0, to, tolen);


}

Posted 7-Nov-14 7:16am

glock123

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Sergey Alexandrovich Kryukov 7-Nov-14 14:43pm

What's the problem? For example, consider using std::string.
—SA

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bling · Answer 1 · 2014-11-07T10:05:00

You code looks to be more C than C++.

One problem I see is your parameters to strcat are backwards. Concatenating message[256] to number[3] isn't safe.

Don't use strcat. Calculate the length and use memcpy.

Since you have three bytes for the numeric prefix, consider using leading zeroes.

sprintf (or sprintf_s) would work here.

C++

char packet[259] = {0};
int number = 13;
const char payload[] = "Payload";
int length = sizeof payload;
// 3 for header, 1 for terminating nul.
if ( (length + 4) >= sizeof packet)
    fprintf(stderr, "too long!\n");
sprintf(packet, "%03d%-0.*s", number, length - 1, payload);
sendto(s, packet, sizeof packet, 0, to, tolen);

If your message design allows variable length packets, use the value returned by sprintf as the size of the packet.