How to get the exact message from recv() in winsock programming?

Question

I'm developing a server-client application using Winsock in c++ and have a problem.

For getting the message from the client by the server I use the code below.

int result;
char buffer[200];

while (true)
{
    result = recv(client, buffer, 200, NULL);

    if (result > 0)
        cout << "\n\tMessage from client: \n\n\t" << message << ";";
}

I send the message "Hello" from the client to the server. However the buffer is actually this:

HelloÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌ

What am I missing?

thank you. What is the null terminator exactly? and Let's say I add a null terminator or any thing like that. How may I retrieve the string from the buffer? would you please show me a sample code? — user3530012, Dec 17 '14 at 14:15
sending null-terminated strings over a network is inviting all kinds of security problems. Writing a protocol from scratch is a big task. You can save yourself a huge amount of work by using something like google protocol buffers to encode/decode the contents of packets. — Richard Hodges, Dec 17 '14 at 14:23
@RC that has exactly the same problem as the original; it requires a null terminator which clearly isn't present. — Alan Stokes, Dec 17 '14 at 14:55
I've actually fixed the problem by adding the null terminator '\0' at the end of the message sent by the client. — user3530012, Dec 17 '14 at 15:04
True Alan, my mistake. memcpy(buffer+result, "\0", 1); // This should work I guess. — RC Brand, Dec 17 '14 at 15:09

Giorgi Moniava · Answer 1 · 2015-11-14T08:27:44.607

8

Since recv might not receive as many bytes as you told it, you typically use a function like this to receive specified number of bytes. Modified from here

int receiveall(int s, char *buf, int *len)
{
    int total = 0;        // how many bytes we've received
    int bytesleft = *len; // how many we have left to receive
    int n = -1;

    while(total < *len) {
        n = recv(s, buf+total, bytesleft, 0);
        if (n <= 0) { break; }
        total += n;
        bytesleft -= n;
    }

    *len = total; // return number actually received here

    return (n<=0)?-1:0; // return -1 on failure, 0 on success
}

It's up to you to null terminate the string if you receive string which is not null terminated.

edited Nov 14 '15 at 08:27

answered Dec 17 '14 at 14:19

Giorgi Moniava

27,046
9
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I actually fixed the problem by adding the null terminator '\0' at the end of the message sent by the client. – user3530012 Dec 17 '14 at 14:35
2

@user3530012: yes but in general you should keep in mind probably that `recv` as I said might not receive all the data you tell it to (read the link I gave for more details) – Giorgi Moniava Dec 17 '14 at 14:39

score 4 · Answer 2 · answered Dec 17 '14 at 14:14

The result tells you how many bytes were received. recv doesn't add a terminator since, in general, network data is binary data which might not be usable as a C-style string.

You can add a terminator yourself, if you know the message won't contain the termination character:

buffer[result] = 0;  // make sure the buffer is large enough

or make a string (or vector, or whatever) from it:

std::string message_str(message, result);

Note that what you receive might not be a single "message", especially if you're uses a stream protocol like TCP. It might contain more than one message, or just the start of one.

score 1 · Answer 3 · answered Jun 19 '18 at 12:25

1

memset(&receive[0], 0, sizeof(receive));
To clear the buffer

answered Jun 19 '18 at 12:25

baranetharen

11
3

1

Please add more description in you'r answer. – Arash Jun 19 '18 at 12:30

score 0 · Answer 4 · edited Mar 22 '17 at 20:48

0

You didn't initialize your buffer

char buffer[200] = {0};

while (true)
{
    result = recv(client, buffer, 200, NULL);

    if (result > 0)
        cout << "\n\tMessage from client: \n\n\t" << message << ";";
    memset(buffer, 0, 200);
}

edited Mar 22 '17 at 20:48

Tim

4,790
4
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answered Mar 22 '17 at 19:41

Natio2

235
1
9

How to get the exact message from recv() in winsock programming?

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