Problems sending an integer array over a socket

Question

I'm sending an integer array from client to server by socket. Like this ;

int arr[3] = {5, 4, 1};


send(sock , &arr, 3 , 0);   //sending the votes count

and receiving in server like so

recv(new_socket, arr1, 3, 0);

I'm getting correct value in arr1[0] but in arr1[1] I'm getting 66 and arr1[2] I'm getting 67

I don't know how this happening and where 66 and 67 is coming from

That 3 is a byte count, not an element count. the two are only synonymous if you're sending simple octets. Chances are you're not even sending one full int, much less three. Unrelated, &arr could just be arr in the send call. Won't really matter, but still. — WhozCraig
– WhozCraig, Commented Apr 25, 2018 at 18:08
note - in general you need to send a length indicator before the message. TCP is a stream protocol not a message protocol, you cannot expect to receive the same messages tht you send, only the same bytes in the same order — pm100
– pm100, Commented Apr 25, 2018 at 18:09

Remy Lebeau · Accepted Answer · 2018-04-26 14:59:53Z

2

send() and recv() operate on bytes only. You are sending and receiving 3 bytes, but your array is actually sizeof(int) * 3 bytes instead (where sizeof(int) is usually 4 on many platforms). You need to use sizeof() on both ends, eg:

send(sock, &arr, sizeof(arr), 0);

recv(new_socket, &arr1, sizeof(arr1), 0);

However, you also have to pay attention to return values, because send() can send fewer bytes than requested, and recv() can receive fewer bytes than requested. TCP is stream-oriented, not message-oriented. You should be calling send() and recv() each in a loop, re-calling each one until all expected bytes have been sent/received in full, eg:

ssize_t sendAll(int sckt, void *data, size_t length)
{
    char *bytes = (char*) data;
    while (length > 0)
    {
        ssize_t sent = send(sckt, bytes, length, 0);
        if (sent == -1) return -1;
        bytes += sent;
        length -= sent;
    }
    return 0;
}

int arr[3] = {5, 4, 1};
sendAll(sock, &arr, sizeof(arr));

ssize_t recvAll(int sckt, void *data, size_t length)
{
    char *bytes = (char*) data;
    while (length > 0)
    {
        ssize_t recvd = recv(sckt, bytes, length, 0);
        if (recvd <= 0) return recvd;
        bytes += recvd;
        length -= recvd;
    }
    return 1;
}

int arr1[3];
recvAll(new_socket, &arr1, sizeof(arr1));

edited Apr 26, 2018 at 14:59

answered Apr 25, 2018 at 18:33

Remy Lebeau

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3 Comments

Mert Can Ergün Over a year ago

also a conversion may be needed if endianness of sender/receiver sides are different

JojOatXGME Over a year ago

Seems to be finde. However, I would use size_t as type for length and ssize_t as type for sent/recvd. It is also a little bit confusing that both functions use different return values. (sendAll: 0/OK, -1/IO Error; recvAll: 1/OK, 0/Unexpected EOF, -1/IO Error)

Remy Lebeau Over a year ago

@JojOatXGME many of the standard socket functions return 0 on success and -1 on error, so I followed that same convention for sendAll(). Since recv() has different semantics, recvAll() is similarly different.

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