Send struct, float and int over UDP sockets in C++

Question

I want to send/receive structs containing float and int variables in client/server socket programs written in C++.

Earlier, the code was written in C, so I was simply sending them as follows:

//structure definition
struct {
    float a;
    float b;
    float c;
    float d[2];

}myStruct_;

//code to send struct
int slen = sizeof(client_sock)

if(recvfrom(sockFD, &myStruct_, sizeof(myStruct_), 0 ,(struct sockaddr *)&client_sock, &slen)<0) {
            printf("Failure while receiving data %d" , WSAGetLastError());
            getchar();
            exit(1);
}

But now in C++, this is giving me this error message:

error: cannot convert '(anonymous struct)*' to 'char *' for argument '2' to 'int recvfrom(SOCKET, char*, int, int, sockaddr*, int*)'

I tried to look for this error, and found out that I have to serialize the struct before sending, and later deserialize the same to get the exact structure. Could you suggest/or provide an example how to serialize and de-serialize it?

if i cast it into (char *) then how do i get the original struct back at the receiver's end? — Anmol Bhatia
At the receiving end you cast the address of an identical struct to char* and copy the receive buffer into that. — Galik
"will the same thing work for float and int variables?" Maybe. Gotchas are endianness and the size of an integer being different on different platforms and compiler toolkits. — user4581301
The problem with floats is they are platform/architecture specific so if you are sending them to a different platform/architecture you might do better sending text rather than binary. — Galik

Remy Lebeau Remy Lebeau · Accepted Answer · 2017-03-31T22:34:11

The code you showed will work fine in C++ with a little tweaking.

In some platforms (like Linux), recvfrom() is defined as expecting a void* pointer to the memory buffer that it will fill in. In other platforms (like Windows), recvfrom() expects a char* pointer instead.

To get rid of the error, simply type-cast the buffer pointer (just like you do with the 5th parameter, where you are passing a sockaddr_in* pointer where a sockaddr* pointer is expected):

recvfrom(sockFD, (char*)&myStruct_, sizeof(myStruct_), 0, (struct sockaddr *)&client_sock, &slen);

Do the same thing when sending a struct:

sendto(sockFD, (char*)&myStruct_, sizeof(myStruct_), 0, (struct sockaddr *)&server_sock, &slen);

Send struct, float and int over UDP sockets in C++

1 Answers