What Is the Correct Way of Reading from a Tcp Socket in C/C++

What is the correct way of reading from a TCP socket in C/C++?

Without knowing your full application it is hard to say what the best way to approach the problem is, but a common technique is to use a header which starts with a fixed length field, which denotes the length of the rest of your message.

Assume that your header consist only of a 4 byte integer which denotes the length of the rest of your message. Then simply do the following.

// This assumes buffer is at least x bytes long,
// and that the socket is blocking.
void ReadXBytes(int socket, unsigned int x, void* buffer)
{
    int bytesRead = 0;
    int result;
    while (bytesRead < x)
    {
        result = read(socket, buffer + bytesRead, x - bytesRead);
        if (result < 1 )
        {
            // Throw your error.
        }

        bytesRead += result;
    }
}

Then later in the code

unsigned int length = 0;
char* buffer = 0;
// we assume that sizeof(length) will return 4 here.
ReadXBytes(socketFileDescriptor, sizeof(length), (void*)(&length));
buffer = new char[length];
ReadXBytes(socketFileDescriptor, length, (void*)buffer);

// Then process the data as needed.

delete [] buffer;

This makes a few assumptions:

• ints are the same size on the sender and receiver.
• Endianess is the same on both the sender and receiver.
• You have control of the protocol on both sides
• When you send a message you can calculate the length up front.

Since it is common to want to explicitly know the size of the integer you are sending across the network define them in a header file and use them explicitly such as:

// These typedefs will vary across different platforms
// such as linux, win32, OS/X etc, but the idea
// is that a Int8 is always 8 bits, and a UInt32 is always
// 32 bits regardless of the platform you are on.
// These vary from compiler to compiler, so you have to 
// look them up in the compiler documentation.
typedef char Int8;
typedef short int Int16;
typedef int Int32;

typedef unsigned char UInt8;
typedef unsigned short int UInt16;
typedef unsigned int UInt32;

This would change the above to:

UInt32 length = 0;
char* buffer = 0;

ReadXBytes(socketFileDescriptor, sizeof(length), (void*)(&length));
buffer = new char[length];
ReadXBytes(socketFileDescriptor, length, (void*)buffer);

// process

delete [] buffer;

I hope this helps.

C TCP write() and read() from sockets defined in main

As user user207421 pointed out, I was accepting a client before connecting.

Also acceptSock = accept(recvSock, (struct sockaddr *)&cli_addr, clilen);

Should be acceptSock = accept(recvSock, (struct sockaddr *)&cli_addr, &clilen);

socklen_t *addrlen parameter of accept() requires pointer, I was not passing a pointer.

Adding the & before the size parameter changes this.

How to read all the data in socket programming (c++)?

Just to complete my comment - this code snippet may help to understand

not tested code

for (i = 0;i < max_clients; i++) {
    sd = clients_list[i];
    if (FD_ISSET(sd,&temp)) {

        char buf[10];
        int n;
        int flag;

        do {
          n = recv(sd, buf, sizeof(buf),0);
          if (n > 0) {
            buf[n] = '\0';
            cout << "From the node: " << buf << endl;
            ioctl(sd, FIONREAD, &flag);
          } else {
            flag = 0;
          }
        } while (flag > 0);

        if (n < 0) {
          perror("recv");
          // in case of an error you may actively close the socket
          // and end transmission by server
          close(sd);
          clients_list[i] = 0;
        } else {
          cout << "Client has currently no further transmission " << endl;
          // don't close socket maybe later new transmission
          // active close socket by server process only if
          // e.g. a timeout has reached 
        }

    }
}

TCP socket Read

Yes! TCP sockets provide a stream interface and it is quite possible that data is delivered in more than one part.

The receiver should continue reading until all data has been received.

while (totalRead < expectedSize) {
    result = read(socketfd,buff + totalRead, expectedSize-totalRead);
    if (result < 0) {
        // nothing read, check for errors
    } else {
        totalRead += result;
        if (result == 0) {
             // Other end shutdown before sending the expected amount of data
        }
    }
}

The same is true for writing:

while (written < totalSize) {
    result = write(socketfd,buff + written, totalSize - written);
    if (result < 0) {
        // nothing written, check for errors
    } else {
        written += result;
    }
}

c++ read unknow amount of data from socket

You asked how to fix your low-level receiving code. That's simple -- just call read once and don't use a while loop. The call to read will return as soon as at least one byte of data can be read. For the number of bytes to read, just pass in the size of your buffer (or the space left in it if there's already some leftover in it from a prior call).

You will also need high-level protocol code, just as you do if you know the number of bytes. That code will probably need to be a bit different. If the chunk of bytes you got includes your stop indication, then process all the bytes up to the stop indication and keep the extra in the buffer for next time. If not, just call read again and get a bunch more bytes.

Your protocol handling code should look sort of like this:

1. Call read and add however many bytes you got to the buffer. (Handle an error or normal close here.)
2. If the data in the buffer does not include the stop indication, go to step 1.
3. Process all the bytes up to the stop indication.
4. Remove the processed bytes from the buffer, leaving any unprocessed bytes.
5. Go to step 2.

Also, the code you gave in the question will spin forever if the other side closes the connection. In this case, read will return zero and you will never be able to read any more data from the connection. You should break out of the loop if read returns zero.

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