c++ - How to know the memcpy() function has successfully copied the memory during completing pakage?

Question

I send a vector containing 120000 from my computer to a server by tcp/ip ssh tunnel. Every time, I send 250 values of type double and the sever send me back them to ensure that I have sent the data correctly. I use the function read() to recieve the data in the server. As we know, read() cannot always recieve all the 250 values (250*8=2000bytes) in one time. Thus, I use the function memcpy() to save the recieved data until it reach 2000 bytes. However, the memcpy only work one times.

For example, I send 250 values (2000bytes). The server recieves 1408 bytes in the 1st time. I use memcpy() to save these 1406 bytes into a array from the buffer. The server recieves 594 bytes in the 2nd time. I use memcpy() to save these 592 bytes into the same array from the buffer. However, I find the 2nd time, memcpy() does not work according to the value send back from server to my computer.

The code c++ in server has two objectives: 1. recieve the 250 data every times. 2. send them back every times.

#include <stdio.h>
#include <iostream>
#include<vector>
#include <math.h>

extern "C"
void useCUDA();

#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

using namespace std;



int main()
{

    vector<double> Y;

    int lread = 250, nMu = 4, ltotal = 120000;


    int sock = socket(AF_INET, SOCK_STREAM, 0);

    struct sockaddr_in serv_addr;
    memset(&serv_addr, 0, sizeof(serv_addr));  
    serv_addr.sin_family = AF_INET; 
    serv_addr.sin_addr.s_addr = inet_addr("127.0.0.1"); 
    serv_addr.sin_port = htons(54321); 
    connect(sock, (struct sockaddr*)&serv_addr, sizeof(serv_addr));


    double* block_buffer_input;
    block_buffer_input = new double[lread];
    double* block_input;
    block_input = new double[lread];
    double* block_buffer_output;
    block_buffer_output = new double[lread];


    while (Y.size() < ltotal)
    {
        int nbyteread = 0;
        int nbytereadtimes = 0;

        while (nbyteread < 8 * lread)
        {

            nbytereadtimes = read(sock, reinterpret_cast<char*>(block_buffer_input), lread * sizeof(double));


            memcpy(block_input + nbyteread, block_buffer_input, nbytereadtimes);

            if (nbytereadtimes != 8 * lread && nbytereadtimes != 0)
                cout << Y.size() << ": " << nbytereadtimes << " " << block_input + nbyteread <<endl;


            nbyteread += nbytereadtimes;
        }

        Y.insert(Y.end(), &block_input[0], &block_input[lread]);

        cout << Y.size() << ": " << nbyteread << endl;


        int Sp = Y.size() - lread;
        for (int i = 0; i != lread; ++i)
        {
            block_buffer_output[i] = Y[Sp + i];
        }
        write(sock, (char*)block_buffer_output, lread * sizeof(double));



    }


    delete[] block_buffer_input;
    delete[] block_input;
    delete[] block_buffer_output;
    close(sock);


    return 0;
}

I want to know why the memcpy() do not work in the 2nd time.

Answer 1

If you adding nbyteread to the pointer to an array of doubles you actually referring to address of nbyteread * sizeof(double) (address of nbyteread element)

memcpy(block_input + nbyteread, block_buffer_input, nbytereadtimes);