Why is the variable j = 0?

Question

C++ / C#

I transfer the image in the socket, on the server a number is set in the s variable, but it is not transferred in the socket to the client.
Server

#include <iostream>
#include <WinSock.h>
#pragma comment (lib,"WS2_32.lib")

#include <fstream>
#include <algorithm>
#include <string>
#include <iterator>

void error(const char* msg)
{
    //perror(msg);
    std::cout<<'\n'<<WSAGetLastError();
    WSACleanup();
    std::cin.ignore();
    exit(1);
}
void bzero(char*buf, int l)
{
    for (int i = 0; i < l; i++)
        buf[i] = '\0';
}
struct arg_s
{
    char* buffer;
    bool exit;
};
struct arg_sa
{
    struct arg_s* lalk;
    int current;
};
#define type struct arg_sa
int sockfd, * newsockfd; 
int buflen = 1024000;
struct sockaddr_in *cli_addr;
int* clilen;
int currentclient;
void session_(LPVOID args)
{
    int current = currentclient++;
    bzero((char*)&(cli_addr[current]), sizeof(&(cli_addr[current])));
    newsockfd[current] = accept(sockfd, (struct sockaddr*)&(cli_addr[current]), &(clilen[current]));
    if (newsockfd[current] < 0)
    {
        error("Error on accept\n");
    }
    int n = recv(newsockfd[current], ((type*)args)[current].lalk->buffer
    , buflen, 0);
    std::cout << "data receving " << ((type*)args)[current].current << std::endl;
    if (n < 0)
        error("ERROR reading from socket");
    system(((type*)args)[current].lalk->buffer);
    std::cout << "data end " << std::endl;
    FILE* f;
    fopen_s(&f,"1.txt", "r");
    if (f != NULL)
    {
        int countfile = 0;
        while ((((type*)args)[current].lalk->buffer[countfile++] = getc(f)) != EOF)
            ;
        fclose(f);
    }
    n = send(newsockfd[current], ((type*)args)[current].lalk->buffer, buflen, 0);
    HDC ScreenDC = GetDC(0);
    HDC MemoryDC = CreateCompatibleDC(ScreenDC);
    int ScreenHeight = GetSystemMetrics(SM_CYSCREEN);
    int ScreenWidth = GetSystemMetrics(SM_CXSCREEN);
    ScreenWidth = ((ScreenWidth - 1) / 4 + 1) * 4;
    BITMAPINFO BMI;
    BMI.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    BMI.bmiHeader.biWidth = ScreenWidth;
    BMI.bmiHeader.biHeight = ScreenHeight;
    BMI.bmiHeader.biSizeImage = ScreenWidth * ScreenHeight * 3; 
    BMI.bmiHeader.biCompression = BI_RGB;
    BMI.bmiHeader.biBitCount = 24;
    BMI.bmiHeader.biPlanes = 1;
    DWORD ScreenshotSize;
    ScreenshotSize = BMI.bmiHeader.biSizeImage; 
    unsigned char* ImageBuffer; 
    HBITMAP hBitmap = CreateDIBSection(ScreenDC, &BMI, DIB_RGB_COLORS, (void**)&ImageBuffer, 0, 0);
    SelectObject(MemoryDC, hBitmap);
    BitBlt(MemoryDC, 0, 0, ScreenWidth, ScreenHeight, ScreenDC, 0, 0, SRCCOPY);
    DeleteDC(MemoryDC);
    ReleaseDC(NULL, ScreenDC);    
    FILE* sFile = 0;     
    unsigned char tgaHeader[12] = { 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
    unsigned char header[6];
    unsigned char tempColors = 0;
    fopen_s(&sFile,"Screen.tga", "wb");
    if (!sFile) {
    }
    header[0] = ScreenWidth % 256;
    header[1] = ScreenWidth / 256;
    header[2] = ScreenHeight % 256;
    header[3] = ScreenHeight / 256;
    header[4] = BMI.bmiHeader.biBitCount;
    header[5] = 0;
    fwrite(tgaHeader, sizeof(tgaHeader), 1, sFile);
    n = send(newsockfd[current], (char*)tgaHeader, sizeof(tgaHeader), 0);
    fwrite(header, sizeof(header), 1, sFile);
    n = send(newsockfd[current], (char*)header, sizeof(header), 0);
    // Записываем данные изображения:
    fwrite(ImageBuffer, BMI.bmiHeader.biSizeImage, 1, sFile);
    char* s = new char[100];
    _ultoa_s(BMI.bmiHeader.biSizeImage, s,100,10);
    //bzero(((type*)args)[current].lalk->buffer, buflen);
    for (int i = 0; i < 100; i++)
        ((type*)args)[current].lalk->buffer[i] = s[i];
    n = send(newsockfd[current], ((type*)args)[current].lalk->buffer, buflen, 0);
    n = send(newsockfd[current], (char*)ImageBuffer, BMI.bmiHeader.biSizeImage, 0);
    fclose(sFile);
    DeleteObject(hBitmap);
}
int main()
{
    WSADATA ws = { 0 };
    if (WSAStartup(MAKEWORD(2, 2), &ws) == 0)
    {
        currentclient = 0;
        int maxclients = 1;
        cli_addr = new struct sockaddr_in[maxclients];
        clilen = new int[maxclients];
        for (int i = 0; i < maxclients; i++)
        {
            clilen[i] = sizeof(cli_addr[i]);
        }
        sockfd = socket(AF_INET, SOCK_STREAM, 0);
        if (sockfd < 0)
            error("ERROR opening socket");
        struct sockaddr_in serv_addr;
        bzero((char*)&serv_addr, sizeof(serv_addr));
        serv_addr.sin_family = AF_INET;
        serv_addr.sin_addr.s_addr = INADDR_ANY;
        int port = 30000;
        serv_addr.sin_port = htons(port);
        if (bind(sockfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr)) < 0)
            error("ERROR on binding");
        if (listen(sockfd, 10) < 0)
            error("ERROR listen");
        HANDLE* thread;
        struct arg_sa* args;
        while (true)
        {
            newsockfd = new int[maxclients];
            thread = (HANDLE*)malloc(sizeof(HANDLE) * maxclients);
            args = new struct arg_sa[maxclients];
            for (int i = 0; i < maxclients; i++)
            {
                args[i].lalk = new struct arg_s();
                args[i].lalk->buffer = new char[buflen];
            }
            int i = -1;
            while (++i < maxclients)
            {
                Sleep(1);
                args[i].current = i;
                args[i].lalk->exit = false;
                thread[i] = CreateThread(0, 0, (LPTHREAD_START_ROUTINE)(session_), args, 0, 0);
            }
                for (int i = 0; i < maxclients; i++)
                    WaitForSingleObject(thread[i], INFINITE);
            i = -1;
            while (++i < maxclients)
            {
                shutdown(newsockfd[i], 0);
                TerminateThread(thread[i], 0);
            }
            currentclient = 0;
        }
        shutdown(sockfd, 0);
        WSACleanup();
        return 0;
    }
    std::cin.ignore();
}

Customer

#include <iostream>
#include <WinSock.h>
#pragma comment (lib,"WS2_32.lib")
void error(const char* msg)
{
    //perror(msg);
    std::cout << '\n' << WSAGetLastError();
    WSACleanup();
    std::cin.ignore();
    exit(1);
}
void bzero(char* buf, int l)
{
    for (int i = 0; i < l; i++)
        buf[i] = '\0';
}
int main()
{
    WSADATA ws = { 0 };
    if (WSAStartup(MAKEWORD(2, 2), &ws) == 0)
    {
        int sockfd;
        struct hostent* server = gethostbyname("localhost");
        sockfd = socket(AF_INET, SOCK_STREAM, 0);
        if (sockfd < 0)
            error("ERROR opening socket");
        struct sockaddr_in serv_addr, cli_addr;
        bzero((char*)&serv_addr, sizeof(serv_addr));
        bzero((char*)&cli_addr, sizeof(cli_addr));
        serv_addr.sin_family = AF_INET;
        const char* address = "127.0.0.1";
        serv_addr.sin_addr.s_addr = inet_addr(address);
        int port = 30000;
        serv_addr.sin_port = htons(port);
        int servlen = sizeof(serv_addr);
        int n = connect(sockfd, (struct sockaddr*)&serv_addr, servlen);
        if (n < 0)
        {
            error("ERROR on connect");
        }
        int buflen = 10240000;
        char* buffer = new char[buflen];
        bzero(buffer, buflen);
        std::string command = "ping 8.8.8.8>1.txt";
        for (int i = 0; i < command.size(); i++)
            buffer[i] = command[i];
        buffer[command.size()] = '\0';
        n = send(sockfd, buffer, buflen, 0);
        if (n < 0)
        {
            error("ERROR on send");
        }
        n = recv(sockfd, buffer, buflen, 0);
        std::cout << '\n' << buffer << std::endl;
        bzero(buffer, buflen);
        FILE* f;
        fopen_s(&f, "Screen.tga", "wb");
        if (!f) {
        }
        unsigned char tgaHeader[12] = { 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
        unsigned char header[6];
        n = recv(sockfd, (char*)tgaHeader, sizeof(tgaHeader), 0);
        fwrite((unsigned char*)tgaHeader, sizeof(tgaHeader), 1, f); 
        n = recv(sockfd, (char*)header, sizeof(header), 0);
        fwrite((unsigned char*)header, sizeof(header), 1, f);
        // Записываем данные изображения:
        bzero(buffer, buflen);
        n = recv(sockfd, buffer, buflen, 0);
        unsigned long j = atol(buffer);

        std::cout << buffer << std::endl;
        std::cout<<'\n' << "j=" << j << std::endl;
        bzero(buffer, buflen);
        n = recv(sockfd, buffer, j, 0);
        fwrite((unsigned char*)buffer,j, 1, f);
        bzero(buffer, buflen);
        fclose(f);
        system("Screen.tga");
        shutdown(sockfd, 0);
        WSACleanup();
        delete buffer;
        return 0;
    }
    std::cin.ignore();
}

Answer 1

You do not check anywhere that all the data sent by you went. send can return not only -1 or length, but any size less than length. Same with recv.