这个插座 select 代码有什么问题？

Question

#include <stdio.h>
#include <time.h>
#include <WinSock2.h>
#include <WS2tcpip.h>
#pragma comment(lib, "WS2_32.lib")

#define IP_ADDRESS "127.0.0.1"
#define PORT        20000
#define BUF_SIZE    64

#undef FD_SETSIZE
#define FD_SETSIZE 10000

void shuffle_buffer(char* buf, size_t size);
SOCKET create_socket();
void send_data(SOCKET sock);

int main()
{
    WSADATA  ws;

    if (WSAStartup(MAKEWORD(2, 2), &ws) != 0)
    {
        printf("Init Windows Socket Failed::%d\n", GetLastError());
        return -1;
    }

    const int CLIENT_SIZE = 1;
    SOCKET socks[CLIENT_SIZE];
    struct timeval tv = { 0, 10 };
    fd_set fd_read, fd_write;
    FD_ZERO(&fd_read);
    FD_ZERO(&fd_write);

    for (int i = 0; i < CLIENT_SIZE; i++) {
        SOCKET sock = create_socket();
        socks[i] = sock;
        FD_SET(sock, &fd_write);
        FD_SET(sock, &fd_read);
    }

    Sleep(1000);

    int number_to_recv = CLIENT_SIZE;
    while (number_to_recv > 0) {
        int ret = select(CLIENT_SIZE, &fd_read, &fd_write, NULL, &tv);
        for (int i = 0; i < CLIENT_SIZE; i++) {
            if (FD_ISSET(socks[i], &fd_read)) {
                char buf[BUF_SIZE];
                int n = recv(socks[i], buf, BUF_SIZE, 0);
                buf[n] = 0;
                printf("%s\n", buf);
                number_to_recv--;
            }

            if (FD_ISSET(socks[i], &fd_write)) {
                send_data(socks[i]);
                FD_CLR(socks[i], &fd_write);
                //Sleep(1);
            }
        }
        //printf("ret and number : %d, %d\n", ret, number_to_recv);
    }

    for (int i = 0; i < CLIENT_SIZE; i++) {
        closesocket(socks[i]);
    }
    WSACleanup();
}

SOCKET create_socket() 
{
    SOCKET cli_sock;
    struct sockaddr_in addr;

    if ((cli_sock = socket(AF_INET, SOCK_STREAM, 0)) == INVALID_SOCKET)
    {
        printf("Create Socket Failed::%d\n", GetLastError());
        return -1;
    }
    //inet_pton
    memset(addr.sin_zero, 0x00, 8);
    addr.sin_family = AF_INET;
    inet_pton(AF_INET, IP_ADDRESS, (void*)(&addr.sin_addr.s_addr));
    addr.sin_port = htons(PORT);

    if (connect(cli_sock, (struct sockaddr*)&addr, sizeof(addr)) == SOCKET_ERROR)
    {
        printf("Connect Error::%d\n", GetLastError());
        return -1;
    }

    return cli_sock;
}

void send_data(SOCKET sock)
{
    const int SEND_SIZE = BUF_SIZE / 2;
    char buf[SEND_SIZE] = { 0 };
    memset(buf, 'a', SEND_SIZE);
    shuffle_buffer(buf, SEND_SIZE);

    if (send(sock, buf, SEND_SIZE, 0) == SOCKET_ERROR)
    {
        printf("Send Info Error::%d\n", GetLastError());
    }
}

void shuffle_buffer(char* buf, size_t size)
{
    for (int i = 0; i < size; i++) {
        buf[i] += int(rand() % 26);
    }
}

Code above is a socket client using select model run on Win10, the problem is after I send data, but I can not receive data(I am sure that server has sent back data), this code below doesn`t run, so what is the problem? Thanks

The first parameter in select is maxfdp , and I know the difference between Win and Unix, so on Windows, this parameter seems not necessary, and I can write data, but can not receive it.

if (FD_ISSET(socks[i], &fd_read)) {
    char buf[BUF_SIZE];
    int n = recv(socks[i], buf, BUF_SIZE, 0);
    buf[n] = 0;
    printf("%s\n", buf);
    number_to_recv--;
}

Answer 1

select removes the sockets from the fd_set if they are not readable/writable. You need to add them back in before the next time you call select .

The reason your code can write data is because sockets start out being writable, so they will still be set in fd_write and your code will write data. They don't start out being readable, if no data has been received yet, so they'll be removed from the fd_read set and then your code stops checking whether they are readable.