What`s wrong with this socket select code? - c++

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.