Server TCP đầu tiên

/* Server echo TCP, chay tren Windows va tren POSIX.
   Windows:  gcc echo_sv.c -o echo_sv.exe -D_WIN32_WINNT=0x0600 -lws2_32
   Linux:    gcc echo_sv.c -o echo_sv                              */

#include <stdio.h>
#include <string.h>
#include <stdint.h>

#if defined(_WIN32)
  #include <winsock2.h>
  #include <ws2tcpip.h>
  typedef int socklen_t;
  typedef SOCKET Sock;
  #define DONG(s)   closesocket(s)
  #define LOI()     WSAGetLastError()
  #define SOCK_XAU  INVALID_SOCKET
#else
  #include <unistd.h>
  #include <errno.h>
  #include <sys/socket.h>
  #include <netinet/in.h>
  #include <arpa/inet.h>
  typedef int Sock;
  #define DONG(s)   close(s)
  #define LOI()     errno
  #define SOCK_XAU  (-1)
#endif

#define CONG    18080
#define TON_DU  10

static int khoi_tao_mang(void) {
#if defined(_WIN32)
    WSADATA d;
    return WSAStartup(MAKEWORD(2, 2), &d) == 0 ? 0 : -1;
#else
    return 0;
#endif
}

static void don_mang(void) {
#if defined(_WIN32)
    WSACleanup();
#endif
}

int main(void) {
    setvbuf(stdout, NULL, _IONBF, 0);

    if (khoi_tao_mang() != 0) {
        fprintf(stderr, "khoi tao mang that bai\n");
        return 1;
    }

    /* 1. socket */
    Sock sv = socket(AF_INET, SOCK_STREAM, 0);
    if (sv == SOCK_XAU) {
        fprintf(stderr, "socket: %d\n", LOI());
        don_mang();
        return 1;
    }

    /* 2. SO_REUSEADDR, PHAI dat truoc bind */
    int co = 1;
    setsockopt(sv, SOL_SOCKET, SO_REUSEADDR, (const char *)&co, sizeof co);

    /* 3. bind */
    struct sockaddr_in dc;
    memset(&dc, 0, sizeof dc);
    dc.sin_family      = AF_INET;
    dc.sin_addr.s_addr = htonl(INADDR_ANY);
    dc.sin_port        = htons(CONG);

    if (bind(sv, (struct sockaddr *)&dc, sizeof dc) < 0) {
        fprintf(stderr, "bind: %d\n", LOI());
        DONG(sv); don_mang();
        return 1;
    }

    /* 4. listen */
    if (listen(sv, TON_DU) < 0) {
        fprintf(stderr, "listen: %d\n", LOI());
        DONG(sv); don_mang();
        return 1;
    }

    printf("server lang nghe tren cong %d\n", CONG);

    /* 5. accept trong vong lap */
    for (int lan = 0; lan < 2; ++lan) {
        struct sockaddr_in kh;
        socklen_t kh_co = sizeof kh;

        Sock c = accept(sv, (struct sockaddr *)&kh, &kh_co);
        if (c == SOCK_XAU) {
            fprintf(stderr, "accept: %d\n", LOI());
            continue;                       /* KHONG thoat, thu tiep */
        }

        char ip[INET_ADDRSTRLEN];
        inet_ntop(AF_INET, &kh.sin_addr, ip, sizeof ip);
        printf("ket noi tu %s:%d\n", ip, ntohs(kh.sin_port));

        char bo_dem[1024];
        int n = recv(c, bo_dem, (int)sizeof bo_dem - 1, 0);
        if (n > 0) {
            bo_dem[n] = '\0';
            printf("nhan %d byte: %s\n", n, bo_dem);
            send(c, bo_dem, n, 0);
        }

        DONG(c);                            /* dong ket noi nay */
    }

    DONG(sv);
    don_mang();
    printf("server dong\n");
    return 0;
}

/* Vi sao TIME_WAIT ton tai:

   1. Goi tin tre. Neu ban dong roi mo ngay mot ket noi moi voi
      DUNG bon so cu, mot goi tin cu con lang thang co the toi noi
      va bi hieu la du lieu cua ket noi moi.

   2. ACK cuoi cung co the mat. Neu no mat, ben kia gui lai FIN,
      va ban phai con song de tra loi. Neu ban da bien mat hoan
      toan thi ben kia nhan duoc RST va bao loi.

   Thoi gian: 2 x MSL (Maximum Segment Lifetime).
     Linux    60 giay
     Windows  240 giay theo mac dinh cu, thuong ngan hon ngay nay */

int co = 1;
setsockopt(sv, SOL_SOCKET, SO_REUSEADDR, (const char *)&co, sizeof co);
/*                                        ^^^^^^^^^^^^^ ep kieu nay
   la vi Windows khai bao tham so la const char*, con POSIX la
   const void*. Ep sang const char* chay dung tren ca hai. */

/* Dat SAU bind thi khong co tac dung gi ca. Trinh bien dich
   khong canh bao, va setsockopt van tra ve 0. */