TCP

flowchart TD
    S["Client → Server<br/>SYN: seq=x"] --> SA["Server → Client<br/>SYN+ACK: seq=y, ack=x+1"]
    SA --> A["Client → Server<br/>ACK: ack=y+1"]

flowchart TD
    F["A → B: FIN"] --> ACK["B → A: ACK"]
    ACK --> BF["B → A: FIN"]
    BF --> LAST["A → B: ACK"]
    LAST --> WAIT["A: TIME_WAIT"]

#define _POSIX_C_SOURCE 200112L
#include <arpa/inet.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <unistd.h>

static void require(int ok, const char *operation) {
    if (!ok) { perror(operation); exit(1); }
}

static int make_socket(void) {
    int fd = socket(AF_INET, SOCK_STREAM, 0);
    require(fd >= 0, "socket");
    struct timeval timeout = {.tv_sec = 2};
    require(setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout)) == 0,
            "receive timeout");
    require(setsockopt(fd, SOL_SOCKET, SO_SNDTIMEO, &timeout, sizeof(timeout)) == 0,
            "send timeout");
    return fd;
}

static void send_all(int fd, const char *data, size_t size) {
    while (size > 0) {
        ssize_t n = send(fd, data, size, 0);
        if (n < 0 && errno == EINTR) continue;
        require(n > 0, "send");
        data += n;
        size -= (size_t)n;
    }
}

static void receive_exact(int fd, char *data, size_t size) {
    while (size > 0) {
        ssize_t n = recv(fd, data, size, 0);
        if (n < 0 && errno == EINTR) continue;
        if (n == 0) { fputs("unexpected EOF\n", stderr); exit(1); }
        require(n > 0, "recv");
        data += n;
        size -= (size_t)n;
    }
}

int main(void) {
    int listener = make_socket();
    struct sockaddr_in address = {
        .sin_family = AF_INET,
        .sin_port = htons(0)
    };
    require(inet_pton(AF_INET, "127.0.0.1", &address.sin_addr) == 1, "inet_pton");
    require(bind(listener, (struct sockaddr *)&address, sizeof(address)) == 0, "bind");
    socklen_t address_size = sizeof(address);
    require(getsockname(listener, (struct sockaddr *)&address, &address_size) == 0,
            "getsockname");
    require(listen(listener, 1) == 0, "listen");

    int client = make_socket();
    require(connect(client, (struct sockaddr *)&address, sizeof(address)) == 0,
            "connect");
    puts("connect 완료, 아직 accept 호출 전");
    int server = accept(listener, NULL, NULL);
    require(server >= 0, "accept");
    struct timeval timeout = {.tv_sec = 2};
    require(setsockopt(server, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout)) == 0,
            "server receive timeout");
    require(setsockopt(server, SOL_SOCKET, SO_SNDTIMEO, &timeout, sizeof(timeout)) == 0,
            "server send timeout");

    send_all(client, "Hello", 5);
    require(shutdown(client, SHUT_WR) == 0, "client shutdown");
    char message[6] = {0};
    receive_exact(server, message, 2);
    printf("서버가 먼저 읽은 2 Byte: %.2s\n", message);
    receive_exact(server, message + 2, 3);
    printf("서버가 이어서 읽은 3 Byte: %.3s\n", message + 2);
    char extra;
    require(recv(server, &extra, 1, 0) == 0, "server EOF");
    puts("클라이언트의 송신은 끝났지만 서버는 응답할 수 있다");
    send_all(server, message, 5);
    require(shutdown(server, SHUT_WR) == 0, "server shutdown");
    receive_exact(client, message, 5);
    printf("클라이언트가 받은 응답: %s\n", message);
    close(server);
    close(client);
    close(listener);
    return 0;
}