使用C语言实现tftp协议的客户端和服务端，并明确定义出操作码

TFTP是一个基于UDP协议的简单文件传输协议，操作码用于区分不同的TFTP操作。以下是TFTP操作码的定义：

1. RRQ（读请求）
2. WRQ（写请求）
3. DATA（数据）
4. ACK（确认）
5. ERROR（错误）

下面分别介绍TFTP客户端和服务端的实现。

TFTP客户端：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

#define MAX_BUF_SIZE 1024

void error(char *msg)
{
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[])
{
    if (argc != 4) {
        fprintf(stderr, "Usage: %s <server_ip> <filename> <mode>\n", argv[0]);
        exit(1);
    }

    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        error("Error opening socket");
    }

    struct sockaddr_in serv_addr;
    bzero(&serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = inet_addr(argv[1]);
    serv_addr.sin_port = htons(69);

    char buf[MAX_BUF_SIZE];
    char *mode = "octet";
    sprintf(buf, "%c%c%s%c%s%c", 0x00, 0x01, argv[2], 0x00, mode, 0x00);
    int n = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&serv_addr, sizeof(serv_addr));
    if (n < 0) {
        error("Error sending request");
    }

    FILE *fp = fopen(argv[2], "wb");
    if (!fp) {
        error("Error opening file for writing");
    }

    int block_num = 0;
    int recv_len = 0;
    struct sockaddr_in cli_addr;
    socklen_t cli_len = sizeof(cli_addr);
    while (1) {
        bzero(buf, MAX_BUF_SIZE);
        n = recvfrom(sockfd, buf, MAX_BUF_SIZE, 0, (struct sockaddr *)&cli_addr, &cli_len);
        if (n < 0) {
            error("Error receiving data");
        }

        int opcode = ntohs(*(unsigned short *)buf);
        if (opcode == 5) {
            char *error_msg = buf + 4;
            fprintf(stderr, "Error: %s\n", error_msg);
            exit(1);
        } else if (opcode == 3) {
            int block = ntohs(*(unsigned short *)(buf + 2));
            if (block == block_num + 1) {
                fwrite(buf + 4, 1, n - 4, fp);
                block_num++;
            }

            sprintf(buf, "%c%c%c%c", 0x00, 0x04, block_num >> 8, block_num & 0xff);
            n = sendto(sockfd, buf, 4, 0, (struct sockaddr *)&cli_addr, cli_len);
            if (n < 0) {
                error("Error sending ACK");
            }

            if (n < MAX_BUF_SIZE - 4) {
                break;
            }
        }
    }

    fclose(fp);
    close(sockfd);

    return 0;
}

TFTP服务端：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

#define MAX_BUF_SIZE 1024

void error(char *msg)
{
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <directory>\n", argv[0]);
        exit(1);
    }

    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        error("Error opening socket");
    }

    struct sockaddr_in serv_addr;
    bzero(&serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(69);

    if (bind(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        error("Error on binding");
    }

    char buf[MAX_BUF_SIZE];
    struct sockaddr_in cli_addr;
    socklen_t cli_len = sizeof(cli_addr);
    while (1) {
        bzero(buf, MAX_BUF_SIZE);
        int n = recvfrom(sockfd, buf, MAX_BUF_SIZE, 0, (struct sockaddr *)&cli_addr, &cli_len);
        if (n < 0) {
            error("Error receiving request");
        }

        int opcode = ntohs(*(unsigned short *)buf);
        if (opcode == 1) {
            char *filename = buf + 2;
            char *mode = buf + 2 + strlen(filename) + 1;
            if (strcmp(mode, "octet") != 0) {
                char *error_msg = "Unsupported mode";
                sprintf(buf, "%c%c%c%c%s%c", 0x00, 0x05, 0x00, 0x00, error_msg, 0x00);
                sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&cli_addr, cli_len);
                continue;
            }

            char path[1024];
            sprintf(path, "%s/%s", argv[1], filename);
            int fd = open(path, O_RDONLY);
            if (fd < 0) {
                char *error_msg = strerror(errno);
                sprintf(buf, "%c%c%c%c%s%c", 0x00, 0x05, 0x00, 0x00, error_msg, 0x00);
                sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&cli_addr, cli_len);
                continue;
            }

            int block_num = 0;
            while (1) {
                bzero(buf, MAX_BUF_SIZE);
                int n = read(fd, buf + 4, MAX_BUF_SIZE - 4);
                if (n < 0) {
                    error("Error reading file");
                }

                if (n == 0) {
                    break;
                }

                *(unsigned short *)buf = htons(3);
                *(unsigned short *)(buf + 2) = htons(block_num + 1);
                sendto(sockfd, buf, n + 4, 0, (struct sockaddr *)&cli_addr, cli_len);

                while (1) {
                    bzero(buf, MAX_BUF_SIZE);
                    n = recvfrom(sockfd, buf, MAX_BUF_SIZE, 0, (struct sockaddr *)&cli_addr, &cli_len);
                    if (n < 0) {
                        error("Error receiving ACK");
                    }

                    opcode = ntohs(*(unsigned short *)buf);
                    if (opcode == 4) {
                        block_num = ntohs(*(unsigned short *)(buf + 2));
                        if (block_num == (int)(*(unsigned char *)(buf + 1))) {
                            break;
                        }
                    }
                }
            }

            close(fd);
        }
    }

    close(sockfd);

    return 0;
}