使用C语言，基于socket编程，不用现成组件实现TFTP服务器程序

在C语言中，利用socket编程实现一个简单的Trivial File Transfer Protocol (TFTP) 服务器，TFTP是一种轻量级的文件传输协议，主要用于小型设备间的文件交换。下面是一个基础的TFTP服务器实现，只包含基本的功能，如接收请求、发送文件、以及一些错误处理。

#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 <errno.h>

#define PORT 69
#define MAX_PACKET_SIZE 512

typedef enum { TFTP_RRQ, TFTP_WRQ } TftpCommand;

struct TftpPacket {
    uint8_t command;
    char filename[100];
    char mode[4]; // 模式，一般为"octet"
};

void send_error(socket_t s, int error_code) {
    char error_msg[] = "ERROR code %d: %s";
    char packet[MAX_PACKET_SIZE];
    snprintf(packet, sizeof(packet), error_msg, error_code, strerror(error_code));
    write(s, packet, strlen(packet));
}

void serve_request(socket_t s) {
    struct TftpPacket packet;
    memset(&packet, 0, sizeof(packet));

    read(s, &packet, sizeof(packet));
    printf("Received request: %s\n", packet.filename);

    if (packet.command == TFTP_RRQ) {
        // 文件读取
        FILE *fp = fopen(packet.filename, "rb");
        if (!fp) {
            send_error(s, ENOENT);
            return;
        }

        char data[MAX_PACKET_SIZE];
        ssize_t bytes_read;
        while ((bytes_read = fread(data, 1, MAX_PACKET_SIZE, fp)) > 0) {
            write(s, data, bytes_read);
        }
        fclose(fp);
    } else if (packet.command == TFTP_WRQ) {
        // 文件写入暂时不做处理，可以简单返回错误
        send_error(s, EPERM);
    } else {
        send_error(s, EPROTO);
    }
}

int main() {
    int server_socket = socket(AF_INET, SOCK_DGRAM, 0);
    if (server_socket == -1) {
        perror("Socket creation failed");
        return 1;
    }

    struct sockaddr_in server_addr;
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(PORT);
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);

    bind(server_socket, (struct sockaddr *)&server_addr, sizeof(server_addr));

    printf("Server started on port %d\n", ntohs(server_addr.sin_port));

    while (true) {
        socklen_t addr_len = sizeof(server_addr);
        struct sockaddr_in client_addr;
        char buffer[MAX_PACKET_SIZE];

        int bytes_received = recvfrom(server_socket, buffer, MAX_PACKET_SIZE, 0, (struct sockaddr *)&client_addr, &addr_len);
        if (bytes_received <= 0) {
            perror("recvfrom failed");
            continue;
        }

        serve_request(server_socket);
    }

    close(server_socket);
    return 0;
}

这个TFTP服务器仅支持RRQ（Read Request）命令，即从服务器获取文件，WRQ（Write Request）功能未实现。实际部署时，你需要处理更复杂的交互，如文件大小确认、数据包顺序、ACK/NACK响应等。