在FreeRTOS中使用任务来实现并发处理。例如，定义一个任务来处理TFTP请求： 完整案例代码

以下是一个使用FreeRTOS和lwIP实现的并发处理的TFTP服务器的完整案例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

#include "lwip/sockets.h"
#include "lwip/sys.h"
#include "lwip/netdb.h"

#include "FreeRTOS.h"
#include "task.h"
#include "semphr.h"
#include "queue.h"

#define TFTP_OPCODE_RRQ 1
#define TFTP_OPCODE_DATA 3
#define TFTP_OPCODE_ACK 4
#define TFTP_OPCODE_ERROR 5

#define TFTP_ERROR_FILE_NOT_FOUND 1
#define TFTP_ERROR_ACCESS_VIOLATION 2
#define TFTP_ERROR_DISK_FULL 3
#define TFTP_ERROR_ILLEGAL_OPERATION 4
#define TFTP_ERROR_UNKNOWN_TRANSFER_ID 5
#define TFTP_ERROR_FILE_EXISTS 6
#define TFTP_ERROR_NO_SUCH_USER 7

#define MAX_FILENAME_LEN 128
#define MAX_DATA_LEN 512

static SemaphoreHandle_t file_mutex;
static QueueHandle_t file_data_queue;

static int read_file(const char *filename, unsigned char *buf, int buflen)
{
    int fd = open(filename, O_RDONLY);
    if (fd < 0) {
        return -1;
    }

    int ret = read(fd, buf, buflen);
    close(fd);

    return ret;
}

static int write_file(const char *filename, unsigned char *buf, int buflen)
{
    int fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0666);
    if (fd < 0) {
        return -1;
    }

    int ret = write(fd, buf, buflen);
    close(fd);

    return ret;
}

static void tftp_task(void *pvParameters)
{
    int sockfd = *(int *)pvParameters;
    struct sockaddr_in client_addr;
    unsigned char buf[1024];

    while (1) {
        // receive request from client
        int len = sizeof(struct sockaddr_in);
        int ret = recvfrom(sockfd, buf, sizeof(buf), 0, (struct sockaddr *)&client_addr, &len);
        if (ret < 0) {
            // error handling
        }

        // process request
        unsigned short opcode = ntohs(*(unsigned short *)buf);
        char *filename = (char *)&buf[2];
        char *mode = filename + strlen(filename) + 1;

        switch (opcode) {
            case TFTP_OPCODE_RRQ:
            {
                xSemaphoreTake(file_mutex, portMAX_DELAY);
                int data_len = read_file(filename, buf, sizeof(buf));
                xSemaphoreGive(file_mutex);

                if (data_len < 0) {
                    buf[1] = TFTP_OPCODE_ERROR;
                    buf[3] = TFTP_ERROR_FILE_NOT_FOUND;
                    sendto(sockfd, buf, 4, 0, (struct sockaddr *)&client_addr, sizeof(struct sockaddr_in));
                } else {
                    int block_num = 1;
                    int data_sent = 0;

                    while (data_sent < data_len) {
                        buf[0] = 0;
                        buf[1] = TFTP_OPCODE_DATA;
                        *(unsigned short *)&buf[2] = htons(block_num);
                        memcpy(&buf[4], &buf[data_sent], MAX_DATA_LEN);

                        int data_to_send = data_len - data_sent;
                        if (data_to_send > MAX_DATA_LEN) {
                            data_to_send = MAX_DATA_LEN;
                        }

                        sendto(sockfd, buf, data_to_send + 4, 0, (struct sockaddr *)&client_addr, sizeof(struct sockaddr_in));

                        int ack_len = recvfrom(sockfd, buf, sizeof(buf), 0, (struct sockaddr *)&client_addr, &len);
                        if (ack_len < 0) {
                            // error handling
                        }

                        if (ntohs(*(unsigned short *)buf) == TFTP_OPCODE_ACK && ntohs(*(unsigned short *)&buf[2]) == block_num) {
                            data_sent += MAX_DATA_LEN;
                            block_num++;
                        }
                    }
                }

                break;
            }

            case TFTP_OPCODE_DATA:
            {
                unsigned short block_num = ntohs(*(unsigned short *)&buf[2]);
                xQueueSend(file_data_queue, &buf[4], 0);

                buf[0] = 0;
                buf[1] = TFTP_OPCODE_ACK;
                *(unsigned short *)&buf[2] = htons(block_num);
                sendto(sockfd, buf, 4, 0, (struct sockaddr *)&client_addr, sizeof(struct sockaddr_in));

                break;
            }

            case TFTP_OPCODE_ERROR:
            {
                // error handling
                break;
            }

            default:
            {
                buf[0] = 0;
                buf[1] = TFTP_OPCODE_ERROR;
                buf[3] = TFTP_ERROR_ILLEGAL_OPERATION;
                sendto(sockfd, buf, 4, 0, (struct sockaddr *)&client_addr, sizeof(struct sockaddr_in));
                break;
            }
        }
    }
}

int main(int argc, char **argv)
{
    file_mutex = xSemaphoreCreateMutex();
    file_data_queue = xQueueCreate(10, MAX_DATA_LEN);

    struct sockaddr_in server_addr;
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        // error handling
    }

    memset(&server_addr, 0, sizeof(struct sockaddr_in));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    server_addr.sin_port = htons(69);

    if (bind(sockfd, (struct sockaddr *)&server_addr, sizeof(struct sockaddr_in)) < 0) {
        // error handling
    }

    xTaskCreate(tftp_task, "tftp_task", 2048, &sockfd, tskIDLE_PRIORITY + 1, NULL);

    vTaskStartScheduler();
}

此代码实现了一个简单的TFTP服务器，可以处理客户端的读取文件请求和数据传输请求。文件的读写操作使用了互斥量来保证一致性，数据的传输使用了消息队列来传递。