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1、引言

在嵌入式开发和数据通信中，YMODEM协议因其可靠性和高效性而被广泛应用。作为一种文件传输协议，YMODEM在设备间的数据交换中发挥着重要作用。然而，市面上现有的YMODEM库大多采用C++或其他高级语言实现，这在某些资源受限的嵌入式环境中可能并不适用。为了解决这一问题，我们开发了一款完全用C语言编写的YMODEM库，旨在提供一个轻量、易用且高效的解决方案。本博客将详细介绍该库的设计原理、实现细节和使用方法，希望为广大嵌入式开发者提供有价值的参考。

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2、理论

YMODEM是一个文件传输协议，它在XMODEM协议的基础上进行了扩展和改进。其主要特点和工作原理如下：

2.1 YMODEM协议的主要特点

数据包传输：

YMODEM使用数据包进行文件传输，每个数据包通常为1024字节（1KB），比XMODEM的128字节要大得多，从而提高了传输效率。

头文件传输：

YMODEM在传输文件数据之前，会先传输一个包含文件名、文件大小和修改时间等信息的头文件。这使得接收方能够提前知道文件的信息并进行相应处理。

错误检测和恢复：

采用CRC（循环冗余校验）进行数据包的错误检测，确保传输的可靠性。如果检测到错误，接收方可以请求重传。

2.2 YMODEM的工作原理

初始化传输：

发送方和接收方建立连接后，发送方首先发送一个包含文件信息的头文件数据包。
接收方接收到头文件后，进行校验并返回一个ACK（确认）信号。

数据包传输：

发送方开始按照顺序发送数据包，每个数据包包含数据内容和校验信息。
接收方接收到数据包后，进行校验。如果数据包无误，接收方返回ACK信号；如果有误，返回NAK（否认）信号，请求重传。

结束传输：

当所有数据包传输完毕后，发送方发送一个EOT（End of Transmission）信号，表示文件传输结束。
接收方接收到EOT信号后，返回一个ACK信号确认，并等待下一个文件的头文件数据包，或者断开连接。

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通过这种方式，YMODEM协议实现了可靠且高效的文件传输，适用于需要稳定数据传输的嵌入式系统和其他应用场景。

3、代码

目前我的代码为了考虑可移植性，先在QT中完成的编写，但是适用与所有的嵌入式程序。

代码分三个部分：
main.c 完成ymodem需要的两个驱动函数，并进行注册和调用接收或发送；
ymodem.c 实现ymodem的接收和发送；
ymodem.h引出对应的函数接口。

开源地址

3.1 main.cpp

#include <QCoreApplication>
#include <QThread>
#include <QDebug>
#include <QtSerialPort/QSerialPort>
#include <QtSerialPort/QSerialPortInfo>
#include "ymodem.h"// 声明一个全局的 QSerialPort 对象
QSerialPort serial;char ser_buff[1024*12] ;
long ser_len= 0;
int getData(char* data, unsigned int len, unsigned int mstime) {if (!serial.isOpen()) {qWarning() << "Serial port is not open.";return -1;}if(ser_len >= len ){memcpy(data, ser_buff, len);ser_len -= len;qWarning() << len;return len;}if (!serial.waitForReadyRead(mstime)) {qWarning() << "Read timeout.";return -1;}// QByteArray readData = serial.read(len);QByteArray readData = serial.readAll();int bytesRead = readData.size();memcpy(&ser_buff[ser_len],readData.data(),bytesRead);ser_len += bytesRead;if (ser_len >= len) {memcpy(data, readData.data(), len);ser_len -= len;memcpy(&ser_buff[0],&ser_buff[len],ser_len);}else{return -1;}qWarning() << len;return len;
}int putData(char* data, unsigned int len, unsigned int mstime) {if (!serial.isOpen()) {qWarning() << "Serial port is not open.";return -1;}QByteArray writeData(data, len);qint64 bytesWritten = serial.write(writeData);if (bytesWritten == -1) {qWarning() << "Write failed.";return -1;}if (!serial.waitForBytesWritten(mstime)) {qWarning() << "Write timeout.";return -1;}qWarning() << writeData.data();return static_cast<int>(bytesWritten);
}int main(int argc, char *argv[]) {QCoreApplication a(argc, argv);// 设置串口参数serial.setPortName("COM7");serial.setBaudRate(QSerialPort::Baud115200);serial.setDataBits(QSerialPort::Data8);serial.setParity(QSerialPort::NoParity);serial.setStopBits(QSerialPort::OneStop);serial.setFlowControl(QSerialPort::NoFlowControl);if (!serial.open(QIODevice::ReadWrite)) {qDebug() << "Failed to open port" << serial.portName();return -1;}else{qDebug() << "open port" << serial.portName();}// 示例使用// char sendData[] = "Hello, Serial Port!";// char receiveData[10];// int bytesSent = putData(sendData, strlen(sendData), 5000);// if (bytesSent > 0) {//     qDebug() << "Bytes sent:" << bytesSent;// } else {//     qWarning() << "Failed to send data.";// }// int bytesReceived = getData(receiveData, sizeof(receiveData), 5000);// if (bytesReceived > 0) {//     qDebug() << "Bytes received:" << bytesReceived;//     qDebug() << "Data received:" << QByteArray(receiveData, bytesReceived);// } else {//     qWarning() << "Failed to receive data.";// }ymodem_register(putData,getData);char buff[1024*502] =" {1,2,3}";// int ret = ymodem_send(buff,sizeof(buff), "2.txt");char name[128] = {};int ret = ymodem_recv(buff,sizeof(buff), name);qWarning() << "code is over."<<ret;// 程序退出前关闭串口serial.close();return 0;// int result = a.exec();}

3.2 ymodem.c

/*** @file ymodem.c* @author zhuxuanlin (you@domain.com)* @brief* @version 0.1* @date 2024-05-22** @copyright Copyright (c) 2024**/#include <stdio.h>
#include <string.h>
#include "ymodem.h"#define SOH 0x01
#define STX 0x02
#define EOT 0x04
#define ACK 0x06
#define NAK 0x15
#define CAN 0x18
#define charC 0x43#define PACKET_SIZE 128
#define PACKET_1K_SIZE 1024
#define PACKET_OVERHEAD 5
#define PACKET_TIMEOUT 2000
#define PACKET_TIMEOUT_TIMES 15typedef enum{YM_FILEHEADER = 2,YM_DATA_TRANS,YM_DATA_OVER,YM_ABORT_TRANS
}ym_process;static ymodem_st ym = {.get_data = 0,.put_data = 0
};
int ymodem_register(int (*put_data)(char* data,unsigned int len,unsigned int mstime),int (*get_data)(char* data,unsigned int len,unsigned int mstime))
{ym.put_data = put_data;ym.get_data = get_data;
}static const unsigned short crc_table[256] =
{0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7,0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6,0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485,0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4,0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823,0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12,0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41,0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0
};static unsigned short crc16(unsigned char *q, int len)
{unsigned short crc = 0;while (len-- > 0)crc = (crc << 8) ^ crc_table[((crc >> 8) ^ *q++) & 0xff];return crc;
}static int __recv_packet(char* data,unsigned int* len, unsigned char packet_num,unsigned int mstime)
{if(len == NULL || data == NULL){return -1;}char ym_packet[PACKET_1K_SIZE+PACKET_OVERHEAD];memset(ym_packet,0,sizeof(ym_packet));int ret = ym.get_data(ym_packet,1,PACKET_TIMEOUT);if(ret < 0 ){return -1;}if(ym_packet[0] != SOH &&  ym_packet[0] != STX){if(ym_packet[0] == EOT){return 0;}return -2;}long packet_len = (ym_packet[0] == SOH)?PACKET_SIZE:PACKET_1K_SIZE;ret = ym.get_data(&ym_packet[1],packet_len+4,PACKET_TIMEOUT);if(ret < 0 ){return -3;}if(!(ym_packet[1] ==  (char)packet_num && ym_packet[2] == (char)(~packet_num))){char ack = NAK;ym.put_data(&ack,1,PACKET_TIMEOUT);return -4;}unsigned short crc = crc16(&ym_packet[3] , packet_len);if(ym_packet[packet_len + 3] == (char)((crc >> 8) & 0xFF) &&ym_packet[packet_len + 4] == (char)(crc & 0xFF)){memcpy(data,&ym_packet[3],packet_len);*len = packet_len;}else{return -6;}return packet_len;
}static int __recv_filename(char* filename,unsigned long* filesize ,unsigned int mstime)
{char ym_packet[PACKET_1K_SIZE];memset(ym_packet,0,sizeof(ym_packet));unsigned long plen = 0;int ret = __recv_packet(ym_packet,&plen,0,mstime);if(ret < 0 || plen != PACKET_SIZE){return -1;}if(filename == NULL || filesize == NULL){for(int i = 0;i<PACKET_SIZE;i++){if(ym_packet[i] != 0){return -2;}}return 1;}int i = 0;while (ym_packet[i] != 0 && i < PACKET_SIZE) {filename[i] = ym_packet[i];i++;}i++;unsigned long fsize = 0;while (ym_packet[i] != 0 && i < PACKET_SIZE) {fsize = fsize * 10 + (ym_packet[i] - '0');i++;}*filesize = fsize;return 0;
}int ymodem_recv(char *buff, long bufsize, char *filename)
{if (!ym.get_data || !ym.put_data) {return -1;  // 无效的功能指针}if(!buff || bufsize <= 0 || !filename ){return -1;}int rerty_times = PACKET_TIMEOUT_TIMES;int ret = -1;char ym_tbuf[2] = {0,0},ym_rchar = 0;long recv_bytes = 0 , single_packet_size = 0,file_size = 0;unsigned char __recv_flags = 0;ym_process process = YM_FILEHEADER;do{switch (process){case YM_FILEHEADER:ym_tbuf[0] = charC;ym.put_data(&ym_tbuf[0],1,PACKET_TIMEOUT);ret = __recv_filename(filename,&file_size,PACKET_TIMEOUT);if(ret < 0){rerty_times--;}else{if(file_size > bufsize){return -2;}ym_tbuf[0] = ACK;ym_tbuf[1] = charC;ym.put_data(ym_tbuf,2,PACKET_TIMEOUT);__recv_flags ++;process = YM_DATA_TRANS;}break;case YM_DATA_TRANS:ret = __recv_packet(&buff[recv_bytes],&single_packet_size,__recv_flags,PACKET_TIMEOUT);if(ret > 0){recv_bytes += single_packet_size;__recv_flags++;ym_tbuf[0] = ACK;ym.put_data(ym_tbuf,1,PACKET_TIMEOUT);if(recv_bytes  > file_size + PACKET_1K_SIZE){int ack = CAN;ym.put_data(&ack,1,PACKET_TIMEOUT);}}else if(ret == 0){process = YM_DATA_OVER;}else{ym_tbuf[0] = NAK;ym.put_data(ym_tbuf,1,PACKET_TIMEOUT);  }break;case YM_DATA_OVER:ym_tbuf[0] = ACK;ym_tbuf[1] = charC;ym.put_data(ym_tbuf,2,PACKET_TIMEOUT);process = YM_ABORT_TRANS;break;case YM_ABORT_TRANS:ret = __recv_filename(NULL,NULL,PACKET_TIMEOUT);if(ret < 0){rerty_times--;}else{ym_tbuf[0] = ACK;ym.put_data(ym_tbuf,1,PACKET_TIMEOUT);return recv_bytes;}break;default:break;}}while(rerty_times != 0);return -1;
}static int __send_packet(char* data,unsigned int len, unsigned char packet_num,unsigned int mstime)
{if(len>PACKET_1K_SIZE || data == NULL){return -1;}char ym_packet[PACKET_1K_SIZE+PACKET_OVERHEAD];memset(ym_packet,0,sizeof(ym_packet));int packet_len = (len<=PACKET_SIZE)?PACKET_SIZE:PACKET_1K_SIZE;ym_packet[0] = (packet_len == PACKET_SIZE)?SOH:STX;ym_packet[1] = packet_num;ym_packet[2] = ~packet_num;memcpy(&ym_packet[3],data,len);unsigned short crc = crc16(&ym_packet[3] , packet_len);ym_packet[packet_len +3] = (crc >> 8) & 0xFF; // Append the higher byte of the CRC.ym_packet[packet_len +4] = crc & 0xFF; // Append the lower byte of the CRC.int ret = ym.put_data(ym_packet,packet_len+PACKET_OVERHEAD,mstime);return (ret > 5)?ret -5:ret;
}static int __send_filename(char* filename,unsigned long filesize ,unsigned int mstime)
{char ym_packet[PACKET_SIZE];memset(ym_packet,0,sizeof(ym_packet));unsigned char plen = 0;if(filename != NULL || filesize != 0){plen = sprintf(ym_packet,"%s", filename); // 文件名plen++;plen += sprintf(&ym_packet[plen], "%d", filesize);return __send_packet(ym_packet,plen,0,mstime);}return __send_packet(ym_packet,PACKET_SIZE,0,mstime);
}int ymodem_send(char *buff,long bufsize, const char *filename)
{if (!ym.get_data || !ym.put_data) {return -1;  // 无效的功能指针}if(!buff || bufsize == 0 || !filename){return -1;}int rerty_times = PACKET_TIMEOUT_TIMES;int ret = -1;char ym_rbuf[2] = {0,0},ym_tchar = 0;long sent_bytes = 0 , single_packet_size = 0;unsigned char __send_flags = 0;ym_process process = YM_FILEHEADER;do{switch (process){case YM_FILEHEADER:ret = ym.get_data(ym_rbuf,1,PACKET_TIMEOUT);if(ret == 1 && ym_rbuf[0] == charC){ret = __send_filename(filename,bufsize,PACKET_TIMEOUT);ret = ym.get_data(ym_rbuf,2,PACKET_TIMEOUT);if(ret == 2 && ym_rbuf[0] == ACK && ym_rbuf[1] == charC){__send_flags++;process = YM_DATA_TRANS; }else if(ret < 0){rerty_times--;}}else{   rerty_times--;}break;case YM_DATA_TRANS:// 单次发送数据量single_packet_size = (bufsize - sent_bytes > PACKET_1K_SIZE) ? PACKET_1K_SIZE : bufsize - sent_bytes;ret = __send_packet(&buff[sent_bytes], single_packet_size,__send_flags,PACKET_TIMEOUT); // 发送数据包ret = ym.get_data(ym_rbuf,1,PACKET_TIMEOUT);if(ret == 1 && ym_rbuf[0] == ACK){__send_flags++;sent_bytes += single_packet_size;if(sent_bytes >= bufsize || sent_bytes < 0){process = YM_DATA_OVER; }}else if(ret == 1 && ym_rbuf[0] == NAK){rerty_times--;// 重发}else if(ret == 1 && ym_rbuf[0] == CAN){break;}else if(ret < 0){rerty_times--;}break;case YM_DATA_OVER:ym_tchar = EOT;ret = ym.put_data(&ym_tchar,1,PACKET_TIMEOUT);ret = ym.get_data(ym_rbuf,1,PACKET_TIMEOUT);if(ret == 1 && ym_rbuf[0] == ACK) {ret = ym.get_data(ym_rbuf,1,PACKET_TIMEOUT);if(ret == 1 && ym_rbuf[0] == charC){process = YM_ABORT_TRANS; }} else if(ret == 1 && ym_rbuf[0] == NAK)  {// 重发}else if(ret < 0){rerty_times--;}      break;case YM_ABORT_TRANS:ret = __send_filename(NULL,0,PACKET_TIMEOUT);ret = ym.get_data(ym_rbuf,1,PACKET_TIMEOUT);if(ret == 1 && ym_rbuf[0] == ACK ){// 正常退出return sent_bytes;}else if(ret < 0){// 部分异常退出return 1;}break;default:break;}}while(rerty_times != 0);return -1;
}

3.3 ymodem.h

#ifndef __YMODEM_H__
#define __YMODEM_H__#ifdef __cplusplus
extern "C" {
#endiftypedef struct 
{/* int -1发送失败，>0 接收字节数，data 数据buff , len 接收数据长度 , 超时时间 ms */int (*get_data)(char* data,unsigned int len,unsigned int mstime);/* int -1发送失败，>0 发送字节数，data 数据buff , len 发送数据长度 , 超时时间 ms */int (*put_data)(char* data,unsigned int len,unsigned int mstime);
}ymodem_st;extern int ymodem_register(int (*put_data)(char* data,unsigned int len,unsigned int mstime),int (*get_data)(char* data,unsigned int len,unsigned int mstime));
extern int ymodem_recv(char *buff, long bufsize, char *filename);
extern int ymodem_send(char *buff, long bufsize, const char *filename);#ifdef __cplusplus
}
#endif
#endif  /* __YMODEM_PORT_H__ */

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4、验证

使用QT和超级终端进行验证。
在电脑中开启虚拟串口，
一端连接QT打开的串口，
一端连接超级终端的串口。
通过虚拟串口模拟两个串口设备的ymodem通信过程。

4.1 ymodem发送

4.2 ymodem接收

经过验证以上代码的ymodem接收和发送都没有问题。

5、移植说明

首先实现这两个驱动函数。

/* int -1发送失败，>0 接收字节数，data 数据buff , len 接收数据长度 , 超时时间 ms */
int (*get_data)(char* data,unsigned int len,unsigned int mstime);
/* int -1发送失败，>0 发送字节数，data 数据buff , len 发送数据长度 , 超时时间 ms */
int (*put_data)(char* data,unsigned int len,unsigned int mstime);

在函数中使用这个函数注册驱动函数给ymodem协议库

    ymodem_register(put_data,get_data);

最后，调用这个两个函数来进行接收或者发送：

    int ret = ymodem_send(buff,sizeof(buff), "2.txt");int ret = ymodem_recv(buff,sizeof(buff), name);

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时间流逝、年龄增长，是自己的磨炼、对知识技术的应用，还有那不变的一颗对嵌入式热爱的心！