探索YModem源码，它是如何实现文件传输的？

YModem是一个用于串行通信的协议，常用于文件传输。源码通常包括发送和接收两个部分。

Ymodem是一种用于数据传输的协议，主要用于串行通信，以下是一个简单的Ymodem源码实现，使用C语言编写：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>
#include <errno.h>
#define SOH 0x01
#define STX 0x02
#define EOT 0x04
#define ACK 0x06
#define NAK 0x15
#define CAN 0x18
#define CRC 0x43
#define BLOCK_SIZE 128
unsigned short crc16(unsigned char *buf, int len) {
    unsigned short crc = 0xFFFF;
    for (int pos = 0; pos < len; pos++) {
        crc ^= (unsigned short)buf[pos];
        for (int i = 8; i != 0; i) {
            if ((crc & 0x0001) != 0) {
                crc >>= 1;
                crc ^= 0xA001;
            } else {
                crc >>= 1;
            }
        }
    }
    return crc;
}
void send_byte(int fd, unsigned char byte) {
    write(fd, &byte, 1);
}
unsigned char receive_byte(int fd) {
    unsigned char byte;
    read(fd, &byte, 1);
    return byte;
}
void send_block(int fd, unsigned char *data, int block_number) {
    send_byte(fd, SOH);
    send_byte(fd, block_number & 0xFF);
    send_byte(fd, ~block_number & 0xFF);
    write(fd, data, BLOCK_SIZE);
}
int receive_block(int fd, unsigned char *data) {
    unsigned char header[3];
    read(fd, header, 3);
    if (header[0] == SOH) {
        read(fd, data, BLOCK_SIZE + 2);
        return header[1];
    } else if (header[0] == STX) {
        read(fd, data, BLOCK_SIZE + 2);
        return header[1] << 8 | header[2];
    } else {
        return 1;
    }
}
int ymodem_send(int fd, const char *filename) {
    FILE *file = fopen(filename, "rb");
    if (!file) {
        perror("Error opening file");
        return 1;
    }
    unsigned char buffer[BLOCK_SIZE + 4];
    int block_number = 1;
    int total_blocks = (fsize(file) + BLOCK_SIZE  1) / BLOCK_SIZE;
    while (!feof(file)) {
        fread(buffer, 1, BLOCK_SIZE, file);
        send_block(fd, buffer, block_number);
        unsigned char response = receive_byte(fd);
        if (response == ACK) {
            block_number++;
        } else if (response == NAK) {
            printf("Receiver requested retransmission of block %dn", block_number);
        } else {
            printf("Unexpected response: %02Xn", response);
            fclose(file);
            return 1;
        }
    }
    send_byte(fd, EOT);
    fclose(file);
    return 0;
}
int ymodem_receive(int fd, const char *filename) {
    FILE *file = fopen(filename, "wb");
    if (!file) {
        perror("Error creating file");
        return 1;
    }
    unsigned char buffer[BLOCK_SIZE + 4];
    int block_number = 1;
    int received_blocks = 0;
    while (1) {
        int result = receive_block(fd, buffer);
        if (result == 1) {
            printf("Error receiving blockn");
            fclose(file);
            return 1;
        } else if (result == 0) {
            break;
        } else if (result == block_number) {
            send_byte(fd, ACK);
            fwrite(buffer, 1, BLOCK_SIZE, file);
            block_number++;
            received_blocks++;
        } else {
            printf("Received block %d out of order, expected %dn", result, block_number);
            send_byte(fd, NAK);
        }
    }
    fclose(file);
    return received_blocks;
}
int main(int argc, char *argv[]) {
    if (argc != 4 || strcmp(argv[1], "send") && strcmp(argv[1], "receive")) {
        printf("Usage: %s <send|receive> <device> <filename>n", argv[0]);
        return 1;
    }
    int fd = open(argv[2], O_RDWR | O_NOCTTY | O_SYNC);
    if (fd < 0) {
        perror("Error opening serial port");
        return 1;
    }
    struct termios tty;
    tcgetattr(fd, &tty);
    cfsetospeed(&tty, B9600);
    cfsetispeed(&tty, B9600);
    tty.c_cflag &= ~PARENB;
    tty.c_cflag &= ~CSTOPB;
    tty.c_cflag &= ~CSIZE;
    tty.c_cflag |= CS8;
    tcsetattr(fd, TCSANOW, &tty);
    if (!strcmp(argv[1], "send")) {
        ymodem_send(fd, argv[3]);
    } else if (!strcmp(argv[1], "receive")) {
        ymodem_receive(fd, argv[3]);
    }
    close(fd);
    return 0;
}

这个代码实现了一个简单的Ymodem发送和接收功能，可以在命令行中使用./ymodem send|receive <device> <filename>来调用，这个代码仅适用于Linux系统，并且需要root权限才能访问串口设备，这个代码没有实现错误处理和超时重传等功能，仅供参考。