#include <stdio.h>

#define BUFFERLEN 3

typedef struct {
    char *lexeme_beginning;
    char *forward;
} CURSOR;

char BUFFER[2][BUFFERLEN+1];

int main(int argc,char* argv[])
{
    CURSOR cursor;
    int nread;
    FILE *pfil= fopen(argv[1],"r");
    
    BUFFER[0][BUFFERLEN]=EOF;
    BUFFER[1][BUFFERLEN]=EOF;
    cursor.lexeme_beginning=&BUFFER[0][0];
    cursor.forward=&BUFFER[0][0];

    if(pfil) {
        nread=fread(&BUFFER[0][0],sizeof(char),BUFFERLEN,pfil);
        while(1) {
            if(*cursor.forward==EOF)return 0;  //詞法分析器的位置
            cursor.forward += 1;
            if(*cursor.forward==EOF) {
                if(cursor.forward==&BUFFER[0][BUFFERLEN]) {
                    nread=fread(&BUFFER[1][0],sizeof(char),BUFFERLEN,pfil);
                    if(nread<BUFFERLEN)BUFFER[1][nread]=EOF;  // 碰巧
                    cursor.forward += 1;
                }
                else if(cursor.forward==&BUFFER[1][BUFFERLEN]) {
                    nread=fread(&BUFFER[0][0],sizeof(char),BUFFERLEN,pfil);
                    if(nread<BUFFERLEN)BUFFER[0][nread]=EOF;  // 碰巧
                    cursor.forward = &BUFFER[0][0];
                }
                else 
                    ;  // 什么也不做，讓詞法分析器接收到EOF詞素
            }
        }
    }

    fclose(pfil);
    return 0;
}