#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define read "E:\\read.txt"
#define write "E:\\write.txt"
typedef struct{
        char ID[11];
        char ID_New[7];
        char name[7];
        char age[3]; 
}DATA;

void substr(char *dest, const char* src, unsigned int start, unsigned int cnt) {
  strncpy(dest, src + start, cnt);
  dest[cnt] = 0;
}

void substr(char *dest, const char* src, unsigned int start, unsigned int cnt);
int main(void)
{
    FILE *readPtr, *writePtr;
    DATA data;
    readPtr = fopen(read, "r");
    writePtr = fopen(write, "a");
    if (!readPtr) {
        printf("read.txt fail to open...\n");
    }
    else if(!writePtr) {
         printf("write.txt fail to open...\n");
    }

    else{ 
         while(fgets(data.ID,11,readPtr) != NULL){                           
               //fseek(readPtr, 0, SEEK_CUR);
               substr(data.ID_New,data.ID,0,6);
               fputs(data.ID_New,writePtr);
               fgets(data.name,7,readPtr);
               fputs(data.name,writePtr);
               fgets(data.age,3,readPtr);     
               fputs(data.age,writePtr);      
               /*
               結果:
               A12345張阿三10
               B123456789李阿
               C1234567老五45

               我認為應該是:
               A12345張阿三10
               B12345李阿四15
               C12345王老五45
               */
         }

    }
    fclose(readPtr);
    fclose(writePtr);
    system("pause");
    return 0;
}