我试了，存不进去啊

随便试试，是不是这效果：

#include <stdio.h>
main()
{
    FILE *fp;
    fp = fopen("file.dat", "wb");
    fwrite("ABCDEF", 6, 1, fp);
    fclose(fp);
    char s[10]={0};
    fp = fopen("file.dat", "rb+");
    fread(s, 6, 1, fp);
    printf("%s\n", s);
    fseek(fp, 2, SEEK_SET);
    fwrite("123", 3, 1, fp);
    fclose(fp);
    fp = fopen("file.dat", "rb");
    fread(s, 6, 1, fp);
    printf("%s\n", s);
    fclose(fp);
   
}