fid与文件中有几个参数无关!
像我显示的参数值fid是3, 猜测LZ已经fopen过两个文件, 但还未fclose, 所以fid是5
from matlab help
Output value fid is a scalar MATLAB integer that you use as a file identifier for all subsequent low-level file input/output routines. If fopen cannot open the file, it returns -1. MATLAB reserves file identifiers 0, 1, and 2 for standard input, standard output, and standard error, respectively.