MATLAB Invalid File Identifier while trying to use fwrite to write to an existing empty text file

Question

fid2 opens an existing text file which is empty . I want to write a new line to this empty file. The reason I used a+ is because I want to use fid2 again later inside a loop to add lines to the txt file.

addpath('home/oa/Desktop/T/K') 
fid2 = fopen( 'r1.txt', 'a+' );

fwrite(fid2,c1);

The message that appears is

Error using fwrite Invalid file identifier. Use fopen to generate a valid file identifier.

Error in coef (line 70) fwrite(fid2,c1);

What is the problem?

Answer 1

try

[fid2,m] = fopen( 'r1.txt', 'a+' );

then check the content of m