fopen error - works for a while but then gives an error - matlab

Question

I have a script that is running a series of for loops, and within these for loops a file is created that is then run using an external program using the script command. In summary it looks like this:
for i=1:n1
for j=1:n2
for k=1:n3
fid=fopen('file.txt','w');
fprintf(fid,'Some commands to pass to external program depending on i j k');
fclose(fid);
system('program file.txt');
end
end
end
The script has in total about 500k cases (n1xn2xn3), and runs fine for a small scenario (about 100 runs), but for the entire script it runs for a while and then returns an error for no apparent reason, giving this error:
fopen invalid file identifier object
There is no obvious reason for this, and Im wondering if anyone could point out what is wrong?

Answer 1

Just a guess: an instance of your external program is reading file.txt and at the same time the next iteration of your nested loop wants to open file.txt for writing. The more instances of your external program are running at the same time, the slower your machine, the more likely becomes this scenario. (called a 'race condition')
Possible solution for this: use a separate text file per case with a unique file name
You should also consider using other ways to call your external function because file handling for 500k cases should be very slow.
Hope that helps,
Eli