How do I write (append) a string to a file?

Could you provide an example?

Function/S CreateAFile()
	Variable refnum
	Open refnum
	if (refnum)
		fprintf refnum, "pi = %g\r", pi
	endif
	FStatus refnum
	String filepath = S_path+S_fileName
	Close refnum
	return filepath
end

Function AddToFile(String filepath)
	Variable refnum
	Open/A refnum as filepath
	if (refnum)
		fprintf refnum, "e = %g\r", e
	endif
	Close refnum
end

Function Test()
	String fpath = CreateAFile()
	AddToFile(fpath)
end

The first function creates a new file, writes one line into it, and returns the full path to that file. The second function takes in a string containing a full path to a file, and adds a line to that file. The function test() calls both of those functions in sequence to create a file with two lines.

Clearly an inefficient way to make a two-line file! But it does illustrate the technique. When you invoke test(), a dialog is put up asking you to give a name to the file and choose a location for it. The function should complete without asking you for more information, since the full path to the file is returned from CreateAFile().

The background is that an open file has a read/write position associated with it in Igor Pro. You can query the file position via FStatus. And also set it via FSetPos. And by using Open/A that file position is set to the end internally, whereas Open just leaves it at the start.

Thank you John. This example is helpful. Probably the "fprintf" is not what I need in my code, as I am looking to 'dump' an entire string into a text file. 

I would say that if you want to write a string to a text file, fprintf is your best option. Just do 'fprintf refnum, mystring' and you should be good to go. I don't know if there is an intrinsic character limit, but I see no problem to 'dump' an entire string. And even if there was a character limit, its just a matter of splitting up the string into parts.

There seems to be a limit of 2400 bytes. The string I am trying to save is 100x larger. :(

then probably you will want to use fbinwrite

Thank you Tony. That solved my issue.