I am using MathLink to call C (resp C++ for using cout...) functions which themselves call an Assembler routine. For the C-part needs a lot of time and its functions are called permanently I first tried not to open and close the connection after each run of the Assembler routine. But then calling Function A sometimes had the effect of calling function B and vice versa! So I included an error message printout - but no MathLink error occured. Nevertheless the information flow broke down,hm. Then I tried out to open and close the connection before/after running the external function. It now seems to work but sometimes the following error message appears: "cannot open object obtained by expansion of ~/filename" whereby filename is the C-part of the connection. Another mystical thing is that often two filename- processes are running at the same time - what is not explainable by the Mathematica code.
I am using both an university server license of Mathematica 4.2 (I think so) and my student license of Mathematica 4.1 - both running on linux systems. The error I reported was seen on the first system but the breakdown of information flow I tried out on both systems.