According to FPL:
Do you mean one should also use rewrite for reading file?
As I have understood your problem, you wanted to write to the file and got an error message "file is not open for writing".
Perhaps `update ( MyFile )' is what you need: Open an existing file for read and write access.
[...] Meanwhile, good new, it is, as you said, only in reporting debugging information that GPC may fails, because, yes, when putting the breakpoint far away, everything works as expected. The actual problem (if one forgett those error of writting files) is that gdb does behave strangely with some part of GPC code. I say some part because I use intensively gdb and for the very greatest part, it works fine.
One problem is that noone in the GPC developing team is familiar enough with GDB to really find out what is wrong in GPC. Any help welcome!
Peter