some questions in psqlodbc

Dear somebody:
hello, I am eric from beijing. And I am a fresh man in psqlodbc. When I am studing at the psqlodbc source code, I have some questions.
In file named "odbcapi.c", the function named "SC_opencheck" are called almost everywhere, and in the function, why return some error when "the cursor is open"? And using git, I saw that change was beening done in 2004-01-06 17:58:22. Oh how old is it, but I can't understande why we can't use a cursor when it is opened or created by some other?
And in function "PGAPI_ExtendedFetch" in another file "results.c", which is called by SQLFetch, why seting cursor to start using function "SC_inc_rowset_start" or function "SC_set_rowset_start"?
Because we will maybe get rowdata next or not?
Could you understand me? My englist is poor, and I am afraid that I couldn't descript my questions clearly. I am sorry to apologize.

Best wishes!
Eric
From Beijing 2014-11-15