Yes, it's confusing. I added some %PUT _user_ statements, and I think that shows something (I'm not sure what). with this code:
%macro TestDoSubl;
%local rc;
%PUT NOTE: Entering macro. x=&x;
%put _user_;
%put;%put;%put;
%LET x=DynamicValue;
%put NOTE: Before Dosubl 1: x=&x;
%put _user_;
%put;
data _null_;
rc=dosubl("");
run;
%put NOTE: After Dosubl 1: x=&x;
%put _user_;
%put;
%put;%put;%put;
%LET x=NewDynamicValue;
%put NOTE: Before Dosubl 2: x=&x;
%put _user_;
%put;
data _null_;
rc=dosubl("");
run;
%put NOTE: After Dosubl 2: x=&x;
%put _user_;
%put;
%put;%put;%put;
%put NOTE: Before Dosubl 3: x=&x;
%put _user_;
data _null_;
rc=dosubl("");
run;
%put NOTE: After Dosubl 3: x=&x;
%put _user_;
%put;
%mend TestDoSubl;
%let x=GlobalValue;
DATA _NULL_;
rc=DOSUBL('%TestDoSubl;');
RUN;
I get:
NOTE: Entering macro. x=GlobalValue
TESTDOSUBL RC
GLOBAL X GlobalValue
GLOBAL X GlobalValue
NOTE: Before Dosubl 1: x=DynamicValue
TESTDOSUBL RC
GLOBAL X DynamicValue
GLOBAL X GlobalValue
NOTE: After Dosubl 1: x=GlobalValue
TESTDOSUBL X GlobalValue
TESTDOSUBL RC
GLOBAL X DynamicValue
GLOBAL X GlobalValue
NOTE: Before Dosubl 2: x=NewDynamicValue
TESTDOSUBL X NewDynamicValue
TESTDOSUBL RC
GLOBAL X DynamicValue
GLOBAL X GlobalValue
NOTE: After Dosubl 2: x=GlobalValue
TESTDOSUBL X GlobalValue
TESTDOSUBL RC
GLOBAL X DynamicValue
GLOBAL X GlobalValue
NOTE: Before Dosubl 3: x=GlobalValue
TESTDOSUBL X GlobalValue
TESTDOSUBL RC
GLOBAL X DynamicValue
GLOBAL X GlobalValue
NOTE: After Dosubl 3: x=DynamicValue
TESTDOSUBL X DynamicValue
TESTDOSUBL RC
GLOBAL X DynamicValue
GLOBAL X GlobalValue
What I see see above is that every time DOSUBL is called above, the value of one of the two global macro variables named X gets copied to the local macro variable X.
But I'm not going to pretend that I understand the inner workings of how DOSUBL moves macro variables back and forth between the main session and the side session. I'm not even sure why the above creates a local macro variable X. Having seen some of the collisions that can occur has led me to stay away from using DOSUBL for production work. But I'm hopeful that in time the good folks at SAS will resolve these scoping issues, and either officially document how DOSUBL transfers macro variables (and system options, and datasets, and other constructs) back and forth between the main session and side session, or write it up in UG papers. There is clearly a lot of power and potential in the idea of a side-session.
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