How to get the Location of SystemFunction,I used the SourceLocation and miss some trouble

hello,I try to Insert some code before a function when it was called.for
char *p,*q;
^ ^
I want to insert some code at this two place,but I failed to handle it.
I used just like this:
VisitCallExpr(CallExpr *CE){
if(FunctionDecl *FD=CE->getDirectCallee())
SourceRange sr = FD->getSourceRange();
SourceLocation ST1 = sr.getBegin();

but I found it make no difference when I used Rewrite.InsertText(ST1,
tempfc, true, true);
trying to insert some code into it.
would you please tell me what's wrong with it and show me how to some this
thank you

I need help! do me a favor