Intel pin: How does RTN_InsertCall works?

Question

The RTN_InsertCall in intel Pin has its definition as

VOID LEVEL_PINCLIENT::RTN_InsertCall(
    RTN     rtn,
    IPOINT  action,
    AFUNPTR funptr,
    ... 
)       

It can call the function funptr with the arguments provided in ... (before or after calling the function represented by rtn).

Example:

void func1(int a) {
    printf("Argument : %d", a);
}

...

RTN_InsertCall(funcRtn, IPOINT_BEFORE, (AFUNPTR)func1, 
               IARG_FUNCARG_ENTRYPOINT_VALUE, 0, 
               IARG_END);

void func2(int a, int b) {
    printf("Argument : %d %d", a, b);
}

...

RTN_InsertCall(funcRtn, IPOINT_BEFORE, (AFUNPTR)func1, 
               IARG_FUNCARG_ENTRYPOINT_VALUE, 0, 
               IARG_FUNCARG_ENTRYPOINT_VALUE, 1, 
               IARG_END);

How does this function work? How to call a non-variadic function(and the number of arguments may vary) inside a variadic function?

Btw, the RTN_InsertCall is used to insert an additional function(func1 / func2) before actually execute the target function(funcRtn). What can we do if we don't know the number of arguments of funcRtn? How to convert the func1 / func2 into variadic functions?