An argument passing convention where the address of an argument variable is passed to a function or procedure, as opposed to passing the value of the argument expression. Execution of the function or procedure may have side-effects on the actual argument as seen by the caller. The C language's "&" (address of) and "*" (dereference) operators allow the programmer to code explicit call-by-reference. Other languages provide special syntax to declare reference arguments (e.g. ALGOL 60).