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Double headed arrows represent reference parameters.
Values are passed from the calling module to the function when the function is called. But values are also passed back from the function to the calling arguments when there are changes to the values of the formal parameters in the function.
This happens because the formal parameters are, in fact, aliases of the corresponding arguments in the calling module. That is, both the arguments of the calling module and the formal parameters of the function share the same address (point to the same location).
When a change happens to the value of a formal parameter in the function, that change also occurs to the value of the calling module because both the calling argument and formal parameter share the same memory location.
Single header arrows pointing down represent value paramenters.
Values are passed from the calling module to the function when the function is called. But values are not passes back from the function to the calling arguments when there are changes to the values of the formal parameters in the function.
This is because the formal parameters and their corresponding arguments point to different memory locations.
When a change occurs to a value of a value formal parameter in the function, that change occurs in a different memory location than that addressed by the corresponding calling argument.
Single headed arrows pointing up represent values returned, not by reference parameters, but by the function's return statement as the function completes execution.
Note: Arguments refer to the parameters of a call when it invokes a function. Those parameters are often called actual parameters because they represent the actual values that are passed to their corresponding formal parameters in the function. The arguments, or actual parameters, may actually be constant values (an integer, or character, or string) or they may be symbolic variables that identify a memory location where actual values may be found.
Formal parameters are the variables declared in the header of a function. Formal parameters are “formal” because they symbolically address memory locations that will receive “actual values” (actual parameters) passed to them from the calling arguments in the call of the function.