A queued call in programming is when a function calls another function to return a value instead of passing a variable. Tail calls are attractive as they don’t use extra stack space and are useful for recursion. However, they can make debugging difficult. The call stack stores function addresses, and a tail call directly passes return values to the calling code, increasing speed and keeping the stack small. Queued calls are useful for recursive functions as they prevent stack overflow.
In computer programming, a queued call is a specific situation within program source code in which a function, subroutine, or procedure returns an expected value by calling another function instead of simply passing a variable that contains the value returned. The name itself indicates that the function called to calculate the return value is at the end, or tail, of the function calling it to provide a return value. A tail call is attractive to some programmers because, with certain optimizations or compiler behaviors, no additional stack space is used to store main function code locations; the tail function is used to generate return value reports directly to the call point where the original function was invoked. Using a tail call is especially useful in situations where recursion is employed, because the amount of stack space used to store caller addresses in cases where recursive calls are nested very deeply could quickly fill up and break the execution of the program. While using queue calls can help increase speed, memory usage, and efficiency in a program, it can also lead to situations where the source code is restructured to use the calls in a way that makes it difficult to debugging and monitoring, especially with recursion cases.
The existence of a call queue is due in large part to how the call stack works within most computer programs and system architectures. The stack, which is like a stack of plates, is a first-in, last-out data structure. When a function, subroutine, or procedure is called, the address from which the call is made, called the stack frame, is stored on the stack. This means that a program that calls function A, which then calls function B, will have two stack frames, one for function B and another below it for function A. When function B finishes executing, its The stack frame is popped off the top of the stack and execution returns to function A, whose frame is removed from the stack on completion, ultimately returning program control to where the first function was originally called.
When a tail call is used, the return statement in one function directly uses the return value of another function as the data to send to the calling code. In the above example, if function A calls function B directly with the return statement, a tail call has been formed. Inside the call stack, instead of having a stack frame for both functions A and B, function B will receive the return address from function A, and function A’s stack frame will be popped and discarded, meaning that function B will pass its return value directly back to the location that called function A without having to first return control to function A. This increases the speed of function calls and helps keep the amount of information on the stack.
The properties of a queue call can make them a very attractive option for recursive functions. A recursive function is a function that calls itself repeatedly to compute a value, as may be the case when traversing a list data structure. No extra stack frames are created for nested function calls, so very deep recursion levels can be run safely without the immediate threat of a stack overflow and possible program termination.
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