Why Doesn't Dereferencing a Function Pointer in C Execute the Function?

Function Pointer Dereferencing: Unraveling the Mystery

In C programming, a function pointer is a variable that stores the address of a function. However, unlike regular variables, dereferencing a function pointer behaves uniquely.

Why does dereferencing a function pointer not produce the expected result? The key lies in understanding how function values operate in rvalue contexts (where they are used as values rather than as locations). In C, function values in rvalue contexts are automatically converted to pointers to their original function value.

Upon dereferencing this pointer with *, the original function value is retrieved. However, this value is immediately reconverted to a pointer, creating an infinite loop of pointer conversions. The code provided in the question illustrates this behavior:

#include <stdio.h>

void hello() { printf("hello"); }

int main(void) { 
    (*****hello)(); 
}

This code essentially calls the hello function five times through a chain of function pointer dereferences. However, these dereferences do not actually execute the function; they merely retrieve the function pointer, which is then converted back to a pointer. The end result is a series of pointer manipulations, but no actual function call.

To understand why this happens, consider a similar experiment:

int x; // Regular variable
int *px = &x; // Pointer to the variable

*px = 5; // Modifying the variable through the pointer

In this code, dereferencing the pointer *px allows us to modify the value of the variable x. However, in the case of a function pointer, dereferencing does not modify the function itself, but rather retrieves its address.

This distinction exists because functions are immutable in C, meaning they cannot be modified. They can only be called or passed around as pointers. Therefore, there is no need for dereferencing a function pointer to modify its behavior.

In summary, dereferencing a function pointer does not execute the function, but rather retrieves its address. This address is then immediately reconverted to a pointer, resulting in an infinite loop of pointer conversions. This behavior is a unique feature of function values in rvalue contexts in C and serves as a convenience for working with function pointers without having to explicitly use ampersand (&) for address referencing.

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