Why Does Repeated Dereferencing of a Function Pointer in C Not Lead to Multiple Function Calls?

The Intriguing Behavior of Function Dereferencing

The act of dereferencing a function pointer in C may appear to produce unexpected results. For example, the code snippet below seems to perform an excessive number of dereferences on a function pointer, but the output remains unchanged:

#include<stdio.h>

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

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

Understanding Function Dereferencing

To understand this behavior, it's crucial to clarify the semantics of function pointers in C. While function names may imply pointers to functions, they actually represent the addresses of the function codes. Dereferencing a function pointer, therefore, does not directly execute the function but instead returns a function designator, which is swiftly converted back to a function pointer.

In the example code, the repeated dereferences of hello do not execute the function repeated times. Instead, each dereference creates a new function designator, which is then converted back to a function pointer. The final expression (*****hello) simply evaluates to a function pointer for hello, and calling it through () executes the function as expected.

Implicit Function Pointer Conversion

Another peculiar aspect of function pointers in C is their implicit conversion between function values and pointers in specific contexts. When a function value appears in an rvalue context (any context where it is used as a value rather than a location), it is automatically converted to a function pointer. This behavior extends to nested function values, where each dereference returns a function pointer pointing to the original function.

Practical Implications

While the behavior of function pointer dereferencing can initially seem puzzling, it can be beneficial in certain scenarios. By avoiding the explicit use of & when passing function pointers, code becomes more concise and readable. Additionally, the symmetry between calling functions through both function pointers and function names allows for seamless transitions between the two forms.

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