技术咨询：两类函数指针与指针函数声明（含extern修饰）的区别

Alright, let's break down these two pairs of C declarations—they look nearly identical but represent completely different constructs, which is a classic gotcha for anyone working with function pointers in C.

第一组：int (*cmp)(void) 与 int *cmp(void) #

Parentheses are everything in C declaration syntax, so let's unpack the core difference here:

int (*cmp)(void) #

This declares a pointer to a function that takes no arguments and returns an int.

• The parentheses wrapping (*cmp) are critical—they force the compiler to interpret cmp as a pointer first, before parsing the function syntax attached to it.
• In simple terms: cmp is a pointer, and it points to a function that takes nothing and spits back an integer.
• Example usage:

  int my_func(void) { return 42; }
  int (*cmp)(void) = &my_func; // Assign the function's address to cmp
  printf("%d\n", (*cmp)()); // Calls my_func, outputs 42 (you can also write cmp() directly!)
  

int *cmp(void) #

This declares a function that takes no arguments and returns a pointer to an int.

• Here, no parentheses wrap *cmp, so the function syntax (cmp(void)) takes priority. The * only applies to the function's return type.
• Plain language: cmp is a function name—call it, and it hands you back a pointer to an integer value.
• Example usage:

  int *cmp(void) {
      static int value = 100;
      return &value; // Return pointer to the static integer
  }
  int *ptr = cmp();
  printf("%d\n", *ptr); // Outputs 100
  

第二组：extern int (*func)(void); 与 extern int *func(void); #

The extern keyword only changes one thing: it tells the compiler that the identifier (func) is defined in another source file or translation unit. The underlying type of each declaration is exactly the same as the first group.

• extern int (*func)(void); → Declares that somewhere else in your codebase, there's a function pointer func (pointing to a void-arg, int-return function) already defined.
• extern int *func(void); → Declares that somewhere else, there's a function func (taking no args, returning an int pointer) already defined.

extern just lets you reference something existing elsewhere without redefining it in the current file.

内容的提问来源于stack exchange，提问作者gnued