signal函数两种等效声明的技术确认与解析

Short answer: Yes, these two declarations are completely equivalent — they describe the exact same function signature for signal(). Let’s break down why, step by step.

First, Let’s Understand the Typedef #

The line:

typedef void (*sighandler_t)(int);

creates a type alias sighandler_t for a specific function pointer type: a pointer to a function that takes an int parameter and returns void. This shorthand exists to simplify the messy syntax of function pointer declarations, making code easier to read.

Declaration 1: The Simplified Version #

Using the typedef, the signal function is declared as:

extern sighandler_t signal(int signum, sighandler_t handler);

Let’s parse this clearly:

• signal is a function that accepts two arguments:
  1. An int (signum, the signal number to handle)
  2. A sighandler_t (handler, the function to run when the signal is triggered)
• The function returns a sighandler_t — this is the previously registered signal handler, so you can restore it later if needed.

Declaration 2: The "Raw" Function Pointer Syntax #

The second declaration skips the typedef and uses the full, unaliased function pointer syntax:

extern void (*signal(int sig, void (*func) (int)))(int);

This looks daunting at first, but let’s unwrap it from the inside out:

1. Start with the function name: signal is a function.
2. Its parameters: (int sig, void (*func)(int)) — it takes an int and a function pointer (func) that accepts an int and returns void.
3. The return type: the entire declaration is wrapped in void (* ... )(int), which means signal returns a pointer to a function that takes an int and returns void.

If you replace every instance of void (*)(int) with our typedef sighandler_t, you end up exactly with Declaration 1. That’s the core reason they’re identical.

Why Two Forms? #

The second form is common in older C code or contexts where a typedef isn’t used (or isn’t available). The typedef version is just a cleaner, more readable way to express the same signature — it hides the verbose function pointer syntax, making the code’s intent clearer at a glance.

内容的提问来源于stack exchange，提问作者Soner from The Ottoman Empire