功能原型范围

#include <stdio.h>

/* The parameter name, apple, has function prototype scope.  These names
   are not significant outside the prototype itself.  This is demonstrated
   below. */

int test_function(int apple);

int main(void)
{
    int orange = 5;

    orange = test_function(orange);
    printf("%d\r\n", orange); //orange = 6

    return 0;
}

int test_function(int fruit)
{
    fruit += 1;
    return fruit;
}

请注意，如果在原型中引入类型名称，则会出现令人费解的错误消息：

int function(struct whatever *arg);

struct whatever
{
    int a;
    // ...
};

int function(struct whatever *arg)
{
    return arg->a;
}

使用 GCC 6.3.0，此代码（源文件 dc11.c）生成：

$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -c dc11.c
dc11.c:1:25: error: ‘struct whatever’ declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
     int function(struct whatever *arg);
                         ^~~~~~~~
dc11.c:9:9: error: conflicting types for ‘function’
     int function(struct whatever *arg)
         ^~~~~~~~
dc11.c:1:9: note: previous declaration of ‘function’ was here
     int function(struct whatever *arg);
         ^~~~~~~~
cc1: all warnings being treated as errors
$

将结构定义放在函数声明之前，或者在函数声明之前将 struct whatever; 添加为一行，并且没有问题。你不应该在函数原型中引入新的类型名称，因为无法使用该类型，因此无法定义或使用该函数。