C++ 为什么 GCC 只有在使用 struct 名称而不是 typedef 时才会抛出错误和不必要的警告

我有一个由两个源文件(farm.c，init.c)和两个相应的头文件(farm.h，init.h)组成的程序.两个源文件都包含头保护，并且彼此都包含，因为它们都需要来自彼此的函数/变量.

初始化.h:

#ifndef INIT_H
#define INIT_H

#include<stdio.h>
#include<stdlib.h>
#include"farm.h"

#define PIG_SOUND "oink"
#define CALF_SOUND "baa"

enum types {PIG, CALF};

typedef struct resources {
    size_t pork;
    size_t veal;
    size_t lamb;
    size_t milk;
    size_t eggs;
} resources;

typedef struct animal {
    size_t legs;
    char* sound;
    int efficiency;
    void (*exclaim)(struct animal*);
    void (*work)(struct animal*, struct resources*);
} animal;

/* I have tried various ways of declaring structs in addition to
   the typedef such as this */

//animal stock;
//animal farm;

void make_pig(struct animal* a, int perf);
void make_calf(struct animal* a, int perf);

#endif

农场h:

#ifndef FARM_H
#define FARM_H

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#include<string.h>
#include"init.h"

/* GCC does not recognise the typedef or struct identifier 
   until these forward declarations have been made in 
   addition to the included init.h header file */

//typedef struct animal animal;
//typedef struct resources resources;

void exclaim(animal* b);
void work(struct animal* b, struct resources* s);

#endif

初始化.c:

#include"init.h"

void make_pig(animal* a, int perf) {
    a->legs = 4;
    a->sound = PIG_SOUND;
    a->efficiency = perf;
    a->exclaim = exclaim;
    a->work = work;
}

void make_calf(animal* a, int perf) {
    a->legs = 4;
    a->sound = CALF_SOUND;
    a->efficiency = perf;
    a->exclaim = exclaim;
    a->work = work;
}

农场c:

#include"farm.h"

int main() {
    return 0;
}

void exclaim(animal* a) {
    for (int i = 0; i < 3; i++) {
        printf("%s ", a->sound);
    }
    printf("\n");
}

void work(animal* a, struct resources* r) {
    if (!strcmp(a->sound, PIG_SOUND)) {
        r->pork += a->efficiency;
    }

    if (!strcmp(a->sound, CALF_SOUND)) {
        r->veal += a->efficiency;
    }
}

在使用C99标准的Linux系统上，使用这两种类型的名称(即struct ani和animal)通常可以很好地工作.然而，当我在这里使用struct ani而不是animal时，对于struct ani和struct resources类型使用的每个实例，我都会得到以下警告.

lib/农场h:10:21: warning: ‘struct ani’ declared inside parameter list will not be visible outside of this definition or declaration
   10 | void exclaim(struct ani* a);
      |                     ^~~
lib/农场h:11:33: warning: ‘struct resources’ declared inside parameter list will not be visible outside of this definition or declaration
   11 | void work(struct ani* a, struct resources* r);

每次使用该窗体的函数指针时，总共有10个警告:

src/初始化.c:17:16: warning: assignment to ‘void (*)(struct ani *)’ from incompatible pointer type ‘void (*)(struct ani *)’ [-Wincompatible-pointer-types]
   17 |     a->exclaim = exclaim;
      |                ^
src/初始化.c:18:13: warning: assignment to ‘void (*)(struct ani *, struct resources *)’ from incompatible pointer type ‘void (*)(struct ani *, struct resources *)’ [-Wincompatible-pointer-types]
   18 |     a->work = work;

有人能解释一下为什么会发生这种行为，以及我如何避免出现问题吗？解决这些错误通常会花费我一段不可行的时间，但我仍然没有真正理解我的错误.