发一下完整做法…….

编写一个函数
用来，把结构体变量A的地址赋给另一个结构体变量B
达到，修改结构体变量B的值就能修改结构体变量A的值的目的
文件已给出，只需编写函数B
/*
@Author: Wang
@Date: 2021-11-27 00:43:55
@LastEditTime: 2021-11-27 00:43:56
@LastEditors: Wang
/
#include"stdio.h"
typedef struct
{
double member_a;
double member_b;
double member_c;
} struct_B; //结构体类型B
typedef struct
{
double member_A;
double member_B;
struct_B *member_C;
} struct_A; //结构体类型A
struct_A struct_variable_A; //结构体变量A
struct_B struct_variable_B = {1.0f, 2.0f, 3.0f}; //结构体变量B
void function_A(struct_A *struct_variable_C); //函数A声明
function_B(); //函数B声明，需要编写的函数声明
int main(void)
{
scanf("%lf,%lf,%lf",&struct_variable_B.member_a,&struct_variable_B.member_b,&struct_variable_B.member_c); //读入结构体变量B的值
function_A(&struct_variable_A); //调用函数A

printf("%lf\n",struct_variable_A.member_C->member_a); //打印结构体变量A中的值
printf("%lf\n",struct_variable_A.member_C->member_b);
printf("%lf\n",struct_variable_A.member_C->member_c);

return 0;
}
//函数A
void function_A(struct_A *struct_variable_C)
{
struct_variable_C->member_C = function_B(); //调用函数B
}
//函数B，需要编写的函数体
function_B()
{
}

题主，函数缺返回值类型

struct_B* function_B()
{
    return &struct_variable_B;
}

完整：

#include "stdio.h"
typedef struct
{
    double member_a;
    double member_b;
    double member_c;
} struct_B; //结构体类型B
typedef struct
{
    double member_A;
    double member_B;
    struct_B *member_C;
} struct_A;                                      //结构体类型A
struct_A struct_variable_A;                      //结构体变量A
struct_B struct_variable_B = {1.0f, 2.0f, 3.0f}; //结构体变量B
void function_A(struct_A *struct_variable_C);    //函数A声明
struct_B* function_B();                                    //函数B声明，需要编写的函数声明
int main(void)
{
    scanf("%lf,%lf,%lf", &struct_variable_B.member_a, &struct_variable_B.member_b, &struct_variable_B.member_c); //读入结构体变量B的值
    function_A(&struct_variable_A);                                                                             //调用函数A
    printf("%lf\n", struct_variable_A.member_C->member_a); //打印结构体变量A中的值
    printf("%lf\n", struct_variable_A.member_C->member_b);
    printf("%lf\n", struct_variable_A.member_C->member_c);

    return 0;
}
//函数A
void function_A(struct_A *struct_variable_C)
{
    struct_variable_C->member_C = function_B(); //调用函数B
}
//函数B，需要编写的函数体
struct_B* function_B()
{
    return &struct_variable_B;
}