开始学习C语言，高人指点

想实现一个初始化双链表。
main.c

 #include <stdio.h>
#include <stdlib.h>
#include "db.h"
int main(){
    pnode *n=NULL;
    n=createlist();
}

db.h

 #ifndef DB_H_INCLUDED
#define DB_H_INCLUDED
struct nod{
    int data;
    struct nod *pre,*next;
};
typedef struct nod *pnode,node;

pnode createlist();
int  insertlist();
int  finded();
int  deletelist();
void print();
int getlen();
#endif // DB_H_INCLUDED

db.c

 #include <stdio.h>
#include <stdlib.h>
#include "db.h"
pnode createlist(){
    pnode *tail=NULL;pnode *newnode=NULL;
    pnode *head=(pnode *)malloc(sizeof(node));
    int length=0;
    if(NULL==head){
        printf("没有分配到空间");
        exit(EXIT_FAILURE);
    }
    head->data=0;
    head->next=NULL;
    head->pre=NULL;
    head=tail;
    printf("请输入双链表的长度");
    scanf("%d",&length);
    for(int i =0;i<length;i++){
        newnode=(pnode *)malloc(sizeof(node));
        if(NULL==newnode){
            printf("新节点分配内存失败");
            exit(EXIT_FAILURE);
        }
        printf("请输入第%d个值",i+1);
        scanf("%d",&newnode->data);
        newnode->next=head->next;
        head->next->pre=newnode;
        head->next=newnode;
        newnode->pre=head;

    }return head;
}

VS2017编译通过，并修复了一些逻辑bug（默认这是为了建立一个有头有尾的双向列表）。你可以试一下，代码如下：
main.c：

#include <stdio.h>
#include <stdlib.h>
#include "db.h"
int main() {
    pnode n ;
    n = createlist();
}

db.h

 #ifndef DB_H_INCLUDED
#define DB_H_INCLUDED
struct nod {
    int data;
    struct nod *pre, *next;
};
typedef struct nod *pnode, node;

pnode createlist();
int  insertlist();
int  finded();
int  deletelist();
void print();
int getlen();
#endif // DB_H_INCLUDED

db.c

 #include <stdio.h>
#include <stdlib.h>
#include "db.h"
pnode createlist() {
    pnode newnode = NULL;
    pnode head = (pnode)malloc(sizeof(node));
    pnode tail = (pnode)malloc(sizeof(node));
    int length = 0;
    if (NULL == head || NULL == tail) {
        printf("没有分配到空间");
        exit(EXIT_FAILURE);
    }
    head->data = 0;
    head->next = tail;
    head->pre = NULL;
    tail->data = 0;
    tail->next = NULL;
    tail->pre = head;
    printf("请输入双链表的长度");
    scanf_s("%d", &length);
    for (int i = 0; i<length; i++) {
        newnode = (pnode)malloc(sizeof(node));
        if (NULL == newnode) {
            printf("新节点分配内存失败");
            exit(EXIT_FAILURE);
        }
        printf("请输入第%d个值", i + 1);
        scanf_s("%d", &(newnode->data));
        head->next->pre = newnode;
        newnode->next = head->next;
        head->next = newnode;
        newnode->pre = head;
    }return head;
}

    应该在在前面添加    #include "db.c"   

可以在每个C文件的最前面增加一个#include "includes.h"

加一个#include "includes.h"

请在程序最开始添加#include "includes.h"，试一下是否可行

请在最开始添加#include "includes.h"，试一下是否可行

VS2017中编译通过，修改了一些逻辑错误：
main.c

 #include "db.h"
int main()
{
    pnode n = NULL;
    n = createlist();
}

db.h

 #pragma once
#ifndef DB_H_INCLUDED
#define DB_H_INCLUDED
struct nod
{
    int data;
    struct nod *pre, *next;
};
typedef struct nod *pnode, node;

pnode createlist();
//int insertlist();
//int finded();
//void print();
//int getlen();
#endif

db.cpp

 #include<stdio.h>
#include<stdlib.h>
#include "db.h"
pnode createlist()
{
    pnode tail = new node;
    pnode head = new node;
    int length = 0;
    if (NULL == head)
    {
        printf("No space created!");
        exit(EXIT_FAILURE);
    }
    head->data = 0;
    head->next = tail;
    head->pre = NULL;
    tail->data = 0;
    tail->next = NULL;
    tail->pre = head;
    printf("Please input the length of list.");
    scanf_s("%d", &length);
    for (int i = 0; i < length; i++)
    {
        pnode newnode = (pnode)malloc(sizeof(node));
        if (NULL == newnode)
        {
            printf("New node space failed.");
            exit(EXIT_FAILURE);
        }
        printf("Please input data of NO.%d node", i + 1);
        scanf_s("%d", &newnode->data);
        newnode->next = tail;
        newnode->pre = tail->pre;
        tail->pre = newnode;
        newnode->pre->next = newnode;
    }
    return head;
}

加一个#include "includes.h"应该就可以了

你可以试着在程序最前面加一个#include"includes.h"

C语言现在都再哪些高级的应用中用到啊

在程序最前面加#include “inchudes.h”

在开头添加 #include 试一下是否可行

在程序最开始加上 #include"includes.h" 导入头文件