C - malloc和free具有动态变化的结构

Question

我在动态变化的结构中移动指针时遇到了麻烦。我已经创建了我的代码，你可以malloc更多的内存，这似乎是工作。我遇到的问题是如何添加到结构中，如何释放内存以及如何从结构移动到结构并打印所有项目。

我正试图测试添加和打印(删除函数似乎不起作用，segfaults)

当我添加到结构体然后打印结构体时，我从我添加的值中得到一个段错误。我不知道我是否正确地从第一个结构体移动到下一个结构体。

#include <stdio.h>
#include <stdlib.h>
#include "pointer.h"
/********************************************
Creates more memory for size (strut * rec+1)
*********************************************/
employee *create(int record){
employee *new_employee = malloc(sizeof(employee) * (record+1));
return new_employee;    
}
/********************************************
Copies the data from one structure to a new structure with 
size "structure" multipled by rec+1
***********************************************/
employee *copy(employee *data, int record){
    employee *new_employee = create(record);
int i;
    for(i = 0; i<record;i++){
        new_employee->first = data->first;
        new_employee->last = data->last;
        new_employee->start_date = data->start_date;
        new_employee->sal = data->sal;
        data++;
    }
    /********************
    Needs to free the old struct
    *********************/
    //deleteData(data, record);
return new_employee;
}
/********************************************
Function prints everything in the struct
*********************************************/
void printStruct(employee *data, int record){
int i;
    for(i = 0; i<record; i++){
        printf("nEntry: %dn", i+1);           
        printf("The employee's name is %s %sn", data->first, data->last);
        printf("The employee was hired on: %sn", data->start_date);
        printf("The employee make $%fnn", data->sal); 
        data++;     
    }
}
/******************************************
Function frees the old data base
*******************************************/
void deleteData(employee *data, int record){
int i;
    for(i = 0; i<record; i++){
        free(data->first);
        free(data->last);
        free(data->start_date);
        data++;
    }
    free(data);
}
/******************************************
Adds an employee to the new structure
*******************************************/
employee *add(employee *data,char *fname, char *lname, char *date, float salary, int record){
employee *employeeDB = create(record);
employeeDB = copy(data, record);
int i;
    employeeDB++;
    employeeDB->first = fname;
    employeeDB->last = lname;
    employeeDB->start_date = date;
    employeeDB->sal = salary;
return employeeDB;
}


/**************************
Starts of the main function
***************************/
int main(void){
    //Keeps track of the number of records that are in the structure
int rec = 0;
    //Keeps the number of accesses to the structure. Even with the one entry   the structure has not been accessed. 
int acc = 0;
    //Holds the input information for the menu
int input;
    //holds the information for inputing user first name
char *fname;
    //holds the information for inputing user last name
char *lname;
    //holds the information for for the startdate
char *start;
    //holds the information for the salary;
float sal;
/*********************************
This next section adds an employee to the record
************************************/
//This creates the first entry to the dynamic structure.
employee *first_employee = create(rec);
first_employee->first = "FIRST";
first_employee->last = "LAST";
first_employee->start_date = "June-20th-2006";
first_employee->sal = 55555.55;
//increase the number of records    
rec = rec+1;
employee *new_employeeDB = add(first_employee, "fname", "lname", "JUNE-20th-2010", 55555.55, rec);
rec = rec + 1;
printStruct(new_employeeDB, rec);

printf("%dn", (sizeof(employee)* rec));

}

Answer 1

第一个问题:Ok…您没有包括雇员类型的声明。我猜first和last被声明为char指针，这是一个错误。你需要它们是固定大小的字符数组，否则这将永远不会工作。为字符串选择一个最大长度，并像这样声明结构。

typedef struct
{
    char name[50];
    char surname[50];
    ....
} employee;

以前的文章:嗯，我必须承认我不太喜欢这种实现方式。我会使用更稳定的方法。

首先，由于它只是一个项目列表，您可以使用双重链表。这将允许您添加和删除复杂度为0(1)的元素。真的很好。

这也允许你从第一个到最后迭代所有的项。

为了做到这一点，我将使用更面向对象的方法:)我知道，我们是在C语言中，但听听这个想法。

typedef struct
{
    MyList* OwnerList;
    Employee* Previous;
    Employee* Next;
    char name[50];
    int age;
} Employee;
typedef struct
{
    Employee* First;
    Employee* Last;
    int Count;
} MyList;
MyList* AllocList() { return calloc(sizeof(MyList), 1); }
void DeleteList(MyList* list)
{
    Employee* current;
    Employee* next;
    for (current = list->First; current != NULL; current = next)
    {
        next = current->Next;
        free(current);
    }
    free(list);
}
int GetCount(const MyList* list)
{
    return list->Count;
}
Employee* AddAmployee(MyList* list)
{
    Employee* result = calloc(sizeof(Employee), 1);
    Employee* last = list->Last;
    if (last != null)
        last->Next = result;
    else
        list->First = result;
    result->Previous = last;
    list->Last = result;
    ++list->Count;
    result->OwnerList = list;
    return result;
}
void RemoveEmployee(Employee* employee)
{
    /* i leave removal for you as exercise :) look for doubly linked list */
    free(employee);
}

现在，迭代所有项很简单:

Employee* current;
for (current = list->First; current != null; current = current->Next)
    printf("%s %dn", current->name, current->age);

Answer 2

您没有使用malloc来分配employee的first, last和start_date属性。因此，当您对deleteData中的指针调用free时，您正在破坏内存。我还会考虑使用链表或其他一些数据结构(如数组)来保存员工记录，从而使您拥有更干净的界面。