如何在 C 中为我的数据结构分配内存

Question

我不明白为什么我的代码会在用户输入数据后打印出垃圾变量。在我的 C 类入门中学习数据结构、链表和内存分配的过程中。感谢您的帮助！

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>

typedef struct AutoMobilesInfo
{
    char *Manufacturer, *ModelNameOfCar, *ColorOfCar;
    int YearOfCar;
    struct AutoMobilesInfo *next;
} Car;

void EnterCarInfo(Car *Info)
{
    Info = (Car *)malloc(sizeof(Car));
    Info->Manufacturer = (char *)malloc(12 *(sizeof(char)));
    Info->ModelNameOfCar = (char *)malloc(12 *(sizeof(char)));
    Info->ColorOfCar = (char *)malloc(12 *(sizeof(char)));

    printf("Please enter the car's manufacturer: \n");
    scanf(" %s", Info->Manufacturer);
    printf("Please enter the car's model: \n");
    scanf(" %s", Info->ModelNameOfCar);
    printf("Please enter the car's color: \n");
    scanf(" %s", Info->ColorOfCar);
    printf("Please enter the year the car was made: \n");
    scanf("%d", &Info->YearOfCar);
}

void PrintedOutCarInfo(Car *host)
{
    host = (Car *)malloc(sizeof(Car));
    host->Manufacturer = (char *)malloc(12 *(sizeof(char)));
    host->ModelNameOfCar = (char *)malloc(12 *(sizeof(char)));
    host->ColorOfCar = (char *)malloc(12 *(sizeof(char)));

    printf("The car's manufacturer is %s\n", host->Manufacturer);
    printf("The car's model is %s\n", host->ModelNameOfCar);
    printf("The color of the car is %s\n", host->ColorOfCar);
    printf("The year the car was made is %d\n", host->YearOfCar);
}

int main()
{
    Car *Car1;
    Car1 = (Car *) malloc(sizeof(Car));
    EnterCarInfo(Car1);
    PrintedOutCarInfo(Car1);
    free(Car1);

    return 0;
}

Answer 1

您正在覆盖PrintedOutCarInfo 中的指针。这一行

host = (Car *)malloc(sizeof(Car));

还有这一行

Info = (Car *)malloc(sizeof(Car));

分配你传入的变量host 和Info。把那个函数改成这个

void PrintedOutCarInfo(Car *host) {
  printf("The car's manufacturer is %s\n", host->Manufacturer);
  printf("The car's model is %s\n", host->ModelNameOfCar);
  printf("The color of the car is %s\n", host->ColorOfCar);
  printf("The year the car was made is %d\n", host->YearOfCar);
}

你也不应该对 malloc 的返回值进行大小写，而是这样做......

 Info = malloc(sizeof(Car));

试试这个...

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <malloc.h>

typedef struct AutoMobilesInfo
{
  char *Manufacturer, *ModelNameOfCar, *ColorOfCar;
  int YearOfCar;
  struct AutoMobilesInfo *next;
} Car;

void EnterCarInfo(Car *Info)
{
  Info->Manufacturer   = malloc(12 * (sizeof(char)));
  Info->ModelNameOfCar = malloc(12 * (sizeof(char)));
  Info->ColorOfCar     = malloc(12 * (sizeof(char)));

  printf("Please enter the car's manufacturer: \n");
  scanf(" %s", Info->Manufacturer);
  printf("Please enter the car's model: \n");
  scanf(" %s", Info->ModelNameOfCar);
  printf("Please enter the car's color: \n");
  scanf(" %s", Info->ColorOfCar);
  printf("Please enter the year the car was made: \n");
  scanf("%d", &Info->YearOfCar);
}

void PrintedOutCarInfo(Car *host) {
  printf("The car's manufacturer is %s\n", host->Manufacturer);
  printf("The car's model is %s\n", host->ModelNameOfCar);
  printf("The color of the car is %s\n", host->ColorOfCar);
  printf("The year the car was made is %d\n", host->YearOfCar);
}

int main()
{
  Car *Car1 = malloc(sizeof(Car));
  EnterCarInfo(Car1);
  PrintedOutCarInfo(Car1);
  free(Car1);
  return 0;
}

Answer 2

该问题是由 PrintedOutCarInfo() 中的内存分配引起的。 在 EnterCarInfo() 中，您正在为 Car 及其成员（字符串）分配必要的内存，并用用户的输入填充它们。 当您在 EnterCarInfo 中再次分配 Car 结构及其成员时，您正在破坏对原始 Car 结构及其成员的引用。 如果删除 EnterCarInfo() 的前 4 行，程序应该可以工作。 附带说明一下，这个应用程序可以用于学习/演示，但是分配一个固定的内存块来接收用户输入而不强制执行大小限制并不是一个好主意。未能检查限制将使您面临缓冲区溢出攻击。 希望这会有所帮助。

Answer 3

正如 Harry 在下面所说，您正在覆盖 PrintedOutCarInfo() 中的指针。该函数应如下所示：

void PrintedOutCarInfo(Car *host)
{
    printf("The car's manufacturer is %s\n", host->Manufacturer);
    printf("The car's model is %s\n", host->ModelNameOfCar);
    printf("The color of the car is %s\n", host->ColorOfCar);
    printf("The year the car was made is %d\n", host->YearOfCar);
}

Malloc() 分配（保留）给定数量的内存供您的应用程序使用，然后返回指向该内存块开始地址的指针。您已经在 EnterCarInfo() 中分配了所需的内存，因此当您在 PrintedOutCarInfo() 中第二次调用 Malloc() 时，您正在分配内存中的另一个空间，然后将 AutoMobilesInfo 的指针指向内存中的那个新位置。 EnterCarInfo() 收集的数据仍在内存中，但它位于您的 AutoMobilesInfo 实例不再引用的旧位置。因此，当您 printf() 数据时，您正在打印新分配的内存块中的任何垃圾。