动态内存分配不匹配

Question

我现在被内存分配困了几个小时了。基本上，我必须添加一个新的图形元素到矢量图形，VectorGraphic的初始化是必要的。

第一个问题是固定的InitVectorGraphic方法内的内存分配?(我认为)现在我坚持的第二个问题是，即使从InitVectorGraphic方法分配内存，pElements在AddGraphicElement方法内没有内存。(即，即使在一个方法(InitVectorGraphic方法)中初始化了它，更改也不会反映在其他方法中)

下面是我的完整代码:

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
enum{ 
    RUNNING = 1 
}; 
struct Point        
{
    int x, y; 
}; 
struct Line        
{ 
    Point start; 
    Point end; 
}; 
struct GraphicElement     
{ 
    enum{ 
        SIZE = 256 
    }; 
    unsigned int numLines;
    Line* pLines; 
    char name[SIZE]; 
}; 
typedef struct         
{ 
    unsigned int numGraphicElements; 
    GraphicElement* pElements; 
}VectorGraphic; 
void InitVectorGraphic(VectorGraphic* image){
    image = (VectorGraphic*)malloc(sizeof(VectorGraphic));
    (*image).pElements = (GraphicElement*)malloc(sizeof(GraphicElement)* 256);
//Problem part 1
    };
void AddGraphicElement(VectorGraphic* image){
    printf("nADDING A Graphic Element"); //Problem part 2
    int index = (*image).numGraphicElements;
    printf("nPlease enter the name of the new GraphicElement(<256 characters): ");
    scanf("%s", &(*image).pElements[index].name);
    printf("How many lines are there in the  new GraphicElement? ");
    scanf("%d", &(*image).pElements[index].numLines);
    (*image).pElements[index].pLines = (Line*)malloc(sizeof(Line)* (*image).pElements[index].numLines);
    for (int i = 0; i < (*image).pElements[index].numLines; i++){
        Line line;
        printf("Please enter the x coord of the start point of line index %d: ", i);
        scanf("%d", &line.start.x);
        printf("Please enter the y coord of the start point of line index  %d: ", i);
        scanf("%d", &line.start.y);
        printf("Please enter the x coord of the end point of line index %d: ", i);
        scanf("%d", &line.end.x);
        printf("Please enter the y coord of the end point of line index %d: ", i);
        scanf("%d", &line.end.y);
        (*image).pElements[index].pLines[i] = line;
    }
    (*image).numGraphicElements = (*image).numGraphicElements + 1;
    printf("Added");
    //add graphicElement to Image
};
void ReportVectorGraphic(VectorGraphic* image){
    printf("nVectorGraphic Report");
    for (int i = 0; i < (*image).numGraphicElements; i++){
        printf("nReporting Graphic Element #%d", i);
        printf("nGraphic Element name: %s", (*image).pElements[i].name);
        for (int j = 0; j < (*image).pElements[i].numLines; j++){
            printf("nLine #%d start x: %d", j, (*image).pElements[i].pLines[j].start.x);
            printf("nLine #%d start y: %d", j, (*image).pElements[i].pLines[j].start.y);
            printf("nLine #%d end x: %d", j, (*image).pElements[i].pLines[j].end.x);
            printf("nLine #%d end y: %d", j, (*image).pElements[i].pLines[j].end.y);
        }       
    }
};
void CleanUpVectorGraphic(VectorGraphic* image){
    free(image);
};
VectorGraphic Image; 

int main()        
{ 
    char response; 
    InitVectorGraphic(&Image); 
    while (RUNNING)               
    { 
        printf("nPlease select an option:n");
        printf("1. Add a Graphic Elementn"); 
        printf("2. List the Graphic Elementsn"); 
        printf("q. Quitn"); printf("CHOICE: ");
        fflush(stdin); 
        scanf("%c", &response); 
        switch (response)                      
        { 
        case '1':AddGraphicElement(&Image); 
            break; 
        case '2':ReportVectorGraphic(&Image);
            break; 
        case 'q':CleanUpVectorGraphic(&Image); 
            return 0;
        default:printf("Please enter a valid optionn"); 
        }
        printf("n"); 
    } 
}

解决方案是什么?

Answer 1

这是指针非常常见的混淆。您需要进行以下更改:

VectorGraphic* Image; //make it a pointer
void InitVectorGraphic(VectorGraphic** image) //expect pointer-to-pointer
{
    *image = (VectorGraphic*)malloc(sizeof(VectorGraphic)); //assign allocated buffer to pointer
    (*image)->pElements = (GraphicElement*)malloc(sizeof(GraphicElement)* 256);
}
InitVectorGraphic(&Image); //call remains the same

在旧代码中，您正在泄漏内存，您可以将image视为局部定义变量，如下所示:

InitVectorGraphic(VectorGraphic* image)
{
  image = malloc(); //Here you are allocating memory to a local variable not to the address Image
  //no free()'ing of image variable
}

按照旧代码的执行步骤只是进行两次赋值，第二次赋值覆盖原始赋值:

VectorGraphic *image = &Image;
image = malloc();