C语言栈实现中malloc与free操作的错误排查求助

问题描述 #

尝试编写C语言代码实现将点坐标动态存入栈，再打印并释放数据，但运行时出现以下问题：

• 程序能打印最后一个条目，打印倒数第二个时弹出“要释放的指针未分配”错误
• 移除free()语句后，仅打印第一行数据，随后无限输出0值
• 将stacktop改为非静态指针也无法解决问题

原代码如下：

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

struct point{
    float x;
    float y;
    float z;
}; typedef struct point POINT;

struct stackPoint{
POINT myPoint;
struct stackPoint *next;
}; typedef struct stackPoint STACKPOINT;

static STACKPOINT *stacktop = NULL;

void printStackElement(POINT aPoint){
printf(" x:%f \t y:%f \t z:%f\n", aPoint.x, aPoint.y, aPoint.z );
}

void push(POINT pushPoint){
STACKPOINT *newElem = malloc(sizeof(STACKPOINT));
stacktop = stacktop +1;
newElem->myPoint = pushPoint;
stacktop = newElem;
}

POINT pop(){
    POINT b = stacktop->myPoint;
    free(stacktop);
    stacktop = stacktop -1;
    return b;
}

int isEmpty(){
    if(stacktop == NULL){
        return 1;
    }
    return 0;
}

POINT readPoint(){
    POINT a;
    printf("Please enter your x-Coordinate: ");
    scanf(" %f", &a.x);
    printf("Please enter your y-Coordinate: ");
    scanf(" %f", &a.y);
    printf("Please enter your z-Coordinate: ");
    scanf(" %f", &a.z);
    return a;
}

int main(){
    char quit = 0;
    while(quit !=1 ){
        printf("\n\n enter 'p' to enter another Point or 'q' to quit: ");
        scanf(" %s", &quit);
        switch(quit){
            case 'p':
                push(readPoint());
                break;
            
            case 'q':
                quit = 1;
                break;

            default:
                break;
        }
    }
    while(isEmpty() == 0){
        printStackElement(pop());
    }
}

错误分析与修复 #

1. 链表栈指针操作逻辑错误（核心问题） #

代码混淆了数组栈和链表栈的实现逻辑：

• push函数中stacktop = stacktop +1;：指针直接加1会指向当前节点内存地址的下一块未知区域，并非链表的下一个节点，直接断裂栈的链表结构，后续访问均为非法内存。
• pop函数中stacktop = stacktop -1;：free当前节点后，指针减1指向未知内存，无法定位上一个节点。

修复方式：
链表栈的push需将新节点的next指向原栈顶，再更新栈顶为新节点；pop需先保存原栈顶的下一个节点，释放当前栈顶后，将栈顶更新为保存的节点。

2. main函数scanf格式符错误 #

scanf(" %s", &quit);中，quit是char类型，%s是读取字符串的格式符，会向quit的内存位置写入多个字节（含字符串结束符），导致内存越界，破坏其他变量内存。

修复方式：
改用%c格式符读取单个字符：scanf(" %c", &quit);

修复后的完整代码 #

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

struct point{
    float x;
    float y;
    float z;
}; typedef struct point POINT;

struct stackPoint{
    POINT myPoint;
    struct stackPoint *next;
}; typedef struct stackPoint STACKPOINT;

static STACKPOINT *stacktop = NULL;

void printStackElement(POINT aPoint){
    printf(" x:%f \t y:%f \t z:%f\n", aPoint.x, aPoint.y, aPoint.z );
}

void push(POINT pushPoint){
    STACKPOINT *newElem = malloc(sizeof(STACKPOINT));
    newElem->next = stacktop;
    newElem->myPoint = pushPoint;
    stacktop = newElem;
}

POINT pop(){
    POINT b = stacktop->myPoint;
    STACKPOINT *temp = stacktop->next;
    free(stacktop);
    stacktop = temp;
    return b;
}

int isEmpty(){
    return stacktop == NULL ? 1 : 0;
}

POINT readPoint(){
    POINT a;
    printf("Please enter your x-Coordinate: ");
    scanf(" %f", &a.x);
    printf("Please enter your y-Coordinate: ");
    scanf(" %f", &a.y);
    printf("Please enter your z-Coordinate: ");
    scanf(" %f", &a.z);
    return a;
}

int main(){
    char quit = 0;
    while(quit != 1 ){
        printf("\n\n enter 'p' to enter another Point or 'q' to quit: ");
        scanf(" %c", &quit);
        switch(quit){
            case 'p':
                push(readPoint());
                break;
            
            case 'q':
                quit = 1;
                break;

            default:
                break;
        }
    }
    while(!isEmpty()){
        printStackElement(pop());
    }
    return 0;
}

验证说明 #

修复后，程序可正常接收多个点坐标，退出后按栈“后进先出”顺序打印所有点，同时正确释放所有动态分配的内存，无内存错误或无限输出问题。

内容的提问来源于stack exchange，提问作者Kyzira