c - 堆栈的链表，head->next 不断变为 null

Question

//Colin James P. Naranjo
//CMSC123 CD-1L
//This program demonstrates postfix evaluation through pop and push functions

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

typedef struct node{                                    //Uses a combination of typedef and tagged structure for the singly linked list
    char value;
    struct node *next;
}stack;


char evaluate(char a, char b, char c){
    int ans;
    if(c == '*'){
        ans = (int)a * (int)b;
    }
    if(c == '+'){
        ans = (int)a + (int)b;
    }
    if(c == '-'){
        ans = (int)a - (int)b;
    }
    if(c == '/'){
        ans = (int)a / (int)b;
    }
    if(c == '%'){
        ans = (int)a % (int)b;
    }
    return (char)ans;
}

char pop(stack *head){          //For popping a value in the stack, create a temporary variable to take over the head   
    stack *temp;
    char x;
    printf("Your sequence is mostly likely not in order.\n");
    temp = head;                        //Then the new head will be the value next to it. Save its value in x then free the temporary variable and return x
    printf("Your sequence is mostly likely not in order.\n");
        head = head->next;

    x = temp->value;
    free(temp);
    return x;
}

void push(stack *head, char op){        //For pushing a value to the stack, create a temporary variable to store the new value
    stack *temp, *h;
    temp=(stack*)malloc(sizeof(stack));                     
    temp->value = op;                   //Tthe temporary value will be the new head, and the previous head will be placed next to it
    if (head == NULL){
         head=temp;
         head->next=NULL;
    }else{
        temp->next=head;
        head=temp;
    }
    h = head;
    while(h!=NULL){
        printf("%c-->",h->value);
        h = h->next;
    }
    printf("\n");
}

main(){
    int i = 0;
    char op[50], a, b, c, answer;

    stack *head = NULL;

    printf("Enter the operators and operands: \n");                         //Asks for the sequence and checks if there's an invalid character
    scanf("%s", op);
    while(op[i] != 0){
        printf("%c\n", op[i]);
        if(op[i] < 48 || op[i] > 57){       
            if(op[i] != '*' && op[i] != '+' && op[i] != '-' && op[i] != '/' && op[i] != '%'){
                printf("You may have entered an invalid character.\n");
                exit(0);
            }
        }
        i++;
    }   
    i = 0;

    while(op[i] != 0){                                                              
            if(op[i] >= 48 && op[i] <= 57){
                printf("test: %c \n", op[i]);
                push (head, op[i]);
                printf("\n");
            }else if(op[i] == '*' || op[i] == '+' || op[i] == '-' || op[i] == '/' || op[i] == '%'){                 
                    push (head, op[i]);
                    if((op[i-1] >= 48 && op[i-1] <= 57) && (op[i-2] >= 48 && op[i-2] <= 57)){
                        printf("test: %c \n", op[i]);           
                        c = pop (head);
                        b = pop (head);
                        a = pop (head);
                        answer = evaluate (a, b, c);
                        printf("test: %d + %d = %d\n", a, b, answer);
                        push (head, answer);
                    }else{
                        printf("Your sequence is mostly likely not in order or is missing something.\n");
                    }   
            }
            i++;
        }
    answer = pop(head);
    printf("%d\n", answer);
}

这是一个栈的程序，其功能是使用链表在头部进行插入和删除。我的问题是它一直存在状态访问冲突，我发现我的程序一直在删除我输入的先前节点。我以前做过这个，我不知道这个是怎么回事。

void push(stack *head, char op){        //For pushing a value to the stack,       create a temporary variable to store the new value
    stack *temp, *h;
    temp=(stack*)malloc(sizeof(stack));                     
    temp->value = op;                   //Tthe temporary value will be the new head, and the previous head will be placed next to it
    h = head;
    if (h == NULL){
         h=temp;
         h->next=NULL;
    }else{
        temp->next=h;
        h=temp;
    }
    h = head;
    while(h!=NULL){
        printf("%c-->",h->value);
        h = h->next;
    }
    printf("\n");
}

这是编辑过的部分，还是不行。唯一被打印的是我添加的最后一个节点，其余的都没有了。

编辑:我不能在 90 分钟内发布另一个问题，所以我要在这里提问。在评估部分，我需要对字符变量做一个算术方程式。将它们设为整数并暂时将它们更改为 char 还是相反更好？上面的代码，在评估部分

Answer 1

您在编写 push 和 pop 时遇到了 C 的按值传递函数调用。函数内部的参数是您传入的外部变量的副本。这意味着...

char pop(stack *head) {
    //...
    head = head->next; // The original pointer is not modified !
    //...
}

... 并且由于您 free() 之前的第一个节点，main() 中的 head 指针现在悬空，并且当您尝试访问现已消失的节点时，最终会出现段错误。

要修复它，只需通过地址传递它，以便函数可以从内部更新它:

char pop(stack **head) {
    stack *oldHead = *head;

    char x = oldHead->value;
    *head = oldHead->next;

    free(oldHead);
    return x;
}

对 push() 做同样的事情并相应地修改调用点。