C编程打印链表，不沿列表移动并崩溃

Question

我正在尝试打印一个链表，它的行为很奇怪，当它应该移动到下一个项目时它不会，例如打印的第一个项目是“Granny Smith”，然后是结构中的其余部分，当它转到下一行时，应该是一种新食物的是“ranny Smith”总是没有 G，并且大多数其他细节都已损坏或乱码。任何帮助将不胜感激。

#define _CRT_SECURE_NO_WARNINGS

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

#define cls system("cls");
#define flush fflush(stdin)
#define pause system("pause");


struct food  // Declaration of struct
{
    char produce[20];
    char type[20];
    char soldBy[20];
    float price;
    int quantityInStock;
    struct food *next;
};


void addPres(struct food** tip, char tokenPro[20], char tokenType[20], char tokenSold[20], float tokenFloat, int tokenInt);
void display ();
void displayMenu ();
int getChoice ();
void readDataIn();




int main()
{
    int choice = 0;
    int ;
    struct food first;
    char *saveFloat, *saveInt;
    char tokenPro[20], tokenType[20], tokenSold[20], savePro[20], saveType[20], saveSold[20], j[1900];
    float tokenFloat, messFloat;
    int danint, tokenInt;
    struct food* a = NULL;
    FILE* file;

    a = NULL; // sets the first item on the list to empty

//readDataIn();
    do
    {
        choice = getChoice();//function to present choices and receive an answer
        switch (choice)//switch routes the choice
        {
        case 1: //to enter a new node on the list
            cls;
            printf("Enter the produce type: ");
            fgets(tokenPro, 20, stdin);
            printf("Enter the type: ");
            fgets(tokenType, 20, stdin);
            printf("Enter the units of measurement: ");
            fgets(tokenSold, 20, stdin);
            printf("\nEnter the price per %s: ", &tokenSold);
            scanf("%f", &tokenFloat );
            printf("\nEnter the quantity in stock: ");
            scanf("%i", &tokenInt );
            //pause;
            addPres(&a, tokenPro, tokenType, tokenSold, tokenFloat, tokenInt);
            break;
        case 2: //to display the linked list
            display(a);

            break;
        case 3: //to reverse the order

            if((file=fopen("AssignmentTwoInput.txt", "r")) != NULL)  //opens files
            {
                while(fgets(j, sizeof(j), file))  //while information is being sent to j from the file
                {
                    strcpy(savePro, strtok(j, ","));
                    strcpy(saveType, strtok(NULL, ","));
                    strcpy(saveSold, strtok(NULL, ","));
                    saveFloat = strtok(NULL, ",");
                    saveInt = strtok(NULL, "\n");
                    messFloat = atof(saveFloat); //ID number is assigned to token2
                    danint = atoi(saveInt); //converts  the value token2 points to into an int (previously read in as an string)
                    addPres(&a, savePro, saveType, saveSold, messFloat, danint); //sends the name and ID to the function to add a person
//printf("\n %s \t %s \t %s \t %f \t %i", savePro, saveType, saveSold, messFloat, danint);
//pause;
                }//end while loop
            }//end if loop
            printf("\nSuccessfully loaded input file\n\n\n"); //message to tell user the back was found and imported successfully
            pause;
            fclose(file);
            break;
        case 4: //to export data

            break;
        case 5: //option to exit

            exit(0);
            break;
        default://error checking
            printf("Please enter a valid choice.\n");
            break;
        }//switch loop close

    }
    while (choice != 5);   //end menu choices


    return 0;
}//end main


void addPres(struct food** tip, char tokenPro[20], char tokenType[20], char tokenSold[20], float tokenFloat, int tokenInt)
{


    struct food* temp =

        (struct food*) malloc(sizeof(struct food));
    strcpy(temp->produce, tokenPro);
    strcpy(temp->type, tokenType);
    strcpy(temp->soldBy, tokenSold);
    temp->price = tokenFloat;
    temp->quantityInStock = tokenInt;

//printf("\n %s \t %s \t %s \t %f \t %i", temp->produce, temp->type, temp->soldBy, temp->price, temp->quantityInStock);
//pause;
    if (tip == NULL) //if the tip node is empty
    {
        tip = temp; //tip node becomes this node
        (*tip)->next = NULL; // since there are no others yet the next node will be empty
    }


    else
    {
        temp->next = tip; // reassigns the tip node to the next one
        (*tip) = temp; //inserts current node as the tip
    }
}

void display(struct food* c)
{
//c = first; //assigns c to the first node on the linked list
    int count = 1;
    struct food *temp;
    temp = c;
    cls;
    printf("==========================================================================\n"); //prints a header to say which info is which
    printf(" Item #   Produce       Type             Sold By         Price    In Stock\n");
    printf("==========================================================================\n");
    while( c != NULL)
    {
        printf("%3i \t", count);
        printf("%s \t", c->produce ); //prints name
        printf("%s \t", c->type ); //prints name
        printf("%s \t", c->soldBy); //prints name
        printf("%-3.2f \t", c->price); //prints ID
        printf("%i\n", c->quantityInStock); //prints ID
        pause;
        count = count + 1;
        c = c->next; //advances to next item in the linked list

        if(c == NULL)
        {
            printf("\n end \n"); //once end is found user is told

            pause;
        }
    }
    printf("\n done \n");
    pause;
}//end display

Answer 1

你犯了三个重要的错误，其中一个好像是复制粘贴了一些代码导致的，

1. 这个printf()语句会导致未定义的行为

   printf("\nEnter the price per %s: ", &tokenSold);
   

   因为您传递的是正确的地址，但指针类型是错误的，所以指针运算将导致 printf() 内部发生未定义的行为

   printf("\nEnter the price per %s: ", tokenSold);
   /* remove the & --------------------^ here */
   

2. 您向 addPres 传递了一个双指针，但您从未取消对它的引用以访问实际的指针，这就是您要修改的内容，它会导致未定义的行为和观察到的效果一个空列表，因为一旦函数返回它并没有改变原来的指针，你应该在这个函数中使用解引用运算符访问双指针，例如而不是

   if (tip == NULL)
   

   必须是

   if (*tip == NULL)
   

   这同样适用于此函数内的每次访问。

   • 我怎么这么快就发现了所有这些问题？

     Because I compiled your programs with compiler warnings enabled and it started showing those problems.

   • 您的代码中还会有其他问题吗？

     Yes probably, but you need to fix these first.

3. 你 fflush(stdin)，一个很常见的错误，似乎来自一些教程或书籍或在线资源，标准明确指出 fflush() 用于输出流，输入流的行为未定义，以下摘自C11草案

   7.21.5.2 The fflush function

   2. If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.