C程序設計打印鏈接列表，不沿列表移動並崩潰

Question

我正在嘗試打印一個鏈表，它的行為很奇怪，假設要移動到下一個項目，它將不會，例如，打印的第一個項目是“ Granny Smith”，然后其余的在結構中（當它進入時）到下一行，應該是新食物的是“蘭尼·史密斯”（Ranny Smith），始終沒有G，其他大多數細節都已損壞或亂碼。 任何幫助將不勝感激。

#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

您有三個重要的錯誤，其中之一似乎是由於復制和粘貼某些代碼而引起的，

1. 此printf()語句將導致未定義的行為

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

   因為您傳遞的是正確的地址，但指針類型錯誤，所以指針算術將導致printf()內部發生未定義的行為

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

2. 您向addPres傳遞了一個雙指針，但是您從未取消對其的引用以訪問實際的poitner，這是您要修改的，這會導致未定義的行為和觀察到的空列表的影響，因為一旦函數返回，它就不會更改原始poitner，則應使用此函數內的dereference運算符訪問double指針，例如，

    if (tip == NULL) 

   它一定要是

    if (*tip == NULL) 

   對於此函數內部的每次訪問也是如此。

   • 我怎么這么快發現所有這些問題？

     因為我在啟用編譯器警告的情況下編譯了程序，所以它開始顯示這些問題。

   • 您的代碼中還會有其他問題嗎？

     可能是的，但是您首先需要解決這些問題。

3. 您fflush(stdin) ，這是一個非常常見的錯誤，似乎來自某些教程，書籍或在線資源，該標准明確指出fflush()用於輸出流，而行為未定義用於輸入流，以下內容摘自C11草案

   7.21.5.2 fflush功能

   2. 如果流指向未輸入最新操作的輸出流或更新流，則fflush函數會使該流的所有未寫入數據都被傳遞到主機環境中，並寫入文件中； 否則，行為是不確定的。