c - 访问成员结构时出现无效数字

Question

我是c语言新手。我将结构成员 (int dd) 添加到文件 (file.txt)，现在我试图读取该文件以访问结构成员 (Day.dd)。但是当我通过 scanf() 函数将 5 添加到一个变量并编译该文件并运行时，我没有得到 (Day = 5)。 我得到 Day = 1325715504

My code is:
#include <stdio.h>
#include <strings.h>
#include <stdlib.h>
struct DailyCost
{
    int dd;

} Day;
FILE *fp;
void add_dailycost();
void display();
void mainmenue();
int main()
{

    int j;
    fp = fopen("/home/mohit/projects/c/gedit_Dailyost/TextFiles/fileb.txt", "w+");
    rewind(fp);
    mainmenue();
    return 0;
}
void mainmenue()
{
    struct DailyCost day1;
    int j;
    printf("Enter 1 to add dailycost and enter 2 display dailycost.");
    scanf("%d", &j);
    switch (j)
    {
    case 1:
        add_dailycost(day1);
        break;
    case 2:
        display(day1);
        break;
    }
}
void add_dailycost(struct DailyCost Day)
{
    if (fp != NULL)
    {
        fseek(fp, 0L, SEEK_END);
    }
    int j;
    printf("Enter Day = ");
    scanf("%d", &Day.dd);
    fwrite(&Day, sizeof(struct DailyCost), 1, fp);
    printf("If menmenue press 1 and exit press 0 = ");
    scanf("%d", &j);
    if (j == 1)
    {
        mainmenue();
    }
    else
    {
        printf("\nThank you");
        fclose(fp);
        exit(0);
    }
}
void display(struct DailyCost Day)
{
    fread(&Day, sizeof(struct DailyCost), 1, fp);
    rewind(fp);
    printf("Day =   %d\n", Day.dd);
    fclose(fp);
}

请指出我在编写程序时做错了什么。以及如何通过使用我的代码来解决这个问题。

Answer 1

根据 Klaus Gütter 的建议使用引用调用，对原始对象进行更改。

#include <stdio.h>
#include <strings.h>
#include <stdlib.h>
struct DailyCost
{
    int dd;

} Day;
FILE *fp;
void add_dailycost();
void display();
void mainmenue();
int main()
{

    int j;
    fp = fopen("/home/mohit/projects/c/gedit_Dailyost/TextFiles/fileb.txt", "w+");
    rewind(fp);
    mainmenue();
    return 0;
}
void mainmenue()
{
    struct DailyCost day1;
    int j;
    printf("Enter 1 to add dailycost and enter 2 display dailycost.");
    scanf("%d", &j);
    switch (j)
    {
    case 1:
        add_dailycost(&day1);
        break;
    case 2:
        display(&day1);
        break;
    }
}
void add_dailycost(struct DailyCost* Day) /*if you pass simply 
  struct DailyCost , all the contents of original objects will be copied 
  into a new memory area, to be used in this function and will be 
  destroyed when this function exits. */
/* if you pass just the address of your object,you are working on 
   the same memory area as you passed from,in this case from mainmenue. */
{
    if (fp != NULL)
    {
        fseek(fp, 0L, SEEK_END);
    }
    int j;
    printf("Enter Day = ");
    scanf("%d", &Day->dd);
    fwrite(Day, sizeof(struct DailyCost), 1, fp);
    printf("If menmenue press 1 and exit press 0 = ");
    scanf("%d", &j);
    if (j == 1)
    {
        mainmenue();
    }
    else
    {
        printf("\nThank you");
        fclose(fp);
        exit(0);
    }
}
void display(struct DailyCost* Day) /* passing the object instead 
  of reference would also work here. But just for the display 
  purpose, you need not to unnecessarily create a new memory. */
{
    fread(Day, sizeof(struct DailyCost), 1, fp);
    rewind(fp);
    printf("Day =   %d\n", Day->dd);
    fclose(fp);
}

如果您还不清楚，请阅读互联网上的按值调用和按引用调用，以及将结构作为参数传递。