Question

Hello程序员注意到我看到了这段代码，但这让我感到困惑fseek(fp, sizeof(e) * (id - 1), SEEK_SET)我不确定sizeof(e)*(id-1)正在做什么？

//program to read a specified employee's record from a random access
//file. The file was previously created and initialized to hold a maximum of 5000 records
//and some data was later stored in the file.
#include <stdio.h>

//Declare Employee structure
struct Employee
{
    int IdNo;
    char FName[20];
    char LName[20];
    float Pay;
};

typedef struct Employee EMP;

void main ()
{
    int id;
    FILE *fp;
    EMP e = {0, "", "", 0.0};
    fp = fopen("Employee.dat", "r+b");
    if (fp != NULL)
    {
        printf("\nEnter employee's id number (1-5000)");
        scanf("%d", &id);
        //locate the record, read it in, then close the file
        fseek(fp, sizeof(e) * (id - 1), SEEK_SET);
        fread(&e, sizeof(e), 1, fp);
        fclose(fp);
        if (e.IdNo != 0)
        {
            printf("Employee's record successfully retrieved from the file\n");
            printf ("Id: %d\n", e.IdNo);
            printf ("First Name: %s\n", e.FName);
            printf ("Last Name: %s\n", e.LName);
            printf ("Pay: %f\n", e.Pay);
        }
        else
            printf("Employee record retrieved from file is empty\n");
    }
    else
        printf("Error - could not open random access file\n");
}

Answer 1

fseek将文件位置移动到新位置。然后，当您从该流中读取时，它将检索该位置的内容。

在您的情况下，位置从头开始为sizeof(e) * (id - 1)（SEEK_SET）。这意味着您的文件中的id记录（sizeof(e)）的位置

第一条记录位于sizeof(e) * (1 - 1)，即0
第二条记录位于sizeof(e) * (2 - 1)，即第一条记录sizeof(e)字节之后
...

因此，当您fseek使用id=2时，它将位于第二条记录，然后将该记录读入名为e的变量。

更新

解决您问题的评论。如果您想要浏览多个记录，可以按记录进行记录

fseek(fp, sizeof(e) * (id - 1), SEEK_SET);
fread(&e, sizeof(e), 1, fp);
// do something with record e
// seek and read further records ...

或一次读取多个记录到结构数组

EMP emps[10];
// ...
fseek(fp, sizeof(emps[0]) * (id - 1), SEEK_SET);
fread(emps, sizeof(emps[0]), 10, fp);
// do something with records in emps

或者你可以在没有干预的情况下阅读一个接一个的记录

fread(&e, sizeof(e), 1, fp);
// do something with first record
fread(&e, sizeof(e), 1, fp);
// do something with second record
// ...

从c中的随机访问文件中读取

1 个答案: