#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define XS 5

struct Student
{
    int xuehao;
    char xingming[10];
    int yuwen;
    int shuxue;
    int yingyu;
    int pingjun;
}xuesheng[XS];
void paixu(struct Student a[], int n)
{
    int i, j;
    for(i = 0; i < XS - 1; i++)
    {
        for(j = i + 1; j < XS; j++)
        {
            if(a[i].pingjun > a[j].pingjun)
            {
                struct Student b;
                b.xuehao = a[i].xuehao;
                strcpy(b.xingming, a[i].xingming);
                b.yuwen = a[i].yuwen;
                b.shuxue = a[i].shuxue;
                b.yingyu = a[i].yingyu;
                b.pingjun = a[i].pingjun;
                a[i].xuehao = a[j].xuehao;
                strcpy(a[i].xingming, a[j].xingming);
                a[i].yuwen = a[j].yuwen;
                a[i].shuxue = a[j].shuxue;
                a[i].yingyu = a[j].yingyu;
                a[i].pingjun = a[j].pingjun;
                a[j].xuehao = b.xuehao;
                strcpy(a[j].xingming, b.xingming);
                a[j].yuwen = b.yuwen;
                a[j].shuxue = b.shuxue;
                a[j].yingyu = b.yingyu;
                a[j].pingjun = b.pingjun;
            }
        }
    }
}

void ceshi(void)   //用这个函数测试读取文件失败,但是我重新写了一个cpp文件测试却是成功的.高手来解答下.
{
    struct Student
    {
        int xuehao;
        char xingming[10];
        int yuwen;
        int shuxue;
        int yingyu;
        int pingjun;
    }xue[XS];
    FILE *fp;
    if((fp = fopen("stu_sort.dat","rb")) == NULL)
    {
        printf("打开文件失败.\n");
        exit(0);
    }
    for(int i = 0; i < XS; i++)
    {
        if(fread(&xue[i],sizeof(xue[i]), 1, fp) != 1)
        {
            printf("xue[%d]提取数据失败.\n",i);
        }
    }
    for(int i = 0; i < XS; i++)
        printf("学号:%d, 姓名:%-6s, 语文成绩:%d, 数学成绩:%d, 英语成绩:%d, 平均分:%d.\n",xue[i].xuehao,xue[i].xingming,xue[i].yuwen,xue[i].shuxue,xue[i].yingyu,xue[i].pingjun);
    fclose(fp);
}

int main()
{
    FILE *fp;
    if((fp = fopen("stud.dat", "rb")) == NULL)
    {
        printf("打开文件失败.\n");
        exit(0);
    }
    for(int i = 0; i < XS; i++)
    {
        if(fread(&xuesheng[i], sizeof(xuesheng[i]), 1, fp) != 1)
        {
            printf("读取文件数据失败.\n");
        }
    }
    for(int i = 0; i < XS; i++)
        printf("学号:%d, 姓名:%-6s, 语文成绩:%d, 数学成绩:%d, 英语成绩:%d, 平均分:%d.\n",xuesheng[i].xuehao,xuesheng[i].xingming,xuesheng[i].yuwen,xuesheng[i].shuxue,xuesheng[i].yingyu,xuesheng[i].pingjun);

    paixu(xuesheng, XS);

    fclose(fp);

    if((fp = fopen("stu_sort.dat", "wb")) == NULL)
    {
        printf("打开文件失败.\n");
        exit(0);
    }

    for(int i = 0; i < XS; i++)
    {
        if(fwrite(&xuesheng[i], sizeof(xuesheng[i]), 1, fp) == 1)
        {
            printf("xuesheng[%d]数据拷贝成功.\n",i);
        }

    }
    ceshi();
    fclose(fp);
    system("pause");
    return 0;
}

在main函数里，你第二次使用fopen()打开文件还没使用fclose()对其关闭，接着又在ceshi函数里对其进行打开操作。没关闭又怎么打开？这个程序请注意main函数和ceshi函数的fp是两个不同的文件指针。他们的作用域不同

额。。。你写的是c，不是c++.为何用cpp格式？
main函数里，打开了这个stu_sort.dat文件。这时，这个文件已经被锁。操作完没关闭。接着你又调用cesh自定义函数。。又打开这个文件操作。。这里肯定操作失败。因为上个操作的文件关闭位置被你写在后面了。

精简一下看看：
void ceshi(void)
{
    //FILE *fp;
    //if((fp = fopen("stu_sort.dat","rb")) == NULL) //stu_sort.dat已经在用

        if(fread(&xue[i],sizeof(xue[i]), 1, fp) != 1)

    //fclose(fp);
}

int main()
{
    FILE *fp;
    if((fp = fopen("stud.dat", "rb")) == NULL)

        if(fread(&xuesheng[i], sizeof(xuesheng[i]), 1, fp) != 1)

    fclose(fp);

    if((fp = fopen("stu_sort.dat", "wb")) == NULL)

        if(fwrite(&xuesheng[i], sizeof(xuesheng[i]), 1, fp) == 1)

    ceshi();

    fclose(fp);
   
}