9.6 文件IO二进制读写

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fseek(文件指针，偏移量，基准);  //偏移量的单位是字节

函数执行成功返回0，执行失败返回一个非0值

注意下面的特殊现象：

返回值是0      fseek(p,2,SEEK_END):  到终点了往后跳是成功的

返回值是-1      fseek(p,-2,SEEK_SET);在起始点往前面跳非法

windows的 b 模式 不影响 \r\n的偏移，只会按真实存在的字节偏移

ftell(p) 返回当前的文件指针位置

#include <stdio.h>
int main()
{
        FILE *fp = fopen("a.txt","r+");
        if(!fp)
        {
                printf("打开文件失败\n");
                return -1;
        }
        int num = fseek(fp,2,SEEK_SET);
        printf("fseek = %d\n",num);
        printf("ftell = %ld\n",ftell(fp));
        char buf[1024];
        while(fgets(buf,sizeof(buf),fp) != NULL)
        {
                printf("%s\n",buf);
        }
        return 0;
}

feof的一个特殊注意点：

#include <stdio.h>
int main(void)
{
        FILE *fp = fopen("a.txt","r");
        while(!feof(fp))
        {
                char tmp = 0;
                fread(&tmp,sizeof(tmp),1,fp);
                printf("%x\n",tmp);
        }
        fclose(fp);
        return 0;
}

abc\n  后面多出一个0  是因为feof到了文件末尾并不返回1，只有越过了文件末尾才返回1。
即以上的fread多读了一次，多输出了一个0。
注意：在linux上统计行数也会多出一行。
解决方案：能不用feof就不用feof
#include <stdio.h>
int main(void)
{
        FILE *fp = fopen("a.txt","r");
        char tmp = 0;
        while(fread(&tmp,sizeof(tmp),1,fp) > 0)
        {
                printf("%x\n",tmp);
        }
        fclose(fp);
        return 0;
}

fread  fwrite(重点):

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);

fread()会返回实际读入的nmemb数目。
fwrite()会返回实际写入的nmemb数目。

fread 和 fwrite拷贝一个二进制文件

这样拷贝是不行的，b.pdf永远都会是1024的倍数

#include <stdio.h>
int main(void)
{
        FILE *fp1 = fopen("a.pdf","r");
        FILE *fp2 = fopen("b.pdf","w");
        char buf[1024];
        while(fread(buf,sizeof(buf),1,fp1) > 0)
        {
                fwrite(buf,sizeof(buf),1,fp2);
        }
        fclose(fp1);
        fclose(fp2);
        return 0;
}
解决方案：
#include <stdio.h>
int main(void)
{
        FILE *fp1 = fopen("a.pdf","r");
        FILE *fp2 = fopen("b.pdf","w");
        char buf[1024];
        int len = 0;
        while((len = fread(buf,1,sizeof(buf),fp1)) > 0)   //fread返回实际读入的第二个参数的数量
        {
                fwrite(buf,1,len,fp2);                     //我读了多少个字节，就写多少个字节
        }
        fclose(fp1);
        fclose(fp2);
        return 0;
}

对二进制文件进行修改，检索，插入等操作

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct student
{
        int ID;
        char name[20];
};
void init_student(char *filename)
{
        struct student st[10] = {
                {1,"刘德华"},
                {2,"周杰伦"},
                {3,"成龙"},
                {4,"范冰冰"},
                {5,"柳岩"},
                {6,"杨幂"},
                {7,"林志玲"},
                {8,"韩寒"},
                {9,"普京"},
                {10,"赵本山"},
        };
        FILE *fp = fopen(filename,"w");
        if(!fp)
        {
                printf("打开文件失败\n");
                return;
        }
        fwrite(st,sizeof(struct student),10,fp);
        fclose(fp);
}
void show_student(char *filename)          //循环输出所有学员信息
{
        FILE * fp = fopen(filename,"r");
        if(!fp)
        {
                printf("打开文件失败\n");
                return;
        }
        struct student st;
        memset(&st,0,sizeof(st));
        while(fread(&st,sizeof(st),1,fp) > 0)
        {
                printf("%d:%s\n",st.ID,st.name);
        }
        fclose(fp);
        printf("***************\n");
}
void update_student(char * filename ,int n) //对二进制文件进行修改
{
        FILE * fp = fopen(filename,"r+");
        if(!fp)
        {
                printf("打开文件失败\n");
                return;
        }
        fseek(fp,(n-1)*sizeof(struct student),SEEK_SET);
        struct student st = {100,"刘威"};
        fwrite(&st,sizeof(st),1,fp);
        fclose(fp);
}
void find_student(char *filename)           //对二进制文件进行检索
{
        FILE * fp = fopen(filename,"r");
        if(!fp)
        {
                printf("打开文件失败\n");
                return;
        }
        int n = 0;
        printf("你输入你需要查找的学生编号：");
        scanf("%d",&n);
        fseek(fp,(n-1)*sizeof(struct student),SEEK_SET);
        struct student st;
        memset(&st,0,sizeof(st));
        fread(&st,sizeof(st),1,fp);
        printf("%d:%s\n",st.ID,st.name);
        fclose(fp);
}
void add_student(char *filename , int n)     //对二进制文件进行插入
{
        FILE * fp = fopen(filename,"r");
        if(!fp)
        {
                printf("打开文件失败\n");
                return;
        }
        struct student *p = calloc(100,sizeof(struct student));
        int index = 0,i = 0;
        while(fread(&p[index],sizeof(struct student),1,fp) > 0)
                index++;
        for(i = index ; i >= n ; i--)
                p[i+1] = p[i];
        p[n].ID = 88;
        strcpy(p[n].name,"习jin平");
        fclose(fp);
        fp = fopen(filename,"w");
        if(!fp)
        {
                printf("打开文件失败\n");
                return;
        }
        fwrite(p,sizeof(struct student),index+1,fp);
        fclose(fp);
}
int main(void)
{
        init_student("a.bin");
        show_student("a.bin");
        //update_student("a.bin",4);
        add_student("a.bin",5);
        show_student("a.bin");
        //find_student("a.bin");
        return 0;
}


把本班的同学读入到结构体数组里，加上序号，并且排序。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
struct student
{
        int ID;
        char name[20];
};
void show_student()
{
        FILE *fp = fopen("student.bin","r");
        while(!fp)
        {
                printf("打开文件失败\n");
                return;
        }
        struct student st;
        memset(&st,0,sizeof(st));
        while(fread(&st,sizeof(st),1,fp) > 0)
        {
                printf("%d:%s\n",st.ID,st.name);
        }
}
void sort_student()
{
        FILE *fp = fopen("student.bin","r");
        while(!fp)
        {
                printf("打开文件失败\n");
                return;
        }
        struct student st[50];
        int index = 0;
        while(fread(&st[index],sizeof(struct student),1,fp) > 0)
                index++;
        int i,j,min;
        struct student tmp;
        for(i = 0 ; i < index ;i++)
        {
                min = i;
                for(j = i+1; j < index ; j++)
                        if(st[min].ID > st[j].ID)
                                min = j;
                if(min != i)
                {
                        tmp = st[min];
                        st[min] = st[i];
                        st[i] = tmp;
                }
        }
        fclose(fp);
        fp = fopen("student.bin","w");
        for(i = 0 ; i < index; i++)
                fwrite(&st[i],sizeof(struct student),1,fp);
        fclose(fp);
}
int main(void)
{
        srand((unsigned)time(NULL));
        FILE *fp1 = fopen("student.txt","r");
        FILE *fp2 = fopen("student.bin","w");
        while(!fp1 || !fp2)
        {
                printf("打开文件失败\n");
                return -1;
        }
        char buf[1024];
        struct student st;
        memset(&st,0,sizeof(st));
        int arr[42] = {0};
        int tmp;
        while(fgets(buf,sizeof(buf),fp1) != NULL)
        {
                if(buf[strlen(buf)-1] == ‘\n‘)
                        buf[strlen(buf)-1] = ‘\0‘;
                tmp = rand()%42;
                while(arr[tmp] == 1)
                        tmp = rand()%42;
                arr[tmp] = 1;
                st.ID = tmp;
                strcpy(st.name,buf);
                fwrite(&st,sizeof(st),1,fp2);
        }
        fclose(fp1);
        fclose(fp2);
        sort_student();
        show_student();
        return 0;
}

9.6 文件IO二进制读写

标签：style   blog   http   color   io   os   ar   for   文件   

原文地址：http://www.cnblogs.com/l6241425/p/3967484.html