C语言 fseek(f,0,SEEK_SET)函数案例详解

来源：jb51　　时间：2021/8/26 12:38:13　　对本文有异议

fseek(f,0,SEEK_SET);

意思是把文件指针指向文件的开头

fseek

函数名: fseek

功能: 重定位流上的文件指针

用法: int fseek(FILE *stream, long offset, int fromwhere);

描述: 函数设置文件指针stream的位置。如果执行成功，stream将指向以fromwhere为基准，偏移offset个字节的位置。如果执行失败(比如offset超过文件自身大小)，则不改变stream指向的位置。

返回值: 成功，返回0，否则返回其他值。

fseek position the file position pointer for the file referenced by stream to the byte location calculated by offset.

程序例:

#include <stdio.h>  
long filesize(FILE *stream);  
int main(void)  
{
  FILE *stream;
  stream = fopen("MYFILE.TXT", "w+");
  fprintf(stream, "This is a test");
  printf("Filesize of MYFILE.TXT is %ld bytes\n", filesize(stream));
  fclose(stream);  return 0;  }
  long filesize(FILE *stream)
  {
	  long curpos, length;
	  curpos = ftell(stream);
	  fseek(stream, 0L, SEEK_END);
 	  length = ftell(stream);
	  fseek(stream, curpos, SEEK_SET);
  	return length;
  }
  int fseek( FILE *stream, long offset, int origin );

第一个参数stream为文件指针

第二个参数offset为偏移量，整数表示正向偏移，负数表示负向偏移

第三个参数origin设定从文件的哪里开始偏移,可能取值为：SEEK_CUR、 SEEK_END 或 SEEK_SET

SEEK_SET：文件开头

SEEK_CUR：当前位置

SEEK_END：文件结尾

其中SEEK_SET,SEEK_CUR和SEEK_END和依次为0，1和2.

简言之：

fseek(fp,100L,0);把fp指针移动到离文件开头100字节处；
fseek(fp,100L,1);把fp指针移动到离文件当前位置100字节处；
fseek(fp,100L,2);把fp指针退回到离文件结尾100字节处。

使用实例：

#include<stdio.h>
#defineN5
typedefstructstudent{
longsno;
charname[10];
floatscore[3];
}STU;
 
voidfun(char*filename,STUn)
{
FILE*fp;
fp=fopen(filename,"rb+");
fseek(fp,-1L*sizeof(STU),SEEK_END);
fwrite(&n,sizeof(STU),1,fp);
fclose(fp);
}
 
voidmain()/*修改覆盖最后一个学生数据*/
{
STUt[N]={{10001,"MaChao",91,92,77},{10002,"CaoKai",75,60,88},
{10003,"LiSi",85,70,78},{10004,"FangFang",90,82,87},
{10005,"ZhangSan",95,80,88}};
STUn={10006,"ZhaoSi",55,70,68},ss[N];
inti,j;FILE*fp;
fp=fopen("student.dat","wb");
fwrite(t,sizeof(STU),N,fp);
fclose(fp);
fp=fopen("student.dat","rb");
fread(ss,sizeof(STU),N,fp);
fclose(fp);
printf("\nTheoriginaldata:\n\n");
for(j=0;j<N;j++)
{
printf("\nNo:%ldName:%-8sScores:",ss[j].sno,ss[j].name);
for(i=0;i<3;i++)printf("%6.2f",ss[j].score[i]);
printf("\n");
}
fun("student.dat",n);
printf("\nThedataaftermodifing:\n\n");
fp=fopen("student.dat","rb");
fread(ss,sizeof(STU),N,fp);
fclose(fp);
for(j=0;j<N;j++)
{
printf("\nNo:%ldName:%-8sScores:",ss[j].sno,ss[j].name);
for(i=0;i<3;i++)printf("%6.2f",ss[j].score[i]);
printf("\n");
}
}