轉(zhuǎn)載請注明出處：https://blog.csdn.net/wl_soft50/article/details/7787521
每天進步一點點-->函數(shù)fseek() 用法
在閱讀代碼時，遇到了很早之前用過的fseek(),很久沒有用了，有點陌生，寫出來以便下次查閱。
函數(shù)功能是把文件指針指向文件的開頭，需要包含頭文件stdio.h
fseek
函數(shù)名: fseek
功 能: 重定位流上的文件指針
用 法: int fseek(FILE *stream, long offset, int fromwhere);
描 述: 函數(shù)設(shè)置文件指針stream的位置。如果執(zhí)行成功，stream將指向以fromwhere為基準，偏移offset個字 節(jié)的位置。如果執(zhí)行失敗(比如offset超過文件自身大小)，則不改變stream指向的位置。
返回值: 成功，返回0，否則返回其他值。
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 );
第一個參數(shù)stream為文件指針
第二個參數(shù)offset為偏移量，整數(shù)表示正向偏移，負數(shù)表示負向偏移
第三個參數(shù)origin設(shè)定從文件的哪里開始偏移,可能取值為：SEEK_CUR、 SEEK_END 或 SEEK_SET
SEEK_SET： 文件開頭
SEEK_CUR： 當前位置
SEEK_END： 文件結(jié)尾
其中SEEK_SET,SEEK_CUR和SEEK_END和依次為0，1和2.
簡言之：
fseek(fp,100L,0);把fp指針移動到離文件開頭100字節(jié)處；
fseek(fp,100L,1);把fp指針移動到離文件當前位置100字節(jié)處；
fseek(fp,100L,2);把fp指針退回到離文件結(jié)尾100字節(jié)處。（根據(jù)評論來看，應該是 fseek(fp,-100L,2) )
使用實例：
#include <stdio.h>
#define N 5
typedef struct student {
long sno;
char name[10];
float score[3];
} STU;
void fun(char *filename, STU n)
{
FILE *fp;
fp = fopen(filename, "rb+");
fseek(fp, -1L*sizeof(STU),SEEK_END);
fwrite(&n, sizeof(STU), 1, fp);
fclose(fp);
}
void main()
{
STU t[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}};
STU n={10006,"ZhaoSi", 55, 70, 68}, ss[N];
int i,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("\nThe original data :\n\n");
for (j=0; j<N; j++)
{
printf("\nNo: %ld Name: %-8s Scores: ",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("\nThe data after modifing :\n\n");
fp = fopen("student.dat", "rb");
fread(ss, sizeof(STU), N, fp);
fclose(fp);
for (j=0; j<N; j++)
{
printf("\nNo: %ld Name: %-8s Scores: ",ss[j].sno, ss[j].name);
for (i=0; i<3; i++)
printf("%6.2f ", ss[j].score[i]);
printf("\n");
}
