在阅读代码时,遇到了很早之前用过的fseek(),很久没有用了,有点陌生,写出来以便下次查阅。
函数功能是把文件指针指向文件的开头,需要包含头文件stdio.h
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:
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#include <stdio.h>
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long filesize(FILE *stream);
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int main(void)
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{
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FILE *stream;
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stream = fopen(
"MYFILE.TXT",
"w+");
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fprintf(stream,
"This is a test");
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printf(
"Filesize of MYFILE.TXT is %ld bytes\n", filesize(stream));
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fclose(stream);
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return
0;
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}
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long filesize(FILE *stream)
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{
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long curpos, length;
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curpos = ftell(stream);
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fseek(stream,
0L, SEEK_END);
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length = ftell(stream);
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fseek(stream, curpos, SEEK_SET);
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return length;
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}
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:
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#include <stdio.h>
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#define N 5
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typedef
struct student {
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long sno;
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char name[
10];
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float score[
3];
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} STU;
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void fun(char *filename, STU n)
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{
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FILE *fp;
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fp = fopen(filename,
"rb+");
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fseek(fp,
-1L*
sizeof(STU),SEEK_END);
-
fwrite(&n,
sizeof(STU),
1, fp);
-
fclose(fp);
-
}
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void main()
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{
-
STU t[N]={ {
10001,
"MaChao",
91,
92,
77}, {
10002,
"CaoKai",
75,
60,
88},
-
{
10003,
"LiSi",
85,
70,
78}, {
10004,
"FangFang",
90,
82,
87},
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{
10005,
"ZhangSan",
95,
80,
88}};
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STU n={
10006,
"ZhaoSi",
55,
70,
68}, ss[N];
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int i,j; FILE *fp;
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fp = fopen(
"student.dat",
"wb");
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fwrite(t,
sizeof(STU), N, fp);
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fclose(fp);
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fp = fopen(
"student.dat",
"rb");
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fread(ss,
sizeof(STU), N, fp);
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fclose(fp);
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printf(
"\nThe original data :\n\n");
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for (j=
0; j<N; j++)
-
{
-
printf(
"\nNo: %ld Name: %-8s Scores: ",ss[j].sno, ss[j].name);
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for (i=
0; i<
3; i++)
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printf(
"%6.2f ", ss[j].score[i]);
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printf(
"\n");
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}
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fun(
"student.dat", n);
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printf(
"\nThe data after modifing :\n\n");
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fp = fopen(
"student.dat",
"rb");
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fread(ss,
sizeof(STU), N, fp);
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fclose(fp);
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for (j=
0; j<N; j++)
-
{
-
printf(
"\nNo: %ld Name: %-8s Scores: ",ss[j].sno, ss[j].name);
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for (i=
0; i<
3; i++)
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printf(
"%6.2f ", ss[j].score[i]);
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printf(
"\n");
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}
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}