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。
函數功能是把檔案指針指向檔案的開頭,需要包含頭檔案stdio.h
fseek 函數名: fseek 功 能: 重定位流上的檔案指針 用 法: int fseek(FILE *stream, long offset, int fromwhere); 描 述: 函數設定檔案指針stream的位置。如果執行成功,stream将指向以fromwhere為基準,偏移offset個字 節的位置。如果執行失敗(比如offset超過檔案自身大小),則不改變stream指向的位置。 傳回值: 成功,傳回0,否則傳回其他值。 fseek position the file position pointer for the file referenced by stream to the byte location calculated by offset. 程式例:
- #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stream為檔案指針 第二個參數offset為偏移量,整數表示正向偏移,負數表示負向偏移 第三個參數origin設定從檔案的哪裡開始偏移,可能取值為:SEEK_CUR、 SEEK_END 或 SEEK_SET SEEK_SET: 檔案開頭 SEEK_CUR: 目前位置 SEEK_END: 檔案結尾 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指針移動到離檔案開頭100位元組處; fseek(fp,100L,1);把fp指針移動到離檔案目前位置100位元組處; fseek(fp,100L,2);把fp指針退回到離檔案結尾100位元組處。 使用執行個體:
- #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);
- 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);
- for (j=0; j<N; j++)
- printf("\nNo: %ld Name: %-8s Scores: ",ss[j].sno, ss[j].name);
- for (i=0; i<3; i++)