fseek函数用法_memcpy函数用法

fseek函数用法_memcpy函数用法转载请注明出处:https://blog.csdn.net/wl_soft50/article/details/7787521每天进步一点点–>函数fseek()用法在阅读代码时,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.hfseek函数名:fseek功能:重定位流上的文件...

转载请注明出处:https://blog.csdn.net/wl_soft50/article/details/7787521

每天进步一点点–>函数fseek() 用法

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:

#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;  
  }  
只听到从架构师办公室传来架构君的声音:
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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。(根据评论来看,应该是 fseek(fp,-100L,2) )
使用实例:

此代码由Java架构师必看网-架构君整理
#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++) [cpp] view plain copy 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++) [cpp] view plain copy printf("%6.2f ", ss[j].score[i]); printf("\n"); }
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