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文字

在头文件<stdio.h>中定义

int fseek(FILE * stream,long offset,int origin);

将文件流的文件位置指示符设置为stream指向的值offset

如果stream以二进制模式打开,则新位置是offset从文件开始处测量的字节(如果origin是)SEEK_SET,从当前文件位置(如果origin是)SEEK_CUR,以及从文件结尾(如果origin是)开始SEEK_END。二进制流不需要支持SEEK_END,特别是如果输出额外的空字节。

如果stream是在文本模式下打开,对于仅支持值offset为零(这与任何工作origin),并通过先前调用返回的值ftell与同一文件(仅与工作相关的物流originSEEK_SET)。

如果stream是面向广域的,则应用文本流和二进制流的限制(ftellSEEK_SET允许使用结果,SEEK_SET和SEEK_CUR允许使用零偏移量,但不允许SEEK_END)。

除了更改文件位置指示符之外,还可以fseek取消ungetc文件结束状态的效果并清除文件结束状态(如果适用)。

如果发生读取或写入错误,则会ferror设置stream()的错误指示符并且文件位置不受影响。

参数

-

文件流进行修改

抵消

-

将位置相对于原点移动的字符数

起源

-

添加了偏移量的位置。它可以具有以下值之一:SEEK_SET,SEEK_CUR,SEEK_END

返回值

0 一旦成功,否则非零值。

注意

在宽流中寻找非终点位置之后,对任何输出函数的下一次调用可能会使文件的其余部分不确定,例如通过输出不同长度的多字节序列。

对于文本流,唯一有效的值offset0(适用于任何origin)和先前调用返回的值ftell(仅适用于SEEK_SET)。

POSIX允许在现有的文件结尾之外寻找。如果在此查找后执行输出,则从间隙读取的数据将返回零字节。在文件系统支持的地方,这将创建一个稀疏文件

fflush如果有任何未写入的数据(但是移位状态是否恢复为实现定义),POSIX还要求fseek首先执行。

带有错误检查的fseek。

#include <stdio.h>#include <stdlib.h>
 int main(void){    /* Prepare an array of f-p values. */
    #define SIZE 5
    double A[SIZE] = {1.,2.,3.,4.,5.};    /* Write array to a file. */
    FILE * fp = fopen("test.bin", "wb");    fwrite(A,sizeof(double),SIZE,fp);    fclose (fp); 
    /* Read the f-p values into array B. */
    double B[SIZE];
    fp = fopen("test.bin","rb"); 
    /* Set the file position indicator in front of third f-p value. */    if (fseek(fp,sizeof(double)*2L,SEEK_SET) != 0)    {       if (ferror(fp))       {          perror("fseek()");          fprintf(stderr,"fseek() failed in file %s at line # %d\n", __FILE__,__LINE__-5);          exit(EXIT_FAILURE);       }    }
 
    int ret_code = fread(B,sizeof(double),1,fp);   /* read one f-p value  */    printf("%.1f\n", B[0]);                        /* print one f-p value */ 
    fclose(fp);    return EXIT_SUCCESS;}

输出:

3.0

参考

  • C11标准(ISO / IEC 9899:2011):

    • 7.21.9.2 fseek函数(p:336-337)

  • C99标准(ISO / IEC 9899:1999):

    • 7.19.9.2 fseek函数(p:302-303)

  • C89 / C90标准(ISO / IEC 9899:1990):

    • 4.9.9.2 fseek函数

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