c - fseek() 跨越文件限制并且不终止

Question

在最后一个 while 循环中，应打印文件中存储的每 5 个字符，但循环无限期地进行并且不会终止。feof() 函数应该在到达 END OF FILE 时返回 1，并且应该退出循环，但循环会无限期地进行。

#include<stdio.h>

main() {
  long n, k;
  char c;
  FILE *fp;
  fp = fopen("RANDOM", "w");

  while ((c = getchar()) != EOF) {
    putc(c, fp);
  }

  n = ftell(fp);
  printf("\nNo. of characters entered by the user is : %ld\n", n);
  fclose(fp);
  fp = fopen("RANDOM", "r");

  while(feof(fp) == 0) {
    n = ftell(fp);
    c = getc(fp);
    printf("The character at %ld position is %c\n", n, c);
    fseek(fp, 4l, 1);
  }
}

Answer 1

来自man fseek() :

A successful call to the fseek() function clears the end-of-file indicator for the stream.

即使您将位置指示器设置在 EOF 后面，fseek() 也会成功。因此很明显，当 fseek() 是循环中的最后一个命令时，while (feof(fp) == 0) 将永远不会终止。

相反，你可以这样做:

for( ;; ) {
    n = ftell(fp);
    if( (c = getc(fp)) == EOF )
         break;
    printf("The character at %ld position is %c\n", n, c);
    fseek(fp, 4l, SEEK_CUR);
}