在while循环的最后一次迭代中，C语言 feo不检查EOF

Question

我有一个使用ungetc和fefo函数的代码，但我注意到fefo没有检查下面是我的代码

 #include<stdio.h>
 int main ()
{
 FILE *fp;
  int c;
 char buffer [200];
 fp = fopen("", "r");
 if( fp == NULL ) 
 {
  perror("Error in opening file");
  return(-1);
  }
 while(!feof(fp)) 
 {
   c = getc (fp);
  if(c==EOF) // **why i need to check for EOF if fefo does?**
   break;
   /* replace ! with + */
   if( c == '!' ) 
   {
     ungetc ('@', fp);
   }
   else 
   {
     ungetc(c, fp);
   }
   fgets(buffer, 255, fp);
   fputs(buffer, stdout);
  }
  return(0);
}

输入为:

   hello !world

如果EOF未显式检查则输出

     hello @world
     hello @world // Bad its repeat

显式检查EOF时的输出

   hello @world // Good

为什么我需要检查EOF和打破当fefo做?

Answer 1

如果我理解正确，您正在从文件中读取缓冲区并在发生'!'时替换'@'。上面的每个评论都为您提供了原因和链接，为什么在while循环中读取字符并使用feof测试是坏的。

将注释放在一起，并稍微清理一下逻辑，下面显示了完成此任务的标准方法。我留下了内联注释代码，以使更改更加明显。看一看，如果你有问题，留下评论:

#include <stdio.h>
#define MAXC 255
int main (int argc, char **argv) {
    FILE *fp = NULL;
    int c;
    char input[MAXC];
    char buffer[MAXC];
    size_t idx = 0;
    if (argc < 2) {
        fprintf (stderr, "n error: insufficient input, filename required.n");
        return 1;
    }
    if (!(fp = fopen (argv[1], "r")))  {
        fprintf (stderr, "n error: file open failed '%s' (%p)n", argv[1], fp);
        return 1;
    }
    while ( (c = getc (fp)) != EOF ) 
    {
        input[idx] = c;
        if (c == '!') c = '@';
        buffer[idx++] = c;
        if (idx == MAXC) {  /* normally realloc if buffer is allocated */
            printf (" warning: maximum size of buffer reached  (%d char)n", MAXC);
            break;
        }
        // {
        //     ungetc ('@', fp);
        // } else {
        //     ungetc (c, fp);
        // }
        // fgets (buffer, 255, fp);
        // fputs (buffer, stdout);
    }
    fclose (fp);
    input[idx] = buffer[idx] = 0;
    // fputs (buffer, stdout);
    printf ("n characters in input & buffer are:nn");
    printf ("n  original : %sn  modified : %snn", input, buffer);
    return 0;
}

$ ./bin/whyfeofbad dat/qbfox2.txt
 characters in input & buffer are:

  original : The quick! brown fox jumps over the lazy! dog. He never says "Hi!", he just saunters up and jumps!
  modified : The quick@ brown fox jumps over the lazy@ dog. He never says "Hi@", he just saunters up and jumps@