如何区分到达 EOF 和到达错误的 fgets

Question

我有一个关于 fgets()

的问题

char *fgets(char *str, int strsize, FILE *stream);

fgets()' 文档说：

On success, the function returns the same str parameter. If the End-of-File is encountered and no characters have been read, the contents of str remain unchanged and a null pointer is returned.

If an error occurs, a null pointer is returned.

你如何区分以上两种情况 - fgets 到达 EOF（文件结束）和读取文件时出错？
此外，当 fgets 读取文件时发生错误时，fgets 是否会跟踪 str 中到目前为止已读取的内容？
如何检查 fgets 是否立即到达 EOF？

Answer 1

提到你的第一个问题：

How do you differentiate between above two situations - fgets reaching EOF(END OF FILE) & error whilst reading file?

如果 fgets() 返回 NULL，则为文件指针调用 ferror()，该文件指针刚刚在 fgets() 调用中使用，返回 NULL。如果 ferror() returns 为非零值，则 fgets() 失败，否则它已到达文件末尾。

示例：

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

#define LINE_LEN_MAX (42)

int main(int argc, char ** argv)
{
   if (1 >= argc)
   {
     errno = EINVAL;
     perror("main() failed");
     exit(EXIT_FAILURE);
   }

   {
     FILE * fp = fopen(argv[1], "r");
     if (NULL == fp)
     {
       perror("fopen() failed");
       exit(EXIT_FAILURE);
     }

     for (char line[LINE_LEN_MAX];
       NULL != fgets(line, LINE_LEN_MAX, fp);)
     {
       printf("%s", line);
     }

     if (0 != ferror(fp))
     {
       perror("fgets() failed");
       exit(EXIT_FAILURE);
     }

     fclose(fp);
   }

   return EXIT_SUCCESS;
 }

另一个问题可以从 the docs:

直接回答

问题 3：

How do you check if fgets immediately reaches EOF?

答案：

If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned.

问题 2：

when an error occurs whilst fgets reads file, does fgets keep track of whatever has been read up to that point in the str?

答案：

If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.

如何区分到达 EOF 和到达错误的 fgets

how to differentiate between fgets reaching EOF and reaching error

c

fgets