如何使用 fgets() 从 stdin 读取？

Question

My professor write a C code about fgets,feof with using stdin. But I don't understand how it works.

fgets(list, 100, stdin);

while (!feof(stdin))
{
    printf("feof!");
    
    fgets(list, 100, stdin);
}

when i write a 'hello', function while is working. why feof(stdin) returns 0?

I think first fgets read stdin buffer all of string(include '\0'). therefore I think while shouldn't work because feof(stdin) returns 1 What am i wrong?

Answer 1

Tip: always check the return value of input functions and most other I/O functions too.

fgets(list, 100, stdin);  // Weak code

---

Yes, calling printf("feof;"); inside the loop is misleading as the end-of-file indicator for stdin is not set given the prior test condition while (!feof(stdin)) .

How to read from stdin with fgets()?

Do not use feof() for this task for primary detection

fgets() returns NULL when:

1. An end-of-file just occurred in the previous input operation.

2. An end-of-file had occurred in the some input operations even before that.

3. An input error just occurred. Examples: file to read is an output stream or a parity error on some serial communication.

while (!feof(stdin)) only detects 1 & 2.

Unless the return value of fgets(list, 100, stdin) is checked, using list may be a problem.

feof() is useful after an I/O function returned the possibility of the need.

Size to the object, avoid magic numbers

---

Good usage:

char list[100];
while (fgets(list, sizeof list, stdin)) {
  printf("%s", list):
}

if (feof(stdin)) {
  puts("End-of-file");
} else if (ferror(stdin)) {
  puts("Error");
} else {
  puts("Unexpected state");
}