Why does EOF behave differently in fgets and read

Question

#include <stdio.h>

#define MAXLINE 4096

int 
main(int argc, char **argv)
{
    char           *s;
    char            buf[MAXLINE];

    s = fgets(buf, MAXLINE, stdin);    // here, if replaced with read(0, buf, MAXLINE);        

    return 0;
}

Input is ：12 ctrl+d

1. fgets doesn't return until input ctrl+d again(That is: 12 ctrl+dctrl+d ). Why doesn't fgets return when it encounts the first EOF ? It seems 12 ctrl+d doesn't work.

2. But when s = fgets(buf, MAXLINE, stdin); is replaced with read(0, buf, MAXLINE); read will return(input is also: 12 ctrl+d ).

Answer 1

Hitting CTRL + d on the terminal:

• simply means flush all the characters in stdin (the input buffer) immediately
• it does NOT trigger an EOF condition on stdin
  (unless the current line/buffer is co-incidentally empty.)

So hitting CTRL+D while running a program ,

• a blocked fgetc() will return if you do it twice consecutively.
  1st = flush currently buffered characters,
  2nd = flush empty buffer; ie EOF condition is valid for fgetc() and it returns .
• a blocked fgetc() will return if you do it once on an empty-line.
  flushes an already empty stdin buffer, ie EOF condition is valid for fgetc() and it returns.
• a blocked read() returns immediately as soon as the input is flushed.

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Checkout the answers to this question for more details.

Answer 2

In common implementations fgets is based on a loop around read . If you call fgets it calls read internally. Typing 1 2 Ctrl+D makes read return the two characters "12" to fgets . That does not yet make a full line, so fgets calls read again. Since we're reading from a terminal device, and not a file for example, read waits for you to type more data. If you type Ctrl+D again, read returns 0 characters, which fgets interprets as an end of file and returns.