caramel1995 caramel1995 - 2 months ago 16
C Question

EOF in Windows command prompt doesn't terminate input stream

Code:

#include <stdio.h>
#define NEWLINE '\n'
#define SPACE ' '

int main(void)
{
int ch;
int count = 0;

while((ch = getchar()) != EOF)
{
if(ch != NEWLINE && ch != SPACE)
count++;
}
printf("There are %d characters input\n" , count);

return 0;
}


Question:


  1. Everything works just fine, it will ignore spaces and newline and output the number of characters input to the screen (in this program I just treat comma, exclamation mark, numbers or any printable special symbol character like ampersand as character too) when I hit the EOF simulation which is
    ^z
    .

  2. But there's something wrong when I input this line to the program. For example I input this:
    abcdefg^z
    , which means I input some character before and on the same line as
    ^z
    . Instead of terminating the program and print out total characters, the program would continue to ask for input.

  3. The EOF terminating character input only works when I specify
    ^z
    on a single line or by doing this:
    ^zabvcjdjsjsj
    . Why is this happening?


Answer

This is true in almost every terminal driver. You'll get the same behavior using Linux.

Your program isn't actually executing the loop until \n or ^z has been entered by you at the end of a line. The terminal driver is buffering the input and it hasn't been sent to your process until that occurs.

At the end of a line, hitting ^z (or ^d on Linux) does not cause the terminal driver to send EOF. It only makes it flush the buffer to your process (with no \n).

Hitting ^z (or ^d on Linux) at the start of a line is interpreted by the terminal as "I want to signal EOF".

You can observe this behavior if you add the following inside your loop:

printf("%d\n",ch);

Run your program:

$ ./test
abc                      <- type "abc" and hit "enter"
97
98
99
10
abc97                    <- type "abc" and hit "^z"
98
99

To better understand this, you have to realize that EOF is not a character. ^z is a user command for the terminal itself. Because the terminal is responsible for taking user input and passing it to processes, this gets tricky and thus the confusion.

A way to see this is by hitting ^v then hitting ^z as input to your program.

^v is another terminal command that tells the terminal, "Hey, the next thing I type - don't interpret that as a terminal command; pass it to the process' input instead".