CODEWITHSUNDEEP
cwindowseof
うちわ 密か
うちわ 密か

Reputation: 57

Value of EOF under windows

I wrote this code :

#include <stdio.h>

int main() {
    int c;

    while ((c = getchar()) != EOF)
        putchar(c);

    return 0;
}

Under windows, I figured out that I have to input ctrl+z so I can stop the program, I also printed the value of EOF and it's -1. However, when I enter -1 instead of ctrl+z the program continues to execute, and I want to understand why. Thank you!

Upvotes: 3

Views: 2822

Answers (4)

user207421
user207421

Reputation: 311052

EOF isn't a 'value'. It is a sentinel defined by the stdio library.

Ctrl/z is a keystroke, that is interpreted by the terminal driver and causes it to notify end of stream to the application reading from it, which causes C read() to return zero, which causes getchar() to return EOF.

The corresponding keystroke on Unix, Linux, etc, is Ctrl/d.

Upvotes: 2

Keith Thompson
Keith Thompson

Reputation: 263657

The getchar() function returns either the numeric value of the character that was just entered (treated as an unsigned char, so it's never negative), or the special value EOF which indicates that there is no more input.

Windows uses Control-Z to trigger and end-of-file condition, which causes getchar() to return the value EOF. But EOF is not a character value; it's specifically chosen to be a value that cannot be a character value.

End-of-file is a condition, not a value. EOF is a numeric value, but not a character value; it's the value returned by getchar() to indicate that input is in an end-of-file condition (or an error condition).

(If you want to read integer values, positive or negative, you need to read a single character at a time and then compute the integer value that the character sequence represents. The integer value -1 can be entered as the character '-' followed by the character '1'. C has various functions, like atoi and the *scanf family, that can do that conversion.)

Upvotes: 3

Paul Gibson
Paul Gibson

Reputation: 634

You need to change the logic just a bit:

#include <stdio.h>
int main() {
int c;
int done = 0;

while (!done)
{
    c = getchar();
    if(c != '-')
        putchar(c);
    else
    {
        c = getchar();
        if (c == '1')//this is an EOF
        {
            done = 1; // and probably putchar the EOF
        }
        else
        {
              putchar('-');
              putchar(c);
        }
    }//else
}//while
return 0;

}

Upvotes: -1

Stephan Lechner
Stephan Lechner

Reputation: 35164

Entering "-1" as text does not return the integer value -1 but two characters, i.e. a '-' (which corresponds to ASCII value 45) and a '1' (which corresponds to ASCII value 49). Both do not compare equal to EOF (which is -1 in decimal).

By definition, you cannot enter something that is consumed as negative value by getchar(), as negative values are defined to represent the "End-of-file".

If you want to read in an integral value (like -1), use scanf:

int num;
if (scanf("%d", &num)==1) { // successfully read one valid integral value?
    printf("you entered number %d:", num);
}

Upvotes: 3

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