c putchar() and getchar()

[Ja4Coltrane]

Hello.

So I am trying to write a simple copy program. For whatever reason, the program will never stop running! Even if I directly enter the value of EOF (-1 in my case), it still goes.



#include <stdio.h>

main()
{
int c;
c = getchar();

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

[zeitghost]

If you're running that in a dos box under Windoze, then EOF is defined as Control Z on standard in.

Try that.

[Ja4Coltrane]

I'm using putty and compiling with gcc over ssh. I tried control z and it outputed "suspended" and ended the program, however that was not what I wanted as it did not continue the program after the while loop (if say, I added printf("done\n"); after the loop). Any suggestions?

[mgb_phys]

On unix, EOF on a console is ctrl D
Ctrl Z puts the program into the background, which isn't what you want if it is taking input from stdin.
To get the program back type 'fg' then kill it with ctrl C

[Ja4Coltrane]

Wow that worked! Thank you very much mgb_phys! Would someone mind explaining in more detail why that works--I mean, if EOF is simply the integer -1, then why is it that letting c = -1 does not end the file. What does control D actually input into the program?

[mgb_phys]

Thats what the operating system is for.
It's job is to read a file off disk and present it to the program a byte at a time, or in your case, read the keyboard convert it into ascii characters and send them to your program You obviously need a way of signalling that you have finished entering your text. One way would be to have a special key 'end' on the keyboard but Unix was designed to work on many different machines with different keyboards so uses ctrl+letter for all these extra characters.

This is also the reason getchar() returns an int not a char. If it only handled chars there wouldn't be any room for the extra commands like EOF.

ps Don't rely on EOF being -1, it can be anything the library and OS choose, always compare it to EOF.

pps. I didn't want to confuse things before, but the more common way to write the code would be:
while ( (c=getchar()) != EOF ) {
putchar(c);
}
Then you don't need the initial getchar() outside the loop. This is a common source of bugs, if it gets removed or other code gets called between it and the test.

[KTC]

Or while ( (c=getchar()) != EOF ) {
putchar(c);
}

[mgb_phys]

Thanks KTC, I fixed the typo.

[Ja4Coltrane]

Interesting. Thank you very much for the help.

[mgb_phys]

Stick at Ja4Coltrane. Starting at this level is important if you ever want to really understand computers whatever language you use.
Just hooking the Java input thing to the Java output thing and not knowing what is actually happening only gets you so far.