Question about 'gets' and EOF

Sharke · 03-05-2009, 09:47 PM

I've never been sure about how EOF works on a system. Take for instance this example program, which counts strings entered until it encounters the word "quit" or the EOF.

Code:

#include <stdio.h>
#include <string.h>
#define SIZE 81
#define LIM 100
#define STOP "quit"

int main(void)
{
  char input[LIM][SIZE];
  int ct = 0;

  printf("Enter up to %d lines (type quit to quit):\n", LIM);

  while (ct < LIM && gets(input[ct]) != NULL && strcmp(input[ct], STOP) != 0)
    ct++;

  printf("%d strings entered\n", ct);
  return 0;
}

Then the following expected behavior is given:

Quote:

1
2
3
4
5
^Z
5 strings entered

But what is happening here?

Quote:

1
2
3
4
5^Z
^Z
^Z
5 strings entered

And here?

Quote:

1
2
3
4
5^Z
6
7
8
9
^Z
8 strings entered

MK27 · 03-05-2009, 10:30 PM

Is this another one of those trick questions at the end of Apocalypse Now where Brando snarls, "I don't seen any EOF at all"?

Because if it is it ain't funny, really.

whiteflags · 03-05-2009, 11:40 PM

The gets function, like fgets, is usually written as a looped call to fread. Your question is answered in the manual:

Quote:

The functions fread() and fwrite() advance the file position indicator
for the stream by the number of bytes read or written. They return the
number of objects read or written. If an error occurs, or the end-of-
file is reached, the return value is a short object count (or zero).

So if you send EOF immediately after some data, it is likely that fread will return "a short object count" instead of zero like you expected, which ultimately determines gets return value. It is no different from what happens when reading a file. You may encounter EOF while you read the last line, but only the call after that fails for certain. This is so the last of the data has an opportunity to be processed.

And remember that when you send EOF from the console there is actually two pieces of data to read: the EOF, and a newline. If the newline weren't there, input would fail like you expect.

vyn · 03-11-2009, 08:39 AM

Quote:

1
2
3
4
5^Z
6
7
8
9
^Z
8 strings entered

I have the same question: how do I prevent the above from happening? I want the input to terminate on pressing Ctrl-D/Z. The behaviour should be like the following:

Quote:

1
2
3
4
5^Z
5 strings entered

The Unix cat program seems to be able to do it. How do I do the same thing in C?

**laserlight** · 03-11-2009, 08:41 AM

vyn, start a new thread and post your current code.

03-05-2009, 10:30 PM	#2
MK27 critical genius Join Date: Jul 2008 Location: SE Queens Posts: 5,203	Is this another one of those trick questions at the end of Apocalypse Now where Brando snarls, "I don't seen any EOF at all"? Because if it is it ain't funny, really. __________________ "A man can't just sit around." -- Larry Walters
MK27 is offline

03-11-2009, 08:41 AM	#5
laserlight C++ Witch Join Date: Oct 2003 Location: Singapore Posts: 11,353	vyn, start a new thread and post your current code. __________________ C + C++ Compiler: MinGW port of GCC Build + Version Control System: SCons + Bazaar Look up a C/C++ Reference and learn How To Ask Questions The Smart Way
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