Thread: fflush( ) questions

  1. #1
    Registered User
    Join Date
    Aug 2006
    Posts
    62

    fflush( ) questions

    I read the FAQ entry on why not to use fflush(stdin), but all it said was that the behavior is undefined...so I'm sort of confused? Like what kind of horrible consequences could arise?

    Also I was trying to clear the screen and someone suggested using fflush(stdout), but that only seems to work the first time I call it, and after that nothing happens. Why exactly is that? I mean I know I could use clrscr(), or system("CLS") since dev doesn't have clrscr().

    So I guess those are my only two questions. Somewhat trivial, but it was something that was sort of bugging me so I felt the urge to ask ^_^

  2. #2
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    Posts
    14,826
    The most likely thing to happen is that it simply won't work, making it pointless to do. fflush doesn't clear a screen. It writes everything in the buffer to the stream. For example, if you've done:
    Code:
    printf("hello world");
    There is no newline, and so if the buffer is not full, it doesn't have to display what you've asked it to. If you want to make sure it does, you can flush the stream; forcing it to write everything that hasn't been written yet.


    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
    C++まいる!Cをこわせ!
    Join Date
    Oct 2007
    Location
    Inside my computer
    Posts
    24,654
    For the first, undefined behavior means just that. Behavior which isn't defined. We don't know what's going to happen. Your computer could explode. Or nothing could happen.
    fflush(stdout) doesn't clear the screen. It only makes sure that whatever is in the output buffer gets shown on the screen. There should be an additional entry in the FAQ about how to clear the screen, I do believe.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  4. #4
    Registered User
    Join Date
    Feb 2010
    Posts
    72
    but what's the point of creating fflush function if it would have undefined behavior?

  5. #5
    Officially An Architect brewbuck's Avatar
    Join Date
    Mar 2007
    Location
    Portland, OR
    Posts
    7,398
    Quote Originally Posted by nik2 View Post
    but what's the point of creating fflush function if it would have undefined behavior?
    The behavior is only undefined when applied to an INPUT stream.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  6. #6
    spurious conceit MK27's Avatar
    Join Date
    Jul 2008
    Location
    segmentation fault
    Posts
    8,300
    The fact that fflush(stdin) will generally work does not change the fact that you are flushing an INPUT stream, potentially resulting in undefined behaviour.

    Once you intentionally introduce undefined behaviour into your program, you cannot be sure that it is not somehow the cause of any and all subsequent problems you may have -- meaning debugging with confidence is impossible.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  7. #7
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    Posts
    14,826
    Quote Originally Posted by nik2 View Post
    but what's the point of creating fflush function if it would have undefined behavior?
    The function takes a file handle. It doesn't know if the file has been open for reading or writing.

    Consider all of the string functions. They all take pointers to characters as their arguments. None of them actually know how much space is really being pointed at. It's up to you to use them correctly.
    Code:
    char x, y;
    
    strcpy( &x, &y );
    This function sees two pointers to characters, and says "Ok, I can use those...". It doesn't know that x and y are singular characters. It doesn't know if they are valid strings (they could actually BE valid strings--a single nul character is a valid string).

    The same goes for fflush. It gets a FILE* and expects you to pass it the right thing. It's not its fault if you pass it something incorrectly. If you really want to, you can crash your program with most standard library functions.


    Quzah.
    Hope is the first step on the road to disappointment.

  8. #8
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    Location
    The edge of the known universe
    Posts
    38,128
    > Like what kind of horrible consequences could arise?
    Because C is the kind of language where if you make statements like "I want to hang myself", it will happily give you the rope to do it (see Quzah's string example).

    There is no safety net - you need to have some sense of what it is you're actually trying to do. Writing a syntactically valid C program is pretty easy - about 10% of the effort. Making sure it behaves within all the other rules (like not flushing input streams) is where you differentiate yourself from the crowd.

    Further, just because you made it run without crashing, and got the results you expected only puts you at about 75% done. Porting the code to other systems (or just changing the optimisation flags) often reveals yet more problems with the code.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  9. #9
    Registered User
    Join Date
    Aug 2006
    Posts
    62
    Okay I suppose I understand what you're all getting at. I think at this point in my knowledge it's one of those things I just shouldn't do. I don't use the fflush "method" since my compiler has the clrscr() function(and the few times I've asked about it everyone has practically had a conniption), but I was really curious as to why it was practically a taboo...and I think you guys cleared it up ^_^

    Salem, I find it funny and ironic that your signature reads this: "If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut." It's probably referring to something else, but it's still ironic xD

  10. #10
    C++まいる!Cをこわせ!
    Join Date
    Oct 2007
    Location
    Inside my computer
    Posts
    24,654
    I believe it refers to that if you invoke undefined behavior, you have to expect trouble. Things that fail to work properly, for example.
    Such as flush stdin. It will not work the same across all compilers and platforms. If you port undefined behavior, you have to expect that they'll break.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. questions....so many questions about random numbers....
    By face_master in forum C++ Programming
    Replies: 2
    Last Post: 07-30-2009, 08:47 AM
  2. A very long list of questions... maybe to long...
    By Ravens'sWrath in forum C Programming
    Replies: 16
    Last Post: 05-16-2007, 05:36 AM
  3. Several Questions, main one is about protected memory
    By Tron 9000 in forum C Programming
    Replies: 3
    Last Post: 06-02-2005, 07:42 AM
  4. When and where to use fflush (stdout)?
    By Micko in forum C Programming
    Replies: 8
    Last Post: 02-18-2005, 11:58 AM
  5. Trivial questions - what to do?
    By Aerie in forum A Brief History of Cprogramming.com
    Replies: 23
    Last Post: 12-26-2004, 09:44 AM