Problem with EOF when using pipes and stdin

Discussion in 'C++' started by Yandos, Sep 14, 2005.

  1. Yandos

    Yandos Guest

    Hello all,

    I have maybe a trivial question, but I cannot think out what is wrong :(
    How do i detect EOF correctly when i read from stdin? Am I doing it
    wrong?

    <pipetest.c>
    #include <stdio.h>
    int main(void) {
    char ch;
    while (!feof(stdin)) { // <-- EOF appears to be true when character 0x1A is on stdin
    ch=getchar(); // <-- variable ch has value of 0xFF when 0x1A was on stdin
    putchar(ch);
    }
    return(0);
    }

    <256.tmp>
    contains values from 0 to 255

    I compiled and run from cmd prompt: pipetest.exe < 256.tmp > out.tmp

    <out.tmp>
    00000000 00 01 02 03 04 05 06 07 08 09 0D 0A 0B 0C 0E 0F
    00000010 10 11 12 13 14 15 16 17 18 19 FF

    Why the program ended on 0x1A character and outputted FF instead?

    How do I correctly write program which can be used with pipes? I use
    free borland turbo c++ 1.01 compiler on windows xp.

    Thanks anyone for kind help,
    Y.
     
    Yandos, Sep 14, 2005
    #1
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  2. Yandos

    Yandos Guest

    Now I know I need to read and write files in binary mode. I know how to open file on disk in binary mode,
    but how do i open stdin and stdout in binary? (AFAIK these files are yet opened)

    Thanks anyone for kind help,
    Yandos
     
    Yandos, Sep 14, 2005
    #2
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  3. Yandos

    Guest Guest

    As apparent from another post of yours, I think you are learning C, not C++.
    You may find better answers at comp.lang.c and/or alt.comp.lang.learn.c-c++
    getchar actually returns int. What you are doing is truncating that value
    and outputting something else.
    I don't know... :(
    You are using a compiler that was released in 1991! :)

    You should consider getting a more modern compiler. For the Windows
    environment; there is a free one from Microsoft, a free one from Borland,
    and a very nice free development environment named Dev-C++ that which uses
    gcc as the compiler.

    Ali
     
    Guest, Sep 14, 2005
    #3
  4. Yandos

    Artie Gold Guest

    Bzzzt. make that:
    int ch; /* since EOF, by definition is a value *not* representable
    by char (this is why getchar returns int) */
    Bzzzt. feof() will return true only *after* a read attempt returns EOF
    (this ain't Pascal).
    *That* is off topic here; pipes are not part of standard C++.
    I recommend that you see the C FAQ, as these are functions (getchar(),
    etc) from the C library inherited into C++.

    HTH,
    --ag
     
    Artie Gold, Sep 14, 2005
    #4
  5. First of all, both questions you posted were centered on the
    (mis)behavior of a C, (not C++,) program. If you have C questions
    you'll get a better responses and more patience in a C related group.

    Now to your question: Borland's Turbo C++ 1.0 is an old compiler,
    (I believe I bought it in 1990?) written at a time were compatibility
    with MS-DOS (plain DOS, not Windows) was important. Many details of
    the run time library ensure compatibility with data files created by
    other DOS programs, many of which tried to be compatible with files
    created under DOS, CPM-86 and CPM-80.

    All these last three operating systems use a CONTROL-Z character
    (0x1A) as an end-of-file marker in text files. The (historic) reason
    being that CPM-80 kept track of file sizes only as the number of 128
    byte blocks. To know exactly where a text file ended in the last
    block, the convention of using CTRL-Z (a non-printable character) was
    established.

    So your program stops reading when it receives an indication it
    reached the end of file. The 0xFF following it is an error code
    supplied when you try to read from a file beyond the logical
    end-of-file.

    Also note that before reaching CTRL-Z, the newline char (0x0A) was
    expanded into carriage return - newline (0x0D 0x0A) and that the
    carriage return following it was discarded. Opening the file/pipe in
    binary mode would correct all these problems

    I am sure there must be a way of modifying your program to tell BC++
    1.0 to use binary mode, but it would be a waste of time. You can get
    other (more modern) free compilers, and your time would be better
    spent learning to do this on LCC-Win32, Digital Mars C/C++, GCC, or
    even Microsoft compilers. (They are free, not Visual Studio, but the
    underlying compilers.)
    Roberto Waltman

    [ Please reply to the group, ]
    [ return address is invalid. ]
     
    Roberto Waltman, Sep 14, 2005
    #5
  6. Yandos

    Yandos Guest

    Right. I don't know what's the difference :( I thought i have c++ compiler, so best place is to ask here :)
    Simple and stupid, sorry :( I will read more about that difference and try to figure right group next time.
    Damn, how do you know all this? :) This is very interesting. Now I unerstand...
    Roberto, thank you again for very useful reply. I glad you realized I'm absoulte newbie and handled me
    with care :) I have learned a lot from your words. The reason why i'm starting with old borland compiler
    is that i knew it is free and i don't yet know the syntax of c and compiler is helping me a lot to correct
    "grammar" :) (I have learned pascal before) One big thank you! ;)

    Y.
     
    Yandos, Sep 14, 2005
    #6
  7. Yandos

    Yandos Guest

    Right, thank you for hint. I will read more about the difference and next time try to figure the correct
    group ;)
    IC, thanks. Anyway, the result is still the same, no matter if i use int or char. But I will keep that in mind.
    Thank you again for hint, i will try it. I have started with borland c++ because i have learned turbo
    pascal before, and knew that borland had released c++ 1.01 for free use - and i needed some "gui"
    because i need a lot of help with syntax.

    Y.
     
    Yandos, Sep 14, 2005
    #7
  8. Yandos

    Yandos Guest

    IC, thanks :) But on the result there's no difference, but i will now be more careful on data types.
    Pascal would have warned me...
    Not really. Also when 0x1A is received, because stdin is opened in text mode by default in turbo c++
    1.01 because of compatibility reasons (Roberto right explained that in different thread)
    IC, sorry.
    Thank you for help, I have a lot to learn yet :) And apologies for off topic...
    Y.
     
    Yandos, Sep 14, 2005
    #8
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