if EOF = -1, can't a valid character == EOF and cause problems?

Discussion in 'C Programming' started by Kobu, Mar 3, 2005.

  1. Kobu

    Kobu Guest

    My question is about EOF (which is (int)-1 on the
    implementation I use). Type char on my implementation
    is 'signed' too.

    How is it that the input library functions that return
    EOF can be used properly if something like ASCII 255
    is read from the input file? 255 would be returned as -1,
    and would be compared against EOF and cause problems.

    What's the point of declaring a int to hold the return
    value from these functions, when a char(on my implementation)
    can represent a -1(EOF on my representation) also?

    Example;

    char c_char;
    while( (c_char = getchar()) != EOF )
    { /* something */ }

    will work just as *badly* as:

    int c_int;
    while( (c_int= getchar()) != EOF )
    { /* something */ }


    I'm not complaining about the AMBIGUITY of
    (int)EOF vs.(int)(Ascii 255 returned as -1)
    but wondering why people suggent capturing the
    return value from functions like getchar into
    an int, when the int doesn't help us get rid
    of this ambiguous case?

    I've tried to research this to death and wrap my
    mind around a possible answer, but I have failed
    miserably. I need someone to help me turn on the
    flickering lightbulb.

    Much thanks in advance
     
    Kobu, Mar 3, 2005
    #1
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  2. You just answered your own question. The functions return int
    precisely because of the need to distinguish between -1 and 255.
    No, it won't. The second version will work much better. You see,
    although as a signed char, 255 is the same as -1, as a signed int,
    255 is 255 and -1 is -1, and never the twain shall meet.
    But it does help.
    I hope I have been of help.
     
    Joona I Palaste, Mar 3, 2005
    #2
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  3. Kobu

    John Guest

    In the case where integers and characters are not the same
    length, you'll find that char 255 appears as an integer 255
    and -1 appears as -1.

    This is a widely used interview question, so read up on it.

    Also learn how to swap bytes without using a temporary, how
    to implement atoi() and how to reverse a list and you're mostly there!

    Now, there are 5 pirates ....
     
    John, Mar 3, 2005
    #3
  4. getchar() returns the next character from stdin *as an unsigned char*
    converted to int. If the character read has the value 255, getchar()
    will return 255, even if plain char is signed. At end-of-file,
    getchar() returns the distinct value EOF (-1 on your system).

    If you assign the result of getchar() to a variable of type
    char, you lose the ability to distinguish between 255 and -1.

    (There's still a potential problem on exotic systems where char and
    int are the same size, but you almost certainly don't need to worry
    about that.)
     
    Keith Thompson, Mar 3, 2005
    #4
  5. Actually I think you've confused the Control Flags value -1 for the Data
    being read. You can actually read both at the same time. ( 0x0ff, and EOF
    flag)
     
    DHOLLINGSWORTH2, Mar 4, 2005
    #5
  6. Kobu

    Old Wolf Guest

    Please don't top-post, and please don't answer questions
    if you have no idea what you are talking about.
     
    Old Wolf, Mar 4, 2005
    #6
  7. Kobu

    pete Guest

    Keith Thompson wrote:
    Use feof,
    to distinguish cases when getchar is supposed to return EOF
    from cases where there is an end of file condition.
     
    pete, Mar 4, 2005
    #7
  8. Kobu

    Al Bowers Guest

    In C, EOF is only indicated after an input routine has tried to read,
    and has reached end-of-file. So the input function (getchar in this
    case) must first reach the end of the file and return EOF and the
    end-of-file indicator for the stream is set. Calling function
    feof before the input function has reached the end of the file will
    not indicate that the next getchar call will return EOF.
    Read faq question 12.2:
    http://www.eskimo.com/~scs/C-faq/q12.2.html
     
    Al Bowers, Mar 4, 2005
    #8
  9. Kobu

    Dan Pop Guest

    1. If there is an end of file condition, getchar() is supposed to
    return EOF, so there is no distinction.

    2. If you don't want to rely on EOF, you have to use both ferror() and
    feof().

    Dan
     
    Dan Pop, Mar 4, 2005
    #9
  10. Kobu

    Jesse Meyer Guest

    If you figure out how to do it in standard C, I'd be curious to
    know.
     
    Jesse Meyer, Mar 4, 2005
    #10
  11. The xor trick should actually work for type unsigned char, as long as
    you ensure that you're not swapping a variable with itself.

    But really, the best answer to "How do I swap variables without using
    a temporary" is "Use a temporary".
     
    Keith Thompson, Mar 4, 2005
    #11
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