beginner question about storing binary data in buffers, seeing binarydata in a variable, etc

Discussion in 'C++' started by darren, Jul 4, 2008.

  1. darren

    darren Guest

    Hi there

    Im working on an assignment that has me store data in a buffer to be
    sent over the network. I'm ignorant about how C++ stores data in an
    array, and types in general.

    If i declare an array of chars that is say 10 bytes long:
    char buff[10];
    does this mean that i can safely store 80 bits of data?

    When i think of an array of chars, i think each spot in the array as a
    sequence of 8 1's or 0's. Is this a correct visualization? I guess
    my question here is why do most buffers seem to be implemented as char
    arrays? Can any binary value between 0 and 255 be safely put into a
    char array slot (00000000 to 11111111). Why not implement a buffer
    using uint8_t ?

    Obviously I have a very loose grasp on how buffers are saving data,
    and how a receiver gets this data on their end. I understand the
    sockets stuff, just not the buffer-specific stuff. Any enlightenment
    would be most appreciated.

    thanks.
    darren, Jul 4, 2008
    #1
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  2. darren

    James Kanze Guest

    Re: beginner question about storing binary data in buffers, seeingbinary data in a variable, etc

    On Jul 4, 11:35 pm, "Alf P. Steinbach" <> wrote:
    > * darren:
    > > Im working on an assignment that has me store data in a
    > > buffer to be sent over the network. I'm ignorant about how
    > > C++ stores data in an array, and types in general.


    > A textbook would be good resource.


    Most probably don't say anything about it, because it's
    unspecified. Except for the specifications of how pointer
    arithmetic works within arrays.

    [...]
    > > I guess my question here is why do most buffers seem to be
    > > implemented as char arrays?


    > Are they?


    Transmission buffers, yes. char[] or unsigned char[] are really
    your only two choices. (I generally use unsigned char, but the
    C++ standard does try to make char viable as well. And on most
    typical architectures, where converting between char and
    unsigned char doesn't change the bit pattern, both work equally
    well in practice.)

    > > Can any binary value between 0 and 255 be safely put into a
    > > char array slot (00000000 to 11111111).


    > It depends what you're really asking.


    > If you provided a concrete example and what you expected as
    > result, one could say whether that was correct or not.


    > But a char has guranteed at least 256 possible bitpatterns
    > (minimum 8 bits), yes.


    On the other hand, I think in theory, char could be signed 1's
    complement, and assigning a negative 0 (0xFF) could force it to
    possitive (which would mean that you could never get 0xFF by
    assignment---but you could memcpy it in). I think: I'm too lazy
    to verify in the standard, and of course, any implementation
    that actually did this would break so much code as to be
    unviable.

    > > Why not implement a buffer using uint8_t ?


    > That's not presently a standard C++ type.


    It's still a viable alternative.

    It's possible to write code for binary network protocols in a
    perfectly portable manner. It's rarely worth it, since it
    entails some very complex work arounds for what are, in the end,
    very rare and exotic machines that most of us don't have to deal
    with. Thus, I know that much of the networking software I write
    professionally will fail on a machine with an unusual convertion
    of unsigned to signed (i.e. which isn't 2's complement, and
    doesn't just use the underlying bit pattern).

    --
    James Kanze (GABI Software) email:
    Conseils en informatique orientée objet/
    Beratung in objektorientierter Datenverarbeitung
    9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
    James Kanze, Jul 5, 2008
    #2
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  3. darren

    James Kanze Guest

    Re: beginner question about storing binary data in buffers, seeingbinary data in a variable, etc

    On Jul 5, 10:48 am, "Alf P. Steinbach" <> wrote:
    [...]
    > > On the other hand, I think in theory, char could be signed 1's
    > > complement, and assigning a negative 0 (0xFF) could force it to
    > > possitive (which would mean that you could never get 0xFF by
    > > assignment---but you could memcpy it in). I think: I'm too lazy
    > > to verify in the standard, and of course, any implementation
    > > that actually did this would break so much code as to be
    > > unviable.


    > In that case any implementation for 1's complement that used
    > 1's complement also for signed 'char' would be unviable... :)


    As it happens, in the two implementations I'm aware of where
    signed integers are not 2's complement, plain char is unsigned,
    thus avoiding the problem. (If one of the goals of plain char
    is to contain text characters, then it really should be unsigned
    anyway. Historically, however, making char unsigned had a
    non-negligible runtime cost on a PDP-11, and since back then,
    all the world was PDP-11, and the only character set which
    counted was ASCII, which only uses the lower 7 bits...)

    > It makes an interesting case for dropping that support in the
    > standard, and go for requirement of two's complement for all
    > signed integral types.


    Perhaps a better choice would be to require that plain char be
    unsigned, so that you could safely use it with the results of
    e.g. istream::get() or fgetc(). (Or both, but I don't think
    you'll find much support for either in the committee.)

    > >>> Why not implement a buffer using uint8_t ?


    > >> That's not presently a standard C++ type.


    > > It's still a viable alternative.


    > Yes, and the main reason for "why not" is that it's not a
    > standard C++ type.


    > > It's possible to write code for binary network protocols in a
    > > perfectly portable manner. It's rarely worth it, since it
    > > entails some very complex work arounds for what are, in the end,
    > > very rare and exotic machines that most of us don't have to deal
    > > with. Thus, I know that much of the networking software I write
    > > professionally will fail on a machine with an unusual convertion
    > > of unsigned to signed (i.e. which isn't 2's complement, and
    > > doesn't just use the underlying bit pattern).


    > Tune up the warning level, perhaps? <g>


    What I'm waiting for is a machine which will core dump if the
    conversion fails (i.e. doesn't result in the same value). I
    don't really expect to see it, however, given that one standard
    idiom in C is things like:

    int ch = fgetc( input ) ;
    while ( ch != EOF && someOtherConditions( ch ) ) {
    *p ++ = ch ; // Where p is a char*...
    }

    It's a bit surprising that something this widespread is
    implementation defined, and may result in an implementation
    defined signal (according to the C standard---the C++ standard
    still has the imprecisions of C90). Because it is so
    widespread, however, I don't expect to see a compiler which
    doesn't support it anytime soon. (As I said, all of the
    "exotic" architectures that I know make plain char unsigned,
    which effectively removes the "implementation defined" here.)

    --
    James Kanze (GABI Software) email:
    Conseils en informatique orientée objet/
    Beratung in objektorientierter Datenverarbeitung
    9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
    James Kanze, Jul 5, 2008
    #3
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