how to define an 8 bit integer

Discussion in 'C Programming' started by DanielJohnson, Oct 25, 2008.

  1. I have seen many legacy code use uint8_t or uint16_t to override the
    default compiler setting of an integer's length.

    I am using gcc on Linux and a sizeof(int) gives me 4. I want the
    ability to define an 8, 16 or 32 bit integer.

    I tried using uint8_t but the gcc doesn't like it. Do I need to modify
    any setting or declare typdefes.

    Please guide me. Your answer is greatly appreciated.

    DanielJohnson, Oct 25, 2008
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  2. DanielJohnson

    James Kuyper Guest

    By default, gcc compiles for for a non-conforming version of C that is
    sort of like C90 with many features that are similar, but not always
    identical, to features that were added to C99. uint8_t was added in C99.
    Use -std=c99 to turn on support for C99. Add -pedantic to come a little
    closer to being fully conforming to C99.

    If for any reason you can't use C99, use the following:

    typedef unsigned char uint8;

    If a compiler supports any unsigned 8-bit integer type, unsigned char
    will be such a type. If the compiler has no 8-bit integer type,
    'unsigned char' is going to be the best approximation possible for that
    James Kuyper, Oct 25, 2008
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  3. All these arcane portability issues have been thought of, solved, and
    painfully debugged by the creators of things like the GNU autotools, so
    why reinvent the wheel?

    Look at the autoconf macros AC_TYPE_INT8_T, AC_TYPE_INT16_T,
    AC_TYPE_INT32_T and AC_TYPE_INT64_T.
    Antoninus Twink, Oct 25, 2008
  4. I think the problem is that he doesn't have "#include <stdint.h>".
    Add that to the top of the file, and uint8_t becomes visible --
    assuming the implementation provides <stdint.h>.

    On my system, this works whether you use gcc's partial C99 mode or not
    -- which is valid behavior, since it's a standard header in C99 and a
    permitted extension in C90.

    Incidentally, using uint8_t or uint16_t doesn't override anything.
    The predefined types are still there, and their sizes don't change for
    a given implementation. uint8_t and uint16_t, if they exist, are
    nothing more than typedefs for existing predefined types (typically
    unsigned char and unsigned short, respectively).

    An implementation note: the <stdint.h> header isn't provided by gcc,
    it's provided by the library. On my system, the library is glibc,
    which does provide <stdint.h>. On another system, a different library
    might not provide this header. I suspect that <stdint.h> will be
    available on *most* modern implementations, but it's not guaranteed
    unless the implementation claims conformance to C99.
    Keith Thompson, Oct 25, 2008
  5. #include <stdint.h>
    Martin Ambuhl, Oct 25, 2008
  6. DanielJohnson

    Bartc Guest

    If he has lots of them (as in an array), then it might be useful to only
    require a quarter or an eighth of the memory for example.

    If he has to talk to some software/hardware that uses specific integer
    widths then again it would be handy.
    Bartc, Oct 25, 2008
  7. DanielJohnson

    Phil Carmody Guest

    Why do you need to know his reasons? Is it that you don't
    believe him? Do you treat all posters with equal mistrust,
    and do you expect others to treat you with the same mistrust?

    He wants the ability to do the above. If he #includes stdint.h,
    he'll have his wants most easily satisfied, no matter what
    his reasons were. If he grabs a decent book in C, then he'll
    probably have his wants satisfied far more quickly in the

    Phil Carmody, Oct 25, 2008
  8. DanielJohnson

    Ian Collins Guest

    Not in my world (that of a driver writer) or that of most embedded
    programmers. Considering a large proportion of C programmers are
    embedded programmers, the need for fixed width types is much greater
    than you think.
    Ian Collins, Oct 25, 2008
  9. DanielJohnson

    CBFalconer Guest

    uint8_t etc. are not guaranteed available. The guaranteed integer
    types are char, short, int, long. C99 adds long long. These can
    all be signed or unsigned.

    Code using uint8_t is inherently non-portable. bytes can be larger
    than 8 bits. See <limits.h> for the sizes available on your
    system, expressed by MAX and MINs for the types.
    CBFalconer, Oct 26, 2008
  10. Using GCC, adding

    #include <stdint.h>

    should do the trick. Like already told by the other posters, it
    may not be avaliable, so add some checks to your source, and
    provide alternative ways to define those types.

    In the end it will boild down to some typedefs from primitive
    types, which have been exactly matched to target architecture
    and compiler. That's how stdint.h works.

    Wolfgang Draxinger
    Wolfgang Draxinger, Oct 26, 2008
  11. [...]

    And if you don't happen to need those macros and conversion functions,
    even on a hosted system, why not use <stdint.h>?
    Keith Thompson, Oct 26, 2008
  12. So? That may have been the intent, but why should a programmer be
    bound by that, or even influenced?

    One standard header contains a few declarations. Another standard
    header contains those same declarations plus some other stuff. If I
    don't need the other stuff, what is the disadvantage of using the
    first header?
    Sure, there's nothing wrong with using <inttypes.h> if you want to.
    I'm just saying that there's nothing wrong with using <stdint.h> if
    you want to.
    Keith Thompson, Oct 26, 2008
  13. PC> Why do you need to know his reasons?

    Because any experienced programmer who's helped others has run into XY
    problems: the person asking for help needs to do X, and thinks
    mistakenly that Y is the way to accomplish it. So the querent asks
    about Y, and the newsgroup spends a lot of time going around in circles
    because Y is really not the right solution to X, but because the querent
    is asking about Y and not X, everyone's time is wasted.

    Asking "Why do you want to do Y?" allows the respondents to say, "Aha!
    That's not the best way to accomplish X -- you'll have a much easier
    time of it if you try Z." If Y is the best way to do X -- it happens
    occasionally -- then help with Y can proceed apace.

    It's not a matter of distrust. Knowing *why* someone wants to do
    something allows respondents to offer alternative solutions that may
    well be better.

    Charlton Wilbur, Oct 26, 2008
  14. I've just switched a project to autoconf/automake, and the result is
    not pretty. The support for non-gcc compilers is weak (how do I say I
    want the highest level of warnings for whatever compiler turns out to
    be available?). The previously short cc commands are now each several
    lines long, making it hard to see warning messages before everything
    has scrolled off the screen. I no longer get errors for undefined
    functions until I run the program. Support for generated files
    (except those produced by known programs like yacc) is poor, and
    doesn't work properly with VPATH on some platforms. And each time I
    try a new platform, I find a bunch of new things that I have to write
    autoconf macros for.

    On balance, it's probably worthwhile, and I don't mean to criticise
    the authors, but prepare yourself for a lot of tedious messing around.

    -- Richard
    Richard Tobin, Oct 27, 2008
  15. [...]

    I would add the following if you expect 32-bit systems...

    typedef char test[
    sizeof(char * 4) == 32 / CHAR_BIT

    shi% happens.
    Chris M. Thomasson, Oct 28, 2008
  16. well, perhaps:

    typedef char tester[
    (sizeof(char) * 4 == 32 / CHAR_BIT) ? 1 : -1

    MAN! I am a fuc%ing retard!!!!!!!!!!!!!!!!!
    Chris M. Thomasson, Oct 28, 2008
  17. DanielJohnson

    Chris Dollin Guest

    I think other-Chris mistyped (see their later posting).
    Assertions don't operate at compile-time, but the imploding
    array trick does.

    'Don't be afraid: /Electra City/
    there will be minimal destruction.' - Panic Room

    Hewlett-Packard Limited registered office: Cain Road, Bracknell,
    registered no: 690597 England Berks RG12 1HN
    Chris Dollin, Oct 28, 2008
  18. DanielJohnson

    Phil Carmody Guest

    sizeof(char) is somewhat redundant - it's defined to be 1.
    So that's a check that CHAR_BIT is 7 or 8. (Of course,
    7 is impossible.) So for that purpose if I absolutely had
    to have such a trap, I'd just keep it simple (no divison,
    no ?:)

    typedef char tester[CHAR_BIT==8];

    Ditto the 32-bit ints condition:

    typedef char tester[CHAR_BIT*sizeof(int)==32];

    However, it does look like some of the C++ Kool-Aid has
    cross-polinated, and I can't say I particularly like
    such bombs.

    If you want to limit yourself to only using 32-bit ints,
    why not code using exact-width integers. If such a type
    can't be found, you'll find out at compile time, without
    need for an obfuscation.

    Phil Carmody, Oct 28, 2008
  19. DanielJohnson

    James Kuyper Guest

    Chris M. Thomasson wrote:
    I understand what you're trying to do there, but wouldn't a #if/#endif
    pair bracking a #error directive do what you're trying to do in a much
    clearer way? In a conforming mode, no compiler can omit the diagnostic
    for an array length of 0, but it's perfectly free to accept the program
    after issuing the diagnostic - I've used compilers with this "feature".
    However, in conforming mode no compiler can accept a translation unit
    containing a #error directive that survives conditional compilation.
    James Kuyper, Oct 28, 2008
  20. because sometimes people aren't describing their actual problem.
    They are describing the problem they are having with their chosen
    solution instead.

    are you always this obnoxious?
    I've dabbled with embedded systems and I also think "the number of
    people who think they need integers of a certain width is much
    than the number who actually do".

    I suspect the same applies to device drivers as well.
    Nick Keighley, Oct 28, 2008
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