convert 64-integer to hex octet string

Discussion in 'C Programming' started by wenmang@yahoo.com, Nov 16, 2006.

  1. Guest

    Hi all,

    I have a confusion for representation of hex number:
    For an unsigned long long number, its max value is:
    unsigned long long maxNum = 18446744073709551615;
    if I do:
    printf("max uint64_t value in hex is %x\n", maxNum);
    it prints as:
    "max uint64_t value in hex is ffffffff"
    but "ffffffff" is the max if I directly input it into MS calculator?
    So, what is hex number for 18446744073709551615? it should be
    0xffffffffffffffff, isn't it?
    The next thing I like to do is to represent the max
    number(18446744073709551615) as octet string, how can I do it?
    I tried following but it seems not right:
    unsigned long long maxNum=18446744073709551615;
    unsigned char hexNumStr[sizeof(unsigned long long)];
    memcpy(&hexNumStr, &maxNum, sizeof(unsigned long long));
    how am I going to print out the content of hexNum as a string?
    thx
     
    , Nov 16, 2006
    #1
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  2. Guest

    correction:
    > but "ffffffff" is NOT the max if I directly input it into MS calculator?
     
    , Nov 16, 2006
    #2
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  3. wrote:
    > Hi all,
    >
    > I have a confusion for representation of hex number:
    > For an unsigned long long number, its max value is:
    > unsigned long long maxNum = 18446744073709551615;


    No, the max value is...

    unsigned long long maxNum = -1;

    Or...

    unsigned long long maxNum = ULLONG_MAX; /* from <limits.h> */

    Which may be more than the number you posted.

    > if I do:
    > printf("max uint64_t value in hex is %x\n", maxNum);


    %x takes an unsigned int. There is no guarantee that maxNum is
    the same as an uint64_t.

    If you want the max of an unsigned long long, you can do
    something like...

    printf("max unsigned long long = %llu\n", -1ull);

    > it prints as:
    > "max uint64_t value in hex is ffffffff"


    That's because you lied to the compiler. [Your format string is a lie
    to the user, but the C standard permits such things. ;-]

    The printf function is a variadic function. You should look up the
    problems with such functions. You should definitely look up
    the specs for printf. Never use a function if you're only guessing
    what the behaviour is going to be.

    --
    Peter
     
    Peter Nilsson, Nov 16, 2006
    #3
  4. "" <> writes:
    > I have a confusion for representation of hex number:
    > For an unsigned long long number, its max value is:
    > unsigned long long maxNum = 18446744073709551615;
    > if I do:
    > printf("max uint64_t value in hex is %x\n", maxNum);
    > it prints as:
    > "max uint64_t value in hex is ffffffff"

    [...]

    The "%x" format expects an unsigned int. To print an unsigned long
    use, "%llx" (assuming your implementation of printf() supports it).

    But uint64_t (if it exists) isn't necessarily the same type as
    unsigned long long. For that, you can use the PRIx64 macro (I think
    that's right) in <inttypes.h>.

    --
    Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
    San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
    We must do something. This is something. Therefore, we must do this.
     
    Keith Thompson, Nov 16, 2006
    #4
  5. CBFalconer Guest

    Peter Nilsson wrote:
    > wrote:
    >>
    >> I have a confusion for representation of hex number:
    >> For an unsigned long long number, its max value is:
    >> unsigned long long maxNum = 18446744073709551615;

    >
    > No, the max value is...
    >
    > unsigned long long maxNum = -1;
    >
    > Or...
    >
    > unsigned long long maxNum = ULLONG_MAX; /* from <limits.h> */
    >
    > Which may be more than the number you posted.
    >
    >> if I do:
    >> printf("max uint64_t value in hex is %x\n", maxNum);

    >
    > %x takes an unsigned int. There is no guarantee that maxNum is
    > the same as an uint64_t.
    >
    > If you want the max of an unsigned long long, you can do
    > something like...
    >
    > printf("max unsigned long long = %llu\n", -1ull);
    >
    >> it prints as:
    >> "max uint64_t value in hex is ffffffff"

    >
    > That's because you lied to the compiler. [Your format string is
    > a lie to the user, but the C standard permits such things. ;-]


    However that %llu specifier will only work if your run time library
    is C99 compliant (to at least some extent). The reason is that
    printf is an interpreter for the format string, and if it doesn't
    understand, it doesn't understand.

    --
    Chuck F (cbfalconer at maineline dot net)
    Available for consulting/temporary embedded and systems.
    <http://cbfalconer.home.att.net>
     
    CBFalconer, Nov 17, 2006
    #5
  6. Tor Rustad Guest

    wrote:
    > Hi all,
    >
    > I have a confusion for representation of hex number:
    > For an unsigned long long number, its max value is:
    > unsigned long long maxNum = 18446744073709551615;


    Ohh... I wouldn't remember that number by hart.

    > if I do:
    > printf("max uint64_t value in hex is %x\n", maxNum);
    > it prints as:
    > "max uint64_t value in hex is ffffffff"


    %x format expect unsigned int
    %lx format expect unsigned long
    %llx format expect unsigned long long (C99)

    > but "ffffffff" is the max if I directly input it into MS calculator?
    > So, what is hex number for 18446744073709551615? it should be
    > 0xffffffffffffffff, isn't it?


    I don't have support for these C99 features myself...

    #include <inttypes.h>
    #include <stdio.h>

    int main(void)
    {
    uint64_t maxNum = UINT64_MAX;

    printf("max uint64_t value in hex is %016"
    PRIx64 "\n", maxNum);

    printf("max uint64_t value in octal is %"
    PRIo64 "\n", maxNum);

    return 0;
    }


    > The next thing I like to do is to represent the max
    > number(18446744073709551615) as octet string, how can I do it?


    Look at snprintf(...) and above.

    --
    Tor <torust AT online DOT no>
     
    Tor Rustad, Nov 17, 2006
    #6
  7. santosh Guest

    wrote:
    > Hi all,
    >
    > I have a confusion for representation of hex number:
    > For an unsigned long long number, its max value is:
    > unsigned long long maxNum = 18446744073709551615;


    Not necessarily. It's maximum value is yieled by the macro ULLONG_MAX
    in limits.h.

    > if I do:
    > printf("max uint64_t value in hex is %x\n", maxNum);
    > it prints as:
    > "max uint64_t value in hex is ffffffff"


    Thats because the 'x' format specifier expects an unsigned int
    argument. So maxNum is automatically converted to a smaller type and
    thus the value is truncated.

    Also the correct format specifiers for the types given in stdint.h,
    (like uint64_t), are given in inttypes.h. Here, maxNum is _not_ of type
    uint64_t as you indicate in your format string. In some implementations
    uint64_t may be a typedef for unsigned long long, but you cannot, and
    should not, make that assumption portably. You should treat both as
    different types.

    To print an unsigned long long variable in hexadecimal format use 'llx'
    as the format specifier.

    > but "ffffffff" is the max if I directly input it into MS calculator?


    Irrelevant. Has to do with the limitations of MS calc.

    > So, what is hex number for 18446744073709551615? it should be
    > 0xffffffffffffffff, isn't it?
    > The next thing I like to do is to represent the max
    > number(18446744073709551615) as octet string, how can I do it?


    printf("maxNum in octal format = %llo\n", maxNum);

    > I tried following but it seems not right:
    > unsigned long long maxNum=18446744073709551615;
    > unsigned char hexNumStr[sizeof(unsigned long long)];
    > memcpy(&hexNumStr, &maxNum, sizeof(unsigned long long));
    > how am I going to print out the content of hexNum as a string?


    Your above method is totally wrong. To print out, in string form, a
    variable, use one of printf(), fprintf(), sprintf(), vsprintf() etc.
     
    santosh, Nov 17, 2006
    #7
  8. CBFalconer Guest

    santosh wrote:
    > wrote:
    >>
    >> I have a confusion for representation of hex number:
    >> For an unsigned long long number, its max value is:
    >> unsigned long long maxNum = 18446744073709551615;

    >
    > Not necessarily. It's maximum value is yieled by the macro
    > ULLONG_MAX in limits.h.
    >
    >> if I do:
    >> printf("max uint64_t value in hex is %x\n", maxNum);
    >> it prints as:
    >> "max uint64_t value in hex is ffffffff"

    >
    > Thats because the 'x' format specifier expects an unsigned int
    > argument. So maxNum is automatically converted to a smaller type
    > and thus the value is truncated.


    No it isn't. You have lied to the compiler, and invoked undefined
    behaviour. It could simply cause all your toilets to overflow at
    once.

    --
    Chuck F (cbfalconer at maineline dot net)
    Available for consulting/temporary embedded and systems.
    <http://cbfalconer.home.att.net>
     
    CBFalconer, Nov 17, 2006
    #8
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