printf formatting for hex

Question

This is more of a curious query than an important question  but why when printing hex as an 8 digit number with leading zeros  does this   08X Not display the same result as 0x 08X   When I try to use the former  the 08 formatting flag is removed  and it doesn t work with just 8   Again I was just curious

User · Answer

The   part gives you a 0x in the output string  The 0 and the x count against your  8  characters listed in the 08 part  You need to ask for 10 characters if you want it to be the same   int i   7   printf    010x n   i       gives 0x00000007 printf  0x 08x n   i       gives 0x00000007 printf    08x n   i        gives 0x000007   Also changing the case of x  affects the casing of the outputted characters   printf   04x   4779      gives 12ab printf   04X   4779      gives 12AB

User · Answer

You could always use  quot  p quot  in order to display 8 bit hex numbers  int main  void        uint8 t a      uint32 t b      a 15      b a lt  lt 28      printf  quot  p quot   b       return 0     Output  0xf0000000

User · Answer

The  0x  counts towards the eight character count  You need    010x    Note that   does not append the 0x to 0 - the result will be 0000000000 - so you probably actually should just use  0x 08x  anyway

User · Answer

The   08X conversion must precede the value with 0X  that is required by the standard   There s no evidence in the standard that the   should alter the behaviour of the 08 part of the specification except that the 0X prefix is counted as part of the length  so you might want need to use   010X   If  like me  you like your hex presented as 0x1234CDEF  then you have to use 0x 08X to achieve the desired result   You could use    8X and that should also insert the leading zeroes   Try variations on the following code    include  lt stdio h gt   int main void        int j   0      printf  0x  8X     08X      8X     010x n   j  j  j  j       for  int i   0  i  lt  8  i                  j    j  lt  lt  4     i   6           printf  0x  8X     08X      8X     010x n   j  j  j  j             return 0       On an RHEL 5 machine  and also on Mac OS X  10 7 5   the output was   0x00000000   00000000   00000000   0000000000 0x00000006   0X000006   0X00000006   0x00000006 0x00000067   0X000067   0X00000067   0x00000067 0x00000678   0X000678   0X00000678   0x00000678 0x00006789   0X006789   0X00006789   0x00006789 0x0006789A   0X06789A   0X0006789A   0x0006789a 0x006789AB   0X6789AB   0X006789AB   0x006789ab 0x06789ABC   0X6789ABC   0X06789ABC   0x06789abc 0x6789ABCD   0X6789ABCD   0X6789ABCD   0x6789abcd   I m a little surprised at the treatment of 0  I m not clear why the 0X prefix is omitted  but with two separate systems doing it  it must be standard   It confirms my prejudices against the   option     The treatment of zero is according to the standard      ISO IEC 9899 2011   7 21 6 1 The fprintf function        6 The flag characters and their meanings are            The result is converted to an  alternative form       For x  or X    conversion  a nonzero result has 0x  or 0X  prefixed to it         Emphasis added      Note that using   X will use upper-case letters for the hex digits and 0X as the prefix  using   x will use lower-case letters for the hex digits and 0x as the prefix   If you prefer 0x as the prefix and upper-case letters  you have to code the 0x separately  0x X   Other format modifiers can be added as needed  of course   For printing addresses  use the  lt inttypes h gt  header and the uintptr t type and the PRIXPTR format macro    include  lt inttypes h gt   include  lt stdio h gt   int main void        void  address    amp address       amp address has type void    but it converts to void       printf  Address 0x  12  PRIXPTR   n    uintptr t address       return 0      Example output   Address 0x7FFEE5B29428   Choose your poison on the length     I find that a precision of 12 works well for addresses on a Mac running macOS   Combined with the   to specify the minimum precision  digits   it formats addresses reliably   If you set the precision to 16  the extra 4 digits are always 0 in my experience on the Mac  but there s certainly a case to be made for using 16 instead of 12 in portable 64-bit code  but you d use 8 for 32-bit code

[c] printf() formatting for hex

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