Correct format specifier to print pointer or address

Question

Which format specifier should I be using to print the address of a variable? I am confused between the below lot.

%u - unsigned integer

%x - hexadecimal value

%p - void pointer

Which would be the optimum format to print an address?

User · Answer

You can use  x or  X or  p  all of them are correct    If you use  x  the address is given as lowercase  for example  a3bfbc4 If you use  X  the address is given as uppercase  for example  A3BFBC4   Both of these are correct   If you use  x or  X it s considering six positions for the address  and if you use  p it s considering eight positions for the address  For example

User · Answer

As an alternative to the other  very good  answers  you could cast to uintptr t or intptr t  from stdint h inttypes h  and use the corresponding integer conversion specifiers  This would allow more flexibility in how the pointer is formatted  but strictly speaking an implementation is not required to provide these typedefs

User · Answer

Use  p  for  pointer   and don t use anything else   You aren t guaranteed by the standard that you are allowed to treat a pointer like any particular type of integer  so you d actually get undefined behaviour with the integral formats   For instance   u expects an unsigned int  but what if void  has a different size or alignment requirement than unsigned int        See Jonathan s fine answer   Alternatively to  p  you can use pointer-specific macros from  lt inttypes h gt   added in C99   All object pointers are implicitly convertible to void  in C  but in order to pass the pointer as a variadic argument  you have to cast it explicitly  since arbitrary object pointers are only convertible  but not identical to void pointers    printf  x lives at  p  n    void   amp x

User · Answer

p is the conversion specifier to print pointers  Use this   int a   42   printf   p n    void     amp a     Remember that omitting the cast is undefined behavior and that printing with p conversion specifier is done in an implementation-defined manner

User · Answer

The simplest answer  assuming you don t mind the vagaries and variations in format between different platforms  is the standard  p notation   The C99 standard  ISO IEC 9899 1999  says in   7 19 6 1   8      p The argument shall be a pointer to void  The value of the pointer is   converted to a sequence of printing characters  in an implementation-defined   manner     In C11     ISO IEC 9899 2011     the information is in   7 21 6 1   8    On some platforms  that will include a leading 0x and on others it won t  and the letters could be in lower-case or upper-case  and the C standard doesn t even define that it shall be hexadecimal output though I know of no implementation where it is not   It is somewhat open to debate whether you should explicitly convert the pointers with a  void    cast   It is being explicit  which is usually good  so it is what I do   and the standard says  the argument shall be a pointer to void    On most machines  you would get away with omitting an explicit cast   However  it would matter on a machine where the bit representation of a char   address for a given memory location is different from the  anything else pointer  address for the same memory location   This would be a word-addressed  instead of byte-addressed  machine   Such machines are not common  probably not available  these days  but the first machine I worked on after university was one such  ICL Perq    If you aren t happy with the implementation-defined behaviour of  p  then use C99  lt inttypes h gt  and uintptr t instead   printf  0x   PRIXPTR   n    uintptr t your pointer     This allows you to fine-tune the representation to suit yourself   I chose to have the hex digits in upper-case so that the number is uniformly the same height and the characteristic dip at the start of 0xA1B2CDEF appears thus  not like 0xa1b2cdef which dips up and down along the number too   Your choice though  within very broad limits   The  uintptr t  cast is unambiguously recommended by GCC when it can read the format string at compile time   I think it is correct to request the cast  though I m sure there are some who would ignore the warning and get away with it most of the time     Kerrek asks in the comments      I m a bit confused about standard promotions and variadic arguments  Do all pointers get standard-promoted to void   Otherwise  if int  were  say  two bytes  and void  were 4 bytes  then it d clearly be an error to read four bytes from the argument  non    I was under the illusion that the C standard says that all object pointers must be the same size  so void   and int   cannot be different sizes   However  what I think is the relevant section of the C99 standard is not so emphatic  though I don t know of an implementation where what I suggested is true is actually false         6 2 5 Types        26 A pointer to void shall have the same representation and alignment requirements as a pointer to a character type 39  Similarly  pointers to qualified or unqualified versions of compatible types shall have the same representation and alignment requirements  All pointers to structure types shall have the same representation and alignment requirements as each other  All pointers to union types shall have the same representation and alignment requirements as each other  Pointers to other types need not have the same representation or alignment requirements       39  The same representation and alignment requirements are meant to imply interchangeability as arguments to functions  return values from functions  and members of unions     C11 says exactly the same in the section   6 2 5    28  and footnote 48    So  all pointers to structures must be the same size as each other  and must share the same alignment requirements  even though the structures the pointers point at may have different alignment requirements   Similarly for unions   Character pointers and void pointers must have the same size and alignment requirements   Pointers to variations on int  meaning unsigned int and signed int  must have the same size and alignment requirements as each other  similarly for other types   But the C standard doesn t formally say that sizeof int       sizeof void      Oh well  SO is good for making you inspect your assumptions   The C standard definitively does not require function pointers to be the same size as object pointers   That was necessary not to break the different memory models on DOS-like systems   There you could have 16-bit data pointers but 32-bit function pointers  or vice versa   This is why the C standard does not mandate that function pointers can be converted to object pointers and vice versa   Fortunately  for programmers targetting POSIX   POSIX steps into the breach and does mandate that function pointers and data pointers are the same size        2 12 3 Pointer Types      All function pointer types shall have the same representation as the type pointer to void  Conversion of a function pointer to void   shall not alter the representation  A void   value resulting from such a conversion can be converted back to the original function pointer type  using an explicit cast  without loss of information       Note    The ISO C standard does not require this  but it is required for POSIX conformance    So  it does seem that explicit casts to void   are strongly advisable for maximum reliability in the code when passing a pointer to a variadic function such as printf     On POSIX systems  it is safe to cast a function pointer to a void pointer for printing   On other systems  it is not necessarily safe to do that  nor is it necessarily safe to pass pointers other than void   without a cast

[c] Correct format specifier to print pointer or address?

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