How can one print a size t variable portably using the printf family

Question

I have a variable of type size t  and I want to print it using printf    What format specifier do I use to print it portably    In 32-bit machine   u seems right  I compiled with g   -g -W -Wall -Werror -ansi -pedantic  and there was no warning  But when I compile that code in 64-bit machine  it produces warning    size t x    lt something gt   printf  size    u n   x    warning  format   u  expects type  unsigned int        but argument 2 has type  long unsigned int    The warning goes away  as expected  if I change that to  lu   The question is  how can I write the code  so that it compiles warning free on both 32- and 64- bit machines   Edit  As a workaround  I guess one answer might be to  cast  the variable into an integer that is big enough  say unsigned long  and print using  lu  That would work in both cases  I am looking if there is any other idea

User · Answer

Will it warn you if you pass a 32-bit unsigned integer to a %lu format? It should be fine since the conversion is well-defined and doesn't lose any information.

I've heard that some platforms define macros in <inttypes.h> that you can insert into the format string literal but I don't see that header on my Windows C++ compiler, which implies it may not be cross-platform.

User · Answer

std  size t s   1024  std  cout  lt  lt  s     or any other kind of stream like stringstream

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For those talking about doing this in C   which doesn t necessarily support the C99 extensions  then I heartily recommend boost  format  This makes the size t type size question moot   std  cout  lt  lt  boost  format  Sizeof Var  is  d n     sizeof Var     Since you don t need size specifiers in boost  format  you can just worry about how you want to display the value

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if you want to print the value of a size t as a string you can do this   char text      Lets go fishing in stead of sitting on our but      size t line   2337200120702199116      on windows I64x or I64d others  lld or  llx if it works  zd or  zx    printf  number   I64d n    size t   amp text   printf  text   s n    char       amp line     result is   number  2337200120702199116  text  Lets go fishing in stead of sitting on our but        Edit  rereading the question because of the down votes i noted his problem is not  llu or  I64d but the size t type on different machines see this question https   stackoverflow com a 918909 1755797 http   www cplusplus com reference cstdio printf   size t is unsigned int on a 32bit machine and unsigned long long int on 64bit but  ll always expects a unsigned long long int   size t varies in length on different operating systems while  llu is the same

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As AraK said  the c   streams interface will always work portably       std  size t s   1024    std  cout  lt  lt  s     or any other kind of stream like stringstream     If you want C stdio  there is no portable answer to this for certain cases of  portable   And it gets ugly since as you ve seen  picking the wrong format flags may yield a compiler warning or give incorrect output   C99 tried to solve this problem with inttypes h formats like    PRIdMAX  n   But just as with   zu   not everyone supports c99  like MSVS prior to 2013   There are  msinttypes h  files floating around to deal with this   If you cast to a different type  depending on flags you may get a compiler warning for truncation or a change of sign  If you go this route pick a larger relevant fixed size type  One of unsigned long long and   llu  or unsigned long   lu  should work  but llu may also slow things down in a 32bit world as excessively large    Edit - my mac issues a warning in 64 bit for  llu not matching size t  even though  lu   llu  and size t are all the same size  And  lu and  llu are not the same size on my MSVS2012  So you may need to cast   use a format that matches    For that matter  you can go with fixed size types  such as int64 t  But wait  Now we re back to c99 c  11  and older MSVS fails again  Plus you also have casts  e g  map size   is not a fixed size type    You can use a 3rd party header or library such as boost  If you re not already using one  you may not want to inflate your project that way  If you re willing to add one just for this issue  why not use c   streams  or conditional compilation     So you re down to c   streams  conditional compilation  3rd party frameworks  or something sort of portable that happens to work for you

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For C89  use  lu and cast the value to unsigned long   size t foo      printf  foo    lu n    unsigned long  foo     For C99 and later  use  zu   size t foo      printf  foo    zu n   foo

User · Answer

C99 defines   zd  etc  for that   thanks to the commenters  There is no portable format specifier for that in C    - you could use  p  which woulkd word in these two scenarios  but isn t a portable choice either  and gives the value in hex   Alternatively  use some streaming  e g  stringstream  or a safe printf replacement such as Boost Format   I understand that this advice is only of limited use  and does require C      We ve used a similar approach fitted for our needs when implementing unicode support    The fundamental problem for C is that printf using an ellipsis is unsafe by design - it needs to determine the additional argument s size from the known arguments  so it can t be fixed to support  whatever you got   So unless your compiler implement some proprietary extensions  you are out of luck

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Use the z modifier   size t x        ssize t y        printf   zu n   x       prints as unsigned decimal printf   zx n   x       prints as hex printf   zd n   y       prints as signed decimal

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Looks like it varies depending on what compiler you re using  blech     gnu says  zu  or  zx  or  zd but that displays it as though it were signed  etc   Microsoft says  Iu  or  Ix  or  Id but again that s signed  etc   nbsp  mdash  but as of cl v19  in Visual Studio 2015   Microsoft supports  zu  see this reply to this comment       and of course  if you re using C    you can use cout instead as suggested by AraK

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printf  size    zu n   sizeof thing

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On some platforms and for some types there are specific printf conversion specifiers available  but sometimes one has to resort to casting to larger types   I ve documented this tricky issue here  with example code  http   www pixelbeat org programming gcc int types  and update it periodically with info on new platforms and types

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Extending on Adam Rosenfield s answer for Windows   I tested this code with on both VS2013 Update 4 and VS2015 preview      test c   include  lt stdio h gt   include  lt BaseTsd h gt     see the note below  int main         size t x   1      SSIZE T y   2      printf   zu n   x       prints as unsigned decimal     printf   zx n   x       prints as hex     printf   zd n   y       prints as signed decimal     return 0      VS2015 generated binary outputs      1   1   2   while the one generated by VS2013 says      zu   zx   zd   Note  ssize t is a POSIX extension and SSIZE T is similar thing in Windows Data Types  hence I added  lt BaseTsd h gt  reference   Additionally  except for the follow C99 C11 headers  all C99 headers are available in VS2015 preview   C11 -  lt stdalign h gt  C11 -  lt stdatomic h gt  C11 -  lt stdnoreturn h gt  C99 -  lt tgmath h gt  C11 -  lt threads h gt    Also  C11 s  lt uchar h gt  is now included in latest preview   For more details  see this old and the new list for standard conformance

[c] How can one print a size_t variable portably using the printf family?

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