printf wprintf s S ls char and wchar Errors not announced by a compiler warning

Question

I have tried the following code   wprintf L 1  s n   some string      Good wprintf L 2  s n  L some string      Not good - gt  print only first character of the string printf  3  s n   some string      Good   printf  4  s n  L some string      Doesn t compile printf   n    wprintf L 1  S n   some string      Not good - gt  print some funny stuff wprintf L 2  S n  L some string      Good   printf  3  S n   some string      Doesn t compile printf  4  S n  L some string       Good   And I get the following output   1 some string 2 s 3 some string  1 g1  s  2 some string 4 some string   So  it seems that both wprintf and printf are able to print correctly both a char  and a wchar   but only if the exact specifier is used  If the wrong specifier is used  you might not get a compiling error  nor warning   and end up with wrong behavior  Do you experience the same behaviour   Note  This was tested under Windows  compiled with MinGW and g   4 7 2  I will check gcc later   Edit  I also tried  ls  result is in the comments   printf   n    wprintf L 1  ls n   some string      Not good - gt  print funny stuff wprintf L 2  ls n  L some string      Good    printf  3  ls n   some string      Doesn t compile printf  4  ls n  L some string       Good

User · Accepted Answer

I suspect GCC  mingw  has custom code to disable the checks for the wide printf functions on Windows  This is because Microsoft s own implementation  MSVCRT  is badly wrong and has  s and  ls backwards for the wide printf functions  since GCC can t be sure whether you will be linking with MS s broken implementation or some corrected one  the least-obtrusive thing it can do is just shut off the warning

User · Answer

For s  When used with printf functions  specifies a single-byte or multi-byte character string  when used with wprintf functions  specifies a wide-character string  Characters are displayed up to the first null character or until the precision value is reached   For S  When used with printf functions  specifies a wide-character string  when used with wprintf functions  specifies a single-byte or multi-byte character string  Characters are displayed up to the first null character or until the precision value is reached   In Unix-like platform  s and S have the same meaning as windows platform   Reference  https   msdn microsoft com en-us library hf4y5e3w aspx

User · Answer

At least in Visual C        printf  and other ACSII functions            s represents an ASCII string          S is a Unicode string     wprintf  and other Unicode functions            s is a Unicode string          S is an ASCII string  As far as no compiler warnings  printf uses a variable argument list  with only the first argument able to be type checked   The compiler is not designed to parse the format string and type check the parameters that match   In cases of functions like printf  that is up to the programmer

User · Answer

S seems to conform to The Single Unix Specification v2 and is also part of the current  2008  POSIX specification   Equivalent C99 conforming format specifiers would be  s and  ls

User · Answer

The format specifers matter    s  says that the next string is a narrow string   ascii  and typically 8 bits per character     S  means wide char string  Mixing the two will give  undefined behaviour   which includes printing garbage  just one character or nothing    One character is printed because wide chars are  for example  16 bits wide  and the first byte is non-zero  followed by a zero byte -  end of string in narrow strings  This depends on byte-order  in a  big endian  machine  you d get no string at all  because the first byte is zero  and the next byte contains a non-zero value

User · Answer

Answer A  None of the answers above pointed out why you might not see some of your prints  This is also because here you are dealing with streams  I didn t know this  and stream has something called orientation  Let me cite something from this source        Narrow and wide orientation      A newly opened stream has no orientation  The first call to any I O function   establishes the orientation       A wide I O function makes the stream   wide-oriented  a narrow I O function makes the stream narrow-oriented    Once set  the orientation can only be changed with freopen        Narrow I O   functions cannot be called on a wide-oriented stream  wide I O   functions cannot be called on a narrow-oriented stream  Wide I O   functions convert between wide and multibyte characters as if by   calling mbrtowc and wcrtomb  Unlike the multibyte character strings   that are valid in a program  multibyte character sequences in the file   may contain embedded nulls and do not have to begin or end in the   initial shift state    So once you use printf   your orientation becomes narrow and from this point on you can t get anything out of wprintf   and you realy don t  Unless you use freeopen   which is intended to be used on files      Answer B  As it turns out you can use freeopen   like this   freopen NULL   w   stdout                  To make stream  not defined  again  Try this example    include  lt stdio h gt   include  lt wchar h gt   include  lt locale h gt   int main void           We set locale which is the same as the enviromental variable  LANG en US UTF-8       setlocale LC ALL   en US UTF-8            We define array of wide characters  We indicate this on both sides of equal sign        with  wchar t  on the left and  L  on the right      wchar t y 100    L         a  p     e  a    est   n           We print header in ASCII characters     wprintf L content-type text html  charset utf-8 n n            A newly opened stream has no orientation  The first call to any I O function        establishes the orientation  a wide I O function makes the stream wide-oriented         a narrow I O function makes the stream narrow-oriented  Once set  we must respect        this  so for the time being we are stuck with either printf   or wprintf         wprintf L  S n   y         Conversion specifier  S is not standardized         wprintf L  ls n   y        Conversion specifier  s with length modifier  l is                                 standardized             At this point curent orientation of the stream is wide and this is why folowing        narrow function won t print anything  Whether we should use wprintf   or printf          is primarily a question of how we want output to be encoded       printf  1 n                Print narrow string of characters with a narrow function     printf   s n    2          Print narrow string of characters with a narrow function     printf   ls n  L 3         Print wide string of characters with a narrow function         Now we reset the stream to no orientation      freopen NULL   w   stdout        printf  4 n                Print narrow string of characters with a narrow function     printf   s n    5          Print narrow string of characters with a narrow function     printf   ls n  L 6         Print wide string of characters with a narrow function      return 0

[c] printf, wprintf, %s, %S, %ls, char* and wchar*: Errors not announced by a compiler warning?

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