How to compare strings in C conditional preprocessor-directives

Question

I have to do something like this in C  It works only if I use a char  but I need a string  How can I do this    define USER  jack     jack or queen   if USER     jack   define USER VS  queen   elif USER     queen   define USER VS  jack   endif

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UPDATE  2021 01 04  One thing that has changed since I first posted this in 2014  is the format of  pragma message  Nowadays  the parens are required   pragma message   quot USER    IS  quot  USER   pragma message   quot USER VS IS  quot  USER VS   That said  the 2016 code  using characters  not strings  still works in VS2019  But  as  Artyer points out  the version involving c strcmp will NOT work in ANY modern compiler   UPDATE  2018 05 03  CAVEAT  Not all compilers implement the C  11 specification in the same way  The below code works in the compiler I tested on  while many commenters used a different compiler  Quoting from Shafik Yaghmour s answer at  Computing length of a C string at compile time  Is this really a constexpr   Constant expressions are not guaranteed to be evaluated at compile time  we only have a non-normative quote from draft C   standard section 5 19 Constant expressions that says this though         gt   Note  Constant expressions can be evaluated during translation    end note     That word can makes all the difference in the world  So  YMMV on this  or any  answer involving constexpr  depending on the compiler writer s interpretation of the spec   UPDATED 2016 01 31  As some didn t like my earlier answer because it avoided the whole compile time string compare aspect of the OP by accomplishing the goal with no need for string compares  here is a more detailed answer  You can t   Not in C98 or C99  Not even in C11   No amount of MACRO manipulation will change this  The definition of const-expression used in the  if does not allow strings  It does allow characters  so if you limit yourself to characters you might use this   define JACK  J   define QUEEN  Q    define CHOICE JACK        or QUEEN  your choice   if  J     CHOICE  define USER  quot jack quot   define USER VS  quot queen quot   elif  Q     CHOICE  define USER  quot queen quot   define USER VS  quot jack quot   else  define USER  quot anonymous1 quot   define USER VS  quot anonymous2 quot   endif   pragma message  quot USER    IS  quot  USER  pragma message  quot USER VS IS  quot  USER VS  You can  In C  11  If you define a compile time helper function for the comparison   2021 01 04  CAVEAT  This does not work in any MODERN compiler  See comment by  Artyer      compares two strings in compile time constant fashion constexpr int c strcmp  char const  lhs  char const  rhs         return     0     lhs 0    amp  amp     0     rhs 0      0             lhs 0     rhs 0      lhs 0  - rhs 0             c strcmp  lhs 1  rhs 1         some compilers may require   int lhs 0  -  int rhs 0     define JACK  quot jack quot   define QUEEN  quot queen quot    define USER JACK          or QUEEN  your choice   if 0    c strcmp  USER  JACK    define USER VS QUEEN  elif 0    c strcmp  USER  QUEEN    define USER VS JACK  else  define USER VS  quot unknown quot   endif   pragma message  quot USER    IS  quot  USER  pragma message  quot USER VS IS  quot  USER VS  So  ultimately  you will have to change the way you accomlish your goal of choosing final string values for USER and USER VS  You can t do compile time string compares in C99  but you can do compile time choosing of strings  If you really must do compile time sting comparisons  then you need to change to C  11 or newer variants that allow that feature   ORIGINAL ANSWER FOLLOWS  Try   define jack VS queen  define queen VS jack   define USER jack             jack    or queen  your choice  define USER VS USER   VS     jack VS or queen VS     stringify usage  S USER  or S USER VS  when you need the string form   define S U  S  U   define S  U   U  UPDATE  ANSI token pasting is sometimes less than obvious   -D Putting a single   before a macro causes it to be changed into a string of its value  instead of its bare value  Putting a double    between two tokens causes them to be concatenated into a single token  So  the macro USER VS has the expansion jack VS or queen VS  depending on how you set USER  The stringify macro S      uses macro indirection so the value of the named macro gets converted into a string  instead of the name of the macro  Thus USER   VS becomes jack VS  or queen VS   depending on how you set USER  Later  when the stringify macro is used as S USER VS  the value of USER VS  jack VS in this example  is passed to the indirection step S  jack VS  which converts its value  queen  into a string  quot queen quot   If you set USER to queen then the final result is the string  quot jack quot   For token concatenation  see  https   gcc gnu org onlinedocs cpp Concatenation html For token string conversion  see  https   gcc gnu org onlinedocs cpp Stringification html Stringification  UPDATED 2015 02 15 to correct a typo

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If your strings are compile time constants  as in your case  you can use the following trick    define USER JACK strcmp USER   jack    define USER QUEEN strcmp USER   queen    if  USER JACK    0  define USER VS USER QUEEN  elif USER QUEEN    0  define USER VS USER JACK  endif   The compiler can tell the result of the strcmp in advance and will replace the strcmp with its result  thus giving you a  define that can be compared with preprocessor directives  I don t know if there s any variance between compilers dependance on compiler options  but it worked for me on GCC 4 7 2   EDIT  upon further investigation  it look like this is a toolchain extension  not GCC extension  so take that into consideration

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define USER IS c0 c1 c2 c3 c4 c5 c6 c7 c8 c9   ch0  c0  amp  amp  ch1  c1  amp  amp  ch2  c2  amp  amp  ch3  c3  amp  amp  ch4  c4  amp  amp  ch5  c5  amp  amp  ch6  c6  amp  amp  ch7  c7     define ch0  j   define ch1  a   define ch2  c   define ch3  k    if USER IS  j   a   c   k  0 0 0 0   define USER VS  queen   elif USER IS  q   u   e   e   n  0 0 0   define USER VS  jack   endif   it basically a fixed length static char array initialized manually instead of a variable length static char array initialized automatically always ending with a terminating null char

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The answere by Patrick and by Jesse Chisholm made me do the following    define QUEEN  Q   define JACK  J    define CHECK QUEEN s   s  QUEEN   define CHECK JACK s   s  JACK    define USER  Q        later on in code        if CHECK QUEEN USER    compile queen func     elif CHECK JACK USER    compile jack func     elif  error  unknown user   endif   Instead of  define USER  Q   define USER QUEEN should also work but was not tested  also works and might be easier to handle   EDIT  According to the comment of  Jean-Fran  ois Fabre I adapted my answer

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Use numeric values instead of strings   Finally to convert the constants JACK or QUEEN to a string  use the stringize  and or tokenize  operators

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As already stated above  the ISO-C11 preprocessor does not support string comparison  However  the problem of assigning a macro with the    opposite value    can be solved with    token pasting    and    table access     Jesse   s simple concatenate stringify macro-solution fails with gcc 5 4 0 because the stringization is done before the evaluation of the concatenation  conforming to ISO C11   However  it can be fixed    define P  user  user     VS  define VS user  P   user   define S U  S  U   define S  U   U   define jack VS  queen  define queen VS jack  S  VS  jack   S  jack  S  VS  queen   S  queen    define USER jack             jack    or queen  your choice  define USER VS USER   VS     jack VS or queen VS S  USER  S  USER VS    The first line  macro P     adds one indirection to let the next line  macro VS    finish the concatenation before the stringization  see  Why do I need double layer of indirection for macros    The stringization macros  S   and S     are from Jesse   The table  macros jack VS and queen VS  which is much easier to maintain than the if-then-else construction of the OP is from Jesse    Finally  the next four-line block invokes the function-style macros  The last four-line block is from Jesse   s answer   Storing the code in foo c and invoking the preprocessor gcc -nostdinc -E foo c yields     1  foo c    1   lt built-in gt     1   lt command-line gt     1  foo c    9  foo c   queen   jack   jack   queen      jack   USER VS    The output is as expected  The last line shows that the USER VS macro is not expanded before stringization

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It s simple I think you can just say   define NAME JACK      if NAME    queen

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I don t think there is a way to do variable length string comparisons completely in preprocessor directives   You could perhaps do the following though    define USER JACK 1  define USER QUEEN 2   define USER USER JACK    if USER    USER JACK  define USER VS USER QUEEN  elif USER    USER QUEEN  define USER VS USER JACK  endif   Or you could refactor the code a little and use C code instead

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You can t do that if USER is defined as a quoted string  But you can do that if USER is just JACK or QUEEN or Joker or whatever  There are two tricks to use   Token-splicing  where you combine an identifier with another identifier by just concatenating their characters  This allows you to compare against JACK without having to  define JACK to something variadic macro expansion  which allows you to handle macros with variable numbers of arguments  This allows you to expand specific identifiers into varying numbers of commas  which will become your string comparison   So let s start out with   define JACK QUEEN OTHER u  EXPANSION1 ReSeRvEd   u  1  2  3   Now  if I write JACK QUEEN OTHER USER   and USER is JACK  the preprocessor turns that into EXPANSION1 ReSeRvEd   JACK  1  2  3  Step two is concatenation   define EXPANSION1 a  b  c  d  e  EXPANSION2 a  b  c  d  e   Now JACK QUEEN OTHER USER  becomes EXPANSION2 ReSeRvEd JACK  1  2  3  This gives the opportunity to add a number of commas according to whether or not a string matches   define ReSeRvEd JACK x x x  define ReSeRvEd QUEEN x x  If USER is JACK  JACK QUEEN OTHER USER  becomes EXPANSION2 x x x  1  2  3  If USER is QUEEN  JACK QUEEN OTHER USER  becomes EXPANSION2 x x  1  2  3  If USER is other  JACK QUEEN OTHER USER  becomes EXPANSION2 ReSeRvEd other  1  2  3  At this point  something critical has happened  the fourth argument to the EXPANSION2 macro is either 1  2  or 3  depending on whether the original argument passed was jack  queen  or anything else   So all we have to do is pick it out  For long-winded reasons  we ll need two macros for the last step  they ll be EXPANSION2 and EXPANSION3  even though one seems unnecessary  Putting it all together  we have these 6 macros   define JACK QUEEN OTHER u  EXPANSION1 ReSeRvEd   u  1  2  3   define EXPANSION1 a  b  c  d  e  EXPANSION2 a  b  c  d  e   define EXPANSION2 a  b  c  d       EXPANSION3 a  b  c  d   define EXPANSION3 a  b  c  d       d  define ReSeRvEd JACK x x x  define ReSeRvEd QUEEN x x  And you might use them like this  int main      if JACK QUEEN OTHER USER     1   printf  quot Hello  Jack  n quot     endif  if JACK QUEEN OTHER USER     2   printf  quot Hello  Queen  n quot     endif  if JACK QUEEN OTHER USER     3   printf  quot Hello  who are you  n quot     endif    Obligatory godbolt link  https   godbolt org z 8WGa19  MSVC Update  You have to parenthesize slightly differently to make things also work in MSVC   The EXPANSION  macros look like this   define EXPANSION1 a  b  c  d  e  EXPANSION2  a  b  c  d  e    define EXPANSION2 x  EXPANSION3 x  define EXPANSION3 a  b  c  d       d  Obligatory  https   godbolt org z 96Y8a1

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The following worked for me with clang  Allows what appears as symbolic macro value comparison   error xxx is just to see what compiler really does  Replacing cat definition with  define cat a b  a    b breaks things    define cat a      cat impl a    VA ARGS     define cat impl a      a      VA ARGS     define xUSER jack 0  define xUSER queen 1  define USER VAL cat xUSER  USER    define USER jack    jack or queen   if USER VAL  xUSER jack    error USER jack    define USER VS  queen   elif USER VAL  xUSER queen    error USER queen    define USER VS  jack   endif

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