Using boolean values in C

Question

C doesn t have any built-in boolean types  What s the best way to use them in C

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It is this    define TRUE 1  define FALSE 0

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You could use  Bool  but the return value must be an integer  1 for true  0 for false   However  It s recommended to include  and use bool as in C    as said in this reply from daniweb forum  as well as this answer  from this other stackoverflow question       Bool  C99 s boolean type  Using  Bool directly is only recommended if you re maintaining legacy code that already defines macros for bool  true  or false  Otherwise  those macros are standardized in the  header  Include that header and you can use bool just like you would in C

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This is what I use   enum  false  true   typedef  Bool bool     Bool is a built in type in C  It s intended for boolean values

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You can simply use the  define directive as follows    define TRUE 1  define FALSE 0  define NOT arg   arg    TRUE   FALSE   TRUE typedef int bool    And use as follows   bool isVisible   FALSE  bool isWorking   TRUE  isVisible   NOT isVisible     and so on

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A few thoughts on booleans in C   I m old enough that I just use plain ints as my boolean type without any typedefs or special defines or enums for true false values   If you follow my suggestion below on never comparing against boolean constants  then you only need to use 0 1 to initialize the flags anyway   However  such an approach may be deemed too reactionary in these modern times   In that case  one should definitely use  lt stdbool h gt  since it at least has the benefit of being standardized   Whatever the boolean constants are called  use them only for initialization   Never ever write something like  if  ready    TRUE      while  empty    FALSE        These can always be replaced by the clearer   if  ready      while   empty        Note that these can actually reasonably and understandably be read out loud   Give your boolean variables positive names  ie full instead of notfull   The latter leads to code that is difficult to read easily   Compare  if  full      if   full        with  if   notfull      if  notfull        Both of the former pair read naturally  while  notfull is awkward to read even as it is  and becomes much worse in more complex boolean expressions   Boolean arguments should generally be avoided   Consider a function defined like this  void foo bool option            Within the body of the function  it is very clear what the argument means since it has a convenient  and hopefully meaningful  name   But  the call sites look like  foo TRUE   foo FALSE     Here  it s essentially impossible to tell what the parameter meant without always looking at the function definition or declaration  and it gets much worse as soon if you add even more boolean parameters   I suggest either  typedef enum   OPT ON  OPT OFF   foo option  void foo foo option option     or   define OPT ON true  define OPT OFF false void foo bool option            In either case  the call site now looks like  foo OPT ON   foo OPT OFF     which the reader has at least a chance of understanding without dredging up the definition of foo

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First things first  C  i e  ISO IEC 9899 has had a boolean type for 19 years now  That is way longer time than the expected length of the C programming career with amateur academic professional parts combined when visiting this question  Mine does surpass that by mere perhaps 1-2 years  It means that during the time that an average reader has learnt anything at all about C  C actually has had the boolean data type   For the datatype   include  lt stdbool h gt   and use true  false and bool  Or do not include it  and use  Bool  1 and 0 instead     There are various dangerous practices promoted in the other answers to this thread  I will address them   typedef int bool   define true 1  define false 0   This is no-no  because a casual reader - who did learn C within those 19 years - would expect that bool refers to the actual bool data type and would behave similarly  but it doesn t  For example  double a        bool b   a    With C99 bool   Bool  b would be set to false iff a was zero  and true otherwise  C11 6 3 1 2p1        When any scalar value is converted to  Bool  the result is 0 if the value compares equal to 0  otherwise  the result is 1  59          Footnotes      59  NaNs do not compare equal to 0 and thus convert to 1      With the typedef in place  the double would be coerced to an int - if the value of the double isn t in the range for int  the behaviour is undefined   Naturally the same applies to if true and false were declared in an enum   What is even more dangerous is declaring   typedef enum bool       false  true   bool    because now all values besides 1 and 0 are invalid  and should such a value be assigned to a variable of that type  the behaviour would be wholly undefined   Therefore iff you cannot use C99 for some inexplicable reason  for boolean variables you should use    type int and values 0 and 1 as-is  and carefully do domain conversions from any other values to these with double negation    or if you insist you don t remember that 0 is falsy and non-zero truish  at least use upper case so that they don t get confused with the C99 concepts  BOOL  TRUE and FALSE

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Here is the version that I used   typedef enum   false   0  true    false   bool    Because false only has one value  but a logical true could have many values  but technique sets true to be what the compiler will use for the opposite of false   This takes care of the problem of someone coding something that would come down to this   if  true     false    I think we would all agree that that is not a good practice  but for the one time cost of doing  true    false  we eliminate that problem    EDIT  In the end I used   typedef enum   myfalse   0  mytrue    myfalse   mybool    to avoid name collision with other schemes that were defining true and false   But the concept remains the same    EDIT  To show conversion of integer to boolean   mybool somebool  int someint   5  somebool     someint    The first  right most    converts the non-zero integer to a 0  then the second  left most    converts the 0 to a myfalse value   I will leave it as an exercise for the reader to convert a zero integer      EDIT  It is my style to use the explicit setting of a value in an enum when the specific value is required even if the default value would be the same   Example  Because false needs to be zero I use false   0  rather than false

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Nowadays C99 supports boolean types but you need to  include  lt stdbool h gt    Example    include  lt stdbool h gt   int main           bool arr 2     true  false         printf   d n   arr 0   amp  amp  arr 1        printf   d n   arr 0     arr 1         return 0        Output   0 1

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If you are using C99 then you can use the  Bool type  No  includes are necessary  You do need to treat it like an integer  though  where 1 is true and 0 is false   You can then define TRUE and FALSE    Bool this is a Boolean var   1      or using it with true and false  define TRUE 1  define FALSE 0  Bool var   TRUE

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Just a complement to other answers and some clarification  if you are allowed to use C99    ------- ---------------- ------------------------- --------------------     Name   Characteristic   Dependence in stdbool h          Value          ------- ---------------- ------------------------- --------------------     Bool     Native type       Don t need header                            ------- ---------------- ------------------------- --------------------     bool        Macro                 Yes             Translate to  Bool    ------- ---------------- ------------------------- --------------------     true        Macro                 Yes               Translate to 1      ------- ---------------- ------------------------- --------------------    false        Macro                 Yes               Translate to 0      ------- ---------------- ------------------------- --------------------    Some of my preferences     Bool or bool   Both are fine  but bool looks better than the keyword  Bool  Accepted values for bool and  Bool are  false or true  Assigning 0 or 1 instead of false or true is valid  but is harder to read and understand the logic flow    Some info from the standard     Bool is NOT unsigned int  but is part of the group unsigned integer types  It is large enough to hold the values 0 or 1  DO NOT  but yes  you are able to redefine bool true and false but sure is not a good idea  This ability is considered obsolescent and will be removed in future  Assigning an scalar type  arithmetic types and pointer types  to  Bool or bool  if the scalar value is equal to 0 or compares to 0 it will be 0  otherwise the result is 1   Bool x   9  9 is converted to 1 when assigned to x   Bool is 1 byte  8 bits   usually the programmer is tempted to try to use the other bits  but is not recommended  because the only guaranteed that is given is that only one bit is use to store data  not like type char that have 8 bits available

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Conditional expressions are considered to be true if they are non-zero  but the C standard requires that logical operators themselves return either 0 or 1    Tom   define TRUE  FALSE is bad and is completely pointless   If the header file makes its way into compiled C   code  then it can lead to problems   void foo bool flag         int flag   TRUE  foo flag     Some compilers will generate a warning about the int    bool conversion   Sometimes people avoid this by doing   foo flag    TRUE     to force the expression to be a C   bool   But if you  define TRUE  FALSE  you end up with   foo flag     0     which ends up doing an int-to-bool comparison that can trigger the warning anyway

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From best to worse  Option 1  C99 and newer   include  lt stdbool h gt   Option 2 typedef enum   false  true   bool   Option 3 typedef int bool  enum   false  true     Option 4 typedef int bool   define true 1  define false 0   Explanation  Option 1 will work only if you use C99  or newer  and it s the  quot standard way quot  to do it  Choose this if possible  Options 2  3 and 4 will have in practice the same identical behavior   2 and  3 don t use  defines though  which in my opinion is better   If you are undecided  go with  1

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If you are using a C99 compiler it has built-in support for bool types    include  lt stdbool h gt  int main       bool b   false    b   true      http   en wikipedia org wiki Boolean data type

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Anything nonzero is evaluated to true in boolean operations  so you could just   define TRUE 1  define FALSE 0   and use the constants

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typedef enum       false   0      true   t bool

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A boolean in C is an integer  zero for false and non-zero for true   See also Boolean data type  section C  C    Objective-C  AWK

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You can use a char  or another small number container for it   Pseudo-code   define TRUE  1  define FALSE 0  char bValue   TRUE

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C has a boolean type  bool  at least for the last 10    years   Include stdbool h and true false will work as expected

[c] Using boolean values in C

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