Error initializer element is not constant when trying to initialize variable with const

Question

I get an error on line 6  initialize my foo to foo init  of the following program and I m not sure I understand why   typedef struct foo t       int a  b  c    foo t   const foo t foo init     1  2  3    foo t my foo   foo init   int main         return 0      Keep in mind this is a simplified version of a larger  multi-file project I m working on   The goal was to have a single constant in the object file  that multiple files could use to initialize a state structure   Since it s an embedded target with limited resources and the struct isn t that small  I don t want multiple copies of the source   I d prefer not to use    define foo init   1  2  3     I m also trying to write portable code  so I need a solution that s valid C89 or C99   Does this have to do with the ORGs in an object file   That initialized variables go into one ORG and are initialized by copying the contents of a second ORG   Maybe I ll just need to change my tactic  and have an initializing function do all of the copies at startup   Unless there are other ideas out there

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It s a limitation of the language   In section 6 7 8 4      All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals    In section 6 6  the spec defines what must considered a constant expression   No where does it state that a const variable must be considered a constant expression   It is legal for a compiler to extend this  6 6 10 - An implementation may accept other forms of constant expressions  but that would limit portability   If you can change my foo so it does not have static storage  you would be okay   int main         foo t my foo   foo init      return 0

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In C language  objects with static storage duration have to be initialized with constant expressions  or with aggregate initializers containing constant expressions   A  large  object is never a constant expression in C  even if the object is declared as const    Moreover  in C language  the term  constant  refers to literal constants  like 1   a   0xFF and so on   enum members  and results of such operators as sizeof  Const-qualified objects  of any type  are not constants in C language terminology  They cannot be used in initializers of objects with static storage duration  regardless of their type   For example  this is NOT a constant  const int N   5      N  is not a constant in C      The above N would be a constant in C    but it is not a constant in C  So  if you try doing   static int j   N     ERROR      you will get the same error  an attempt to initialize a static object with a non-constant   This is the reason why  in C language  we predominantly use  define to declare named constants  and also resort to  define to create named aggregate initializers

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This is a bit old  but I ran into a similar issue  You can do this if you use a pointer    include  lt stdio h gt  typedef struct foo t        int a  int b  int c    foo t  static const foo t s FooInit      a 1   b 2   c 3       or a pointer static const foo t  const s pFooInit     amp  const foo t    a 2   b 4   c 6     int main  int argc  char   argv        const foo t  const f1    amp s FooInit      const foo t  const f2   s pFooInit      printf  Foo1    d   d   d n   f1- gt a  f1- gt b  f1- gt c       printf  Foo2    d   d   d n   f2- gt a  f2- gt b  f2- gt c       return 0

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gcc 7 4 0 can not compile codes as below    include  lt stdio h gt  const char   const str1    str1   const char   str2   str1  int main         printf   s -  s n   str1  str2       return 0      constchar c 3 21  error  initializer element is not constant  const char   str2   str1   In fact  a  const char    string is not a compile-time constant  so it can t be an initializer  But a  const char   const  string is a compile-time constant  it should be able to be an initializer  I think this is a small drawback of CLang   A function name is of course a compile-time constant So this code works   void func void        printf  func n      typedef void   func type  void   func type f   func  int main         f        return 0

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Just for illustration by compare and contrast The code is from http   www geeksforgeeks org g-fact-80   The code fails in gcc and passes in g      include lt stdio h gt  int initializer void        return 50     int main         int j      for  j 0 j lt 10 j                  static int i   initializer              The variable i is only initialized to one           printf   value of i    d    i           i              return 0

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I had this error in code that looked like this  int A   1  int B   A   The fix is to change it to this int A   1   define B A  The compiler assigns a location in memory to a variable  The second is trying a assign a second variable to the same location as the first - which makes no sense  Using the macro preprocessor solves the problem

[c] Error "initializer element is not constant" when trying to initialize variable with const

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