typedef fixed length array

Question

I have to define a 24-bit data type I am using char 3  to represent the type  Can I typedef char 3  to type24  I tried it in a code sample  I put typedef char 3  type24  in my header file  The compiler did not complain about it  But when I defined a function void foo type24 val     in my C file  it did complain  I would like to be able to define functions like type24 to int32 type24 val  instead of type24 to int32 char value 3

User · Answer

Here s a short example of why typedef array can be confusingly inconsistent   The other answers provide a workaround    include  lt stdio h gt  typedef char type24 3    int func type24 a            type24 b          printf  sizeof a  is  zu n  sizeof a            printf  sizeof b  is  zu n  sizeof b            return 0     int main void            type24 a          return func a       This produces the output  sizeof a  is 8 sizeof b  is 3   because type24 as a parameter is a pointer    In C  arrays are always passed as pointers    The gcc8 compiler will issue a warning by default  thankfully

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From R   s answer      However  this is probably a very bad idea  because the resulting type   is an array type  but users of it won t see that it s an array type    If used as a function argument  it will be passed by reference  not by   value  and the sizeof for it will then be wrong    Users who don t see that it s an array will most likely write something like this  which fails     include  lt stdio h gt   typedef int twoInts 2    void print twoInts  twoIntsPtr   void intermediate  twoInts twoIntsAppearsByValue    int main          twoInts a      a 0    0      a 1    1      print  amp a       intermediate a       return 0    void intermediate twoInts b        print  amp b      void print twoInts  c       printf   d n d n     c  0     c  1        It will compile with the following warnings   In function    intermediate     warning  passing argument 1 of    print    from incompatible pointer type  enabled by default      print  amp b          note  expected    int     2     but argument is of type    int           void print twoInts  twoIntsPtr                And produces the following output   0 1 -453308976 32767

User · Answer

Arrays can t be passed as function parameters by value in C   You can put the array in a struct   typedef struct type24       char byte 3     type24    and then pass that by value  but of course then it s less convenient to use  x byte 0  instead of x 0    Your function type24 to int32 char value 3   actually passes by pointer  not by value  It s exactly equivalent to type24 to int32 char  value   and the 3 is ignored   If you re happy passing by pointer  you could stick with the array and do   type24 to int32 const type24  value     This will pass a pointer-to-array  not pointer-to-first-element  so you use it as     value  0    I m not sure that s really a gain  since if you accidentally write value 1  then something stupid happens

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Building off the accepted answer  a multi-dimensional array type  that is a fixed-length array of fixed-length arrays  can t be declared with typedef char M  T N       wrong   instead  the intermediate 1D array type can be declared and used as in the accepted answer  typedef char T t M   typedef T t T N    or  T can be declared in a single  arguably confusing  statement  typedef char T N  M    which defines a type of N arrays of M chars  be careful about the order  here

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To use the array type properly as a function argument or template parameter  make a struct instead of a typedef  then add an operator   to the struct so you can keep the array like functionality like so   typedef struct type24     char amp  operator   int i    return byte i       char byte 3     type24   type24 x  x 2     r   char c   x 2

User · Answer

You want  typedef char type24 3     C type declarations are strange that way   You put the type exactly where the variable name would go if you were declaring a variable of that type

User · Answer

The typedef would be  typedef char type24 3     However  this is probably a very bad idea  because the resulting type is an array type  but users of it won t see that it s an array type  If used as a function argument  it will be passed by reference  not by value  and the sizeof for it will then be wrong   A better solution would be  typedef struct type24   char x 3     type24    You probably also want to be using unsigned char instead of char  since the latter has implementation-defined signedness

[c] typedef fixed length array

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