How to create table (array) with extern values?

Question

I would like to create a static (file scope) table of data pointer, data size and data version. The problem is that the data are in external files, but constants in the extern files.

Example:

file1.c

    const unsigned char data1[] =
    {
    0x65, 0xF0, 0xA8, 0x5F, 0x5F,
    0x5F, 0x5F, 0x31, 0x32, 0x2E,
    0x31, 0xF1, 0x63, 0x4D, 0x43, 
    0x52, 0x45, 0x41, 0x54, 0x45,
    0x44, 0x20, 0x42, 0x59, 0x3A,
    0x20, 0x69, 0x73, 0x70, 0x56, 
// ...
    };
    const unsigned int data1_size =
        sizeof(data1) / sizeof(data1[0]);
    const unsigned int data1_version = 1;

file2.c

    const unsigned char data2[] =
    {
    0x20, 0x44, 0x61, 0x74, 0x61,
    0x20, 0x52, 0x6F, 0x77, 0x20,
    0x3D, 0x20, 0x34, 0x38, 0x12, 
//...
    };
    const unsigned int data2_size =
        sizeof(data2) / sizeof(data2[0]);
    const unsigned int data2_version = 1;

main.c

struct Data_Info
{
  unsigned char * data_ptr;
  unsigned int    data_size;
  unsigned int    data_version;
};

extern const unsigned char data1[];
extern const unsigned int  data1_size;
extern const unsigned int  data1_version;

extern const unsigned char data2[];
extern const unsigned int  data2_size;
extern const unsigned int  data2_version;

static struct Data_Info  Data_Info_Table[] =
{
  // How to set this up??
  // The compiler wants constant expressions here,
  //    and the extern is not considered a constant expression
  // This is what I tried:
  { data1, data1_size, data1_version},
  { data2, data2_size, data2_version},
};

int main(void)
{
  return 0;
} 

I am using Green Hills Compiler, 4.2.3. The exact error message:

error #28, expression must have a constant value.

See my earlier post:
http://stackoverflow.com/questions/2580115/c-external-const-ints-in-a-array-of-const-struct

Note: C++ tag is included since this also applies to C++

Answer 1

When the constants are defined, use the extern keyword.

Answer 2

I am not that much of a C expert, but the order of initialization across different translation units is most probably undefined (I know it is undefined in C++, and it would be a huge break of compatibility if the behavior differed in C), so even if it was allowed by the compiler it would be dangerous.

Now, adding the C++ tag is most probably not going to help much, as AFAIK this is not only dependent on the language, but also in the specific version of the language: C89/C99. In C++ it works if you move the initialization to the declaration of the constant in the header file. Again, I am no C expert.

Answer 3

Have you tried? (where X is 0 or 1.)

struct Data_Info    
{    
  unsigned char const * data_ptr;    
  unsigned int    data_size;    
  unsigned int    data_version;    
};    

fileX.h

extern const struct Data_Info data_infoX;

fileX.c

const struct Data_Info data_infoX = { dataX, dataX_size, dataX_version };

main.c

const struct Data_Info const * Data_Info_Table[] = 
{ 
  &data_info1,
  &data_info2
}; 

Answer 4

The problem is that the compiler doesn't know the value to place into the data_size and data_version members of the field. There are a couple of ways you can get around this without too much fuss.

Approach 1:

#include "data1.c"
#include "data2.c"

...
static struct Data_Info  Data_Info_Table[] =
{
  { data1, sizeof(data1), 1},
  { data2, sizeof(data2), 1},
};

Approach 2:

struct Data_Info
{
  unsigned char * data_ptr;
  unsigned int *  data_size;
  unsigned int *  data_version;
};

...

static struct Data_Info  Data_Info_Table[] =
{
  { data1, &data1_size, &data1_version},
  { data2, &data2_size, &data2_version},
};

A third approach, as already demonstrated by jyoung, is to allocate the members in the separate data files and include their addresses in the Data_Info_Table.