Question

I got a code from an old project implementing AES on 802.15.4
It defines the the default key like this:

static uint8_t default_key_source[8] = {0xFF, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};

and afterwards define a table which uses this in the following manner:

uint8_t mac_key_table[34] =
{
    // KeyIdLookupList[1].LookupData : macDefaultKeySource || g_Sec_KeyIndex_1
    default_key_source[0], // LookupData[0]
    default_key_source[1], // LookupData[1]
    default_key_source[2], // LookupData[2]
    default_key_source[3], // LookupData[3]
    default_key_source[4], // LookupData[4]
    default_key_source[5], // LookupData[5]
    default_key_source[6], // LookupData[6]
...
}

I get “expression must have a constant value” error on compilation. which is strange because it used to compile on different compiler (don’t know exactly which, but I’m using IAR embedded workbench for ARM)

I tried adding const before the static declaration of the default_key_source. but it didn’t help, I get that the compiler can’t initialize a structure with variables, however these variables are constants, so what’s the problem here ?

Answer 1

An easy way to deal with such a situation in C is to use a macro, to be sure that you’d have to define the constants only in one place:

#define DEFAULT_KEY_SOURCE 0xFF, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00

// if you really need that variable
static default_key_source[8] = { DEFAULT_KEY_SOURCE };

uint8_t mac_key_table[34] =
{
  DEFAULT_KEY_SOURCE,
  ...
};