[SOLVED] Issue : Initializer element is not constant on C #define

Issue

I am using a #define to call a function and get the following gcc error: "error: initializer element is not constant" on the directive.

Here, the define directive:

#define HASH(STRING_VALUE) (lookup_hash(STRING_VALUE))

Function implementation:

static inline uint32_t lookup_hash(const char *str)
{
    uint32_t hash = 5381;
    int32_t c;

    while ((c = *str++))
    {
        hash = ((hash << 5) + hash) + c;
    }

    return hash;
}

Function call example:

static const asr_handlerTree_t asr_LT_nwk_orders[]=
{
    {HASH("link"), NULL, asr_link_funct},
    {HASH("unlink"), NULL, asr_unlink_funct},

    //  Used at runtime to determinate table limit, keep at table tail
    {(uint32_t) 0, NULL, NULL}
};

Why gcc compiler raise the issue as directive calls should only be replaced by function at pre-processing ?

---

Update: Add structure declaration following answer feedbacks:

typedef struct _asr_handlerTree_type_
{
    const uint32_t hashedIntent_u32;
    const asr_handlerTree_t * branchs;
    const funct_t Func;
} asr_handlerTree_t;

Solution

Variables with static storage duration, i.e. those declared at file scope or with the static keyword, can only be initialized with constant expressions. This means you can't use the values of other variables (although you can use their addresses) or make function calls.

You would need to initialize this particular field with a dummy value, then have a function that is called at the start of main that assigns the values that need to be set at runtime.

static const asr_handlerTree_t asr_LT_nwk_orders[]=
{
    {0, NULL, asr_link_funct},
    {0, NULL, asr_unlink_funct},

    //  Used at runtime to determinate table limit, keep at table tail
    {(uint32_t) 0, NULL, NULL}
};

void set_hashes()
{
    asr_LT_nwk_orders[0].hash = HASH("link");
    asr_LT_nwk_orders[1].hash = HASH("unlink");
}

Answered By - dbush

Answer Checked By - Willingham (WPSolving Volunteer)