initializer element not constant? - c

Question

I am relatively knew to C and only learning pieces of it to publish a Pebble C/PebbleKitJS app to track buses. So far I have the data being processed on a Node server, and I am getting ready to have the data processed by a JS File. MY one problem however lies within the C Code.
This code process data stored in a Key Dictionary sent from JS and assigns it to a variable for use below. By using #define var 9, I can successfully have the .high value set to 9. But through an int var, it fails and throws the error:initializer element not constant?? .
What does this error mean, and what exactly is the difference between static and constant if i don't define it. apparently static vars don't return anything? Some help would be much appreciated.
UPDATE: The problem still isn't fixed. The following new error message occurs in addition to the initializer one. error: (near initialization for 's_data_points[0].high')
int key0_buffer;
void process_tuple(Tuple *t)
{
//Get key
int key = t->key;
//Get integer value, if present
int value = t->value->int32;
//Get string value, if present
char string_value[32];
strcpy(string_value, t->value->cstring);
//Decide what to do
switch(key) {
case key_0:
//Location received
key0_buffer = value;
break;
}
}
static WeatherAppDataPoint s_data_points[] = {
{
.city = "San Diego",
.description = "surfboard :)",
.icon = WEATHER_APP_ICON_GENERIC_WEATHER,
.current = 110,
.high = key0_buffer,
.low = 9,
},
};

Answer 1

Try this instead:
enum { key0_buffer = 9 };
C doesn't provide for runtime computations while initializing global variables. (The concept does exist as a C++ feature called "dynamic initialization.")
The execution model is that it can store all the bytes of global variables in ROM, and then copy any modifiable variables into RAM together with a single memcpy. Assigning one global to another would be more complicated.
#define allows you to substitute the text 9, which is a constant expression.
Many frown upon using text substitutions to avoid variables as primitive, unnecessarily low-level, and potentially inefficient. In this case though, the results should be the same.
In C, enum constants have type int, so they are a suitable substitute. You're out of luck for other types, though.

Answer 2

There is a key difference between the code in your function and the code lower down that defines the static variable. The function code is executable -- these lines are going to get run when the function is called.
Your static declaration of WeatherAppDataPoint is just telling the compiler to create the static variable. The initialization values you are placing in that declaration tell the compiler what value to initialize this data to. That is why they must be constant -- these are the values that get loaded in before anything gets executed.
The #define statement just tells the preprocessor to replace all instances of "var" with the string "9". It is literally the same as a cut and paste operation in a text editor. This gets done before the compiler ever sees the code. This is why you have no problems with that; the compiler sees a literal constant, just as if you'd manually typed the "9" directly into the source code.
In 'C', variables don't return things, they store them. Only functions return things. If you want to have an integer declared somewhere, and then assign it to the value of your static variable, that is an actual executable line of code that will need to occur inside of a function somewhere (ie inside of your process_tuple function). The lines of code below the static declaration aren't executed at run time, they just setup the initial state of the program and tell the compiler how big the variable is.