Conditional execution not working for this case of code? - c

Question

Why the code inside the #ifdef INITIALISATION and #endif is not executed?
int main(void)
{
uint8_t DLEVEL = 10;
#if DLEVEL > 5
#define INITIALISATION
#endif
while (1)
{
#ifdef INITIALISATION
Display(); // This line is never being executing :: please help for a solution
#endif
}
}
edited...
Thanks for the reply
Actually the below mentioned logic of the code is used as a part of my code memory optimization.
Now the code memory size is overflowed, so i need to execute the one time initializations when the SYM_DLEVEL value is 10 and then the value of SYM_DLEVEL is changed from 10 to 2, then the initial sections i need to be automatically commented or disabled and then only the Display_2() function needed to be enable and need to execute.
Is it possible?
#define SYM_DLEVEL 10
int main(void)
{
#if SYM_DLEVEL > 5
Display_1(); // need to execute this line once(before the value changes from 10 t0 2)
#endif
#define SYM_DLEVEL 2 // after this line execution i need to automatically disable the above section and automatically enable the below section
#if SYM_DLEVEL < 5
#define INITIALISATION // need to execute only when value changes from 10 to 2
#endif
while (1)
{
#ifdef INITIALISATION
Display_2();
#endif
}
}

Answer 1

It doesn't execute code because the preprocessor runs in the compiler, while the local variable DLEVEL is assigned during execution.
The preprocessor simply looks for a preprocessor symbol DLEVEL that evidently is undefined.
Undefined symbols are equated to 0, so the symbol INITIALISATION isn't defined.
To make it work define a preprocessor symbol, i.e. SYM_DLEVEL, to be used in the conditional preprocessing, and eventually assign it to the runtime variable. I.e.
#define SYM_DLEVEL 10
int main(void)
{
uint8_t DLEVEL = SYM_DLEVEL;
#if SYM_DLEVEL > 5
#define INITIALISATION
#endif
while (1)
{
#ifdef INITIALISATION
Display(); // This line is now is executed
#endif
}
}
Remember that preprocessing is more or less a text processor (which actuates on base piece of text, tokens, as outlined in comments below), and is executed before the compilation of the code. It doesn't know about runtime variables and their assignments, it only knows symbols defined with a preprocessor directive (#define).
Don't confuse actions as dead code removal or other optimization made by compiler with what the preprocessor can do. The golden rule is: "preprocessing only understand preprocessing objects, C code is a plain text without any special meaning for it".