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C (:fire:declaration VS definition (int x = 1; int x = 2; (global scope:…

- - - - OK when -std=c99 or c11
      - ERROR when -std=c89
- - - - re-definition error
    - - x = 2
    - - x = 2
    - - int main(void){
        static int x = 5;
        {
        static int x = 2;
        printf("%d", x); // x = 2
        }
        }
  - - - pointer to a constant integer
    - - constant pointer to an integer
    - - constant pointer to a constant integer
    - - param: constant pointer to a constant integer
    - - WARNING: ptr defintion discard "const" from variable "i"
  - - - register related variable
      - interrupt related variable
      - multi-thread related variable
    - - optimization: reduce the direct access times to the variable address by storing that value to a system location for quicker accessing.
- - - - In FOR statement, k is treated as integer
        and "0.9 is understood as 0"
  - - - :red_flag:
        The statements after IF should be only 1 statement
        or
        multiple statement covering by {}
  - - - x: is RVALUE
        :<3:(Eg. switch (2) is OK)
        must be integer value
        must not the memory address
      - A, B,...: must be a literal
        MUST NOT duplicate
      - SWITCH is same as IF
        Without {}, only 1 statement is included in SWITCH
        E.g.
        switch (x)
        case A: break
- - - - "extern int x" is a declaration
      - :explode:"extern" and non-extern keywords can exist in both .h and .c files
        
        :warning:
        error if a definition exists in header file
        
        :check:because compiler compiles all source file
        => if the header file is included in multiple source file
        => multi-definition error
        
        Whenever want to extern a variable,
        the declaration with "extern" can be placed in header or source file
      - "int x" is a definition
    - - "static" variable in source file
        
        :warning:cannot be seen except the source file
      - "static" variable in included header
        
        can be seen by the includer file
    - - x not equal to 0.1
        
        0.1 is literal number which is stored as double
        
        x store as float
- - - - => the called method is declared implicitly and MAY CAUSE ERROR
        
        :red_flag: E.g.
        void main(){
        int i = foo();
        printf("%d", i);
        }
        void foo(){
        printf("hello");
        }
        
        => There are warnings and output is hello5
- - - - enummeration and macro can be use, BUT constant