Expressions

While reading in the book "The complete C Reference". I got some notes!

Expressions

• DataTypes:
• char: 1 byte
• int
• float
• double
• void
• Modifiers:
• signed
• unsigned
• long 
• short
• DataTypes with modifiers (some notes)
• the default is "signed"
  Ex:
• char = signed char
• Sizes:
• char: 8 bits
• short int: 16 bits
• long int: 32 bits
• long long int: 64 bits (added by C99)
• float: 32 bits
• double: 64 bits 
• long double: 80 bits (added by C99)
• Identifiers (variable and functions names, labels, ...etc) :
• first character: "_" or a letter
• other characters: "_" or letter or number
• significant charachters:
• internal identifiers (used only in the same file)
• external identifiers (used in more than one file, like global variables and functions)
• C89:
• for internal identifiers: 31 characters are significant
• for external identifiers: 6 characters are significant ("studentName" and "studentNameCon" will be treated as the same identifier!)
• C99:
• for internal identifiers: 63 characters are significant
• for external identifiers: 31 characters are significant
• Variables
• C89: all variables shall be declared in the beginning of the block
• C99: can be defined anyway
• 4 C Scopes:
• File Scope (variables defined in file scope are global)
• Block Scope (variables defined in a block scope, are local to there block, also variables defined in the function definition (formal parameters) are local to the function block scope )
• Prototype Scope (variables declared in function prototype, are local to the prototype)
• Function Scope: applies only to the labels. (so block scope does not apply to labels, function scope is applied instead!)
• the following code is not valid
  void func1(){
    fun1: goto fun2;
  }
  void fun2(){
    fun2: goto fun1;
  }
• the following code is valid
  void fun1(){
   printf("fun1");
      {
         block1: printf("block1");
      }
      goto block1;
  }
• Type Qualifiers:
• const: 
• Saved in ROM
• The program can not change this variable
• Other component other than the program can change the variable (ex: hardware device)
• volatile: 
• To highlight that this variable may be changed, without explicit assignment in the program
  ex:
  may be changed by HW device, by operating system, ...etc
• This prevents compiler optimization like the following
• y = x*3/(5x+2);    =======>     y = x*3/(5x+2);
  z = x*3/(5x+2);                 z = y;
• Storage Class Specifiers:
• extern  ==> the variabale is defined in other file
• In some compilers, it is not mandatory to use "extern" key word
• static
• in file scope ==> the variable is file global (internal)
• in block scope ==> the variable shall be initialized only once at the program startup
• register
• This variable shall be stored in a register if applicable (faster access)
• If no registers are available, define them some how to be faster in operations.
• Allowed for local variables.
• Theoretically, can be ignored by the compiler if not possible, but this seldom happens.
• auto
• The variable is local to the function (no need to write this! all function variables are auto by default)
• Constants
• The compiler fits numeric constants to data types as follows:
• Default fit
• Integer constant: to the smallest data type fits
• 16 >> short
• 100000 >> long
• Floating point constant: to double
• 16.3 >> double
• postfix fit
• 12.9F or 12.9f >> float
• 12.9L or 12.9l >> long double
• 12U or 12u >> unsiged short
• 12L or 12l >> long int
• Hexadecimal and Octal
• 0x10 ==> Hexadecimal
• 010 ==> Octal
• Operators:
• Assignment operator "="
• Multiple Assignment
• x = z = 1;
• Compound assignment
• x += 1;
• Arithmatic operators
• ++ , --
• - (unary operator)
• + , / , %
• + , - 
• Relational and logical operators
• !
• > , >= , < , <=
• == , !=
• &&
• ||
• Bitwise Operators
• & , | , ^ , ~
• Other Operators
• ? (ternary operator)
• pointers: * , &
• pointer operators and unary "-" operator have the highest precedence
• sizeof()
• unary operator
• run time operator
• returns the size of the operand (in bytes)
• the return value is of type size_t, where:
  "size_t" is defined to be "unsigned int", using typedef
• Comma operator
• separates expressions
• the result is the last expression result
• x = (y = 5, y + 1);
  this means:
• put y = 5
• then put x = y + 1
• dot (.) and  arrow (->)
• dot (.)
• for struct/union member reference using the struct
• arrow (->)
• for struct/union member reference using the struct pointer
• () and []
• ()
• for precedence
• []
• for array indexing
• Precedence
• Unary operators and ?
• associate from right to left
• Other operators
• associate from left to right
• Precedence:
• ()    []    ->    . 
• !    ~    ++    --    -    (type)    *:at address    &:address of    sizeof
• *    /    %
• +    -    
• <<    >>
• <    <=    >    >=
• ==    !=
• &
• ^
• |
• &&
• ||
• ?    :
• =    +=    -=    *=    /=    etc
• ,
• Type promotion in expressions:
• char,  short ==> int
• if one operand is long double ==> the other is promoted to long double
• else if one operand is double ==> the other is promoted to double
• else if one operand is float ==> the other is promoted to float
• else if one operand is unsigned long ==> the other is promoted to unsigned long
• else if one operand is long ==> the other is promoted to long
• else if one operand is unsigned int ==> the other is promoted to unsigned int
• Example: 
• unsigned char x = 0xFF
• if (~x == 0x00) // evaluates to false
• because x will be promoted to unsigned int -> 0x000000FF
• ~x = 0xFFFFFF00