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Lifetime of variables in C

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• Life time of Global Static global, static local, Local and Parameter variables in C
  • The life time of the variables in C:

    Kind of variable	Life time in C
    Global variable	During the entire execution of the program.
    static global variable	During the entire execution of the program.
    static local variable	During the entire execution of the program.
    Parameter variable	From the start of execution of the function to the end of the execution of the function
    Local variable	From the definition of the variable in the function to the end of the execution of the function

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• (1) Global, (2) static global and (3) static local variables: created using the .skip assembler directive
  • Fact:

    The memory space for global, static global and static local variables are reserved using: Assembler directives that are similar to the .skip directive of ARM !!!

    That's why:

    The life time of global, static global and static local variables is the entire execution of the program

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• (1) Parameter variables and (2) local variables: created on the system stack
  • Recall from assembler programming:

    The parameter variables are created (= pushed) on the statck when a function is called The local variables are created on the stack by decrementing the stack pointer in the prelude of the a function The parameter and local variables are popped (= destroyed !) when the function returns

    That's why the life time of parameter and local variables is the duration of the function call !!!

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• When is a variable initialized ??
  • Initialized variable

    In high level programming language, we can define initialized variables Exmaple: int x = 4444; // initialized variable

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  • When will a initialized variable be given an initial value:

    The initialization of a initialized variable occurs each time that the variable is created !!!!

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  • Consequence:

    Local variables and parameter variables: are initialized each time a function is called (Because Local variables and parameter variables are created each time a function is called !!!) Global variables, static global variables and static local variables: are initialized only once (Because global variables, static global variables and static local variables are created once only at the start of the program execution !!!)

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• Warning: the kind of variable has significant impact of program behavior
  • Illustrative example of the initialization behavior of local and static local variables in C:

    #include <stdio.h> void f( ) { int i = 1234; // Normal local (created when f() is called) static int j = 1234; // Static local (created using ds, ONCE !) i++; j++; printf( "i = %d, j = %d\n", i, j ); } int main( int argc, char* argv[] ) { f(); f(); f(); f(); f(); }

    Output:

    i = 1235, j = 1235 // i "forgets" i = 1235, j = 1236 // j remembers !!! i = 1235, j = 1237 i = 1235, j = 1238 i = 1235, j = 1239

    Explanation:

    The variable i is a local variable and it is created each time the function f( ) is invoked Therefore: The variable i is initialized to 1234 each time the function f( ) is called !!!       So when we add 1 to i with i++, the variable i will contain 1235 The variable j is a static local variable and it is created once (when the program begins execution) Therefore: The variable j is not initialized to 1234 when the function f( ) is called !!!       So when we add 1 to i with j++, the variable j will keep increasing

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/kinds-vars2.c

    How to run the program:

    To compile:   gcc kinds-vars2.c          To run: ./a.out