C language learning global variables

A. Global variables
• defined outside of the function variables are global
• Global variables have a global scope and lifetime
• They have nothing to do with any function
• they can be used in any internal function

II. Global variables are initialized
• do not initialize global variables get a value of 0
• pointer will get a NULL value
• Use only with known values compile-time to initialize global variables
• their initialization occurs before the main function

#include  <stdio.h>

int f(void);

int gAll = 12;

int main(int argc, char const *argv[])
{
    printf("in %s gAll=%d\n",__func__,gAll);  //__func__打印函数名字 
    f();
    printf("agn in %s gAll=%d\n",__func__,gAll);
    return 0;
}

int f(void)
{
    printf("in %s gAll=%d\n",__func__,gAll);
    gAll +=2;
    printf("agn in %s gAll=%d\n",__func__,gAll);
    return gAll;
}

Hidden global variables
• If there is an internal function variables and global variables of the same name, then the global variables are hidden

III. Static local variable
• The local variables defined in the plus static modifier becomes static local variable

• When leaving the function of static local variable will continue to exist and retain their value

• initialize static local variable is only the first time you enter this function to do, the future will hold value when you left when entering the function

• static local variable is actually a special global variables
• They are located in the same area of memory
• static local variable with global survival, local scope within the function

• static here means local scope (locally accessible)

#include  <stdio.h>

int f(void);

int gAll =12;

int main(int argc,char const *argv[])
{
    f();
//    f();
//    f();
    return 0;
}
int f(void)
{
    int k =0;
    static int all =1;
    printf("&gAll=%p\n",&gAll);
    printf("& All =% the p-\ the n- " , & All); // static local variable is actually a special global variables 
    printf ( " & k =% the p-\ the n- " , & k); 
    printf ( " in% S All =% d \ the n- " , __ __ FUNC, All); 
    All + = 2 ; 
    the printf ( " AGN in% S% All D = \ n- " , __ __ FUNC, All);
     return All; 
}

IV. * Return pointer function
• Returns the local address of a variable is dangerous
• return the global variable or static local variable address is safe
• Return malloc function in the memory is safe, but likely to cause problems

• The best practice is to return the incoming pointer

#include <stdio.h>
int *f(void);
void g(void);

int main(int argc, char const *argv[])
{
    int *p= f();
    printf("*p=%d\n",*p);
    g();
    printf("*p=%d\n",*p);
    return 0;
}

int *f(void)
{
    int i=12;
    printf("&i=%p\n",&i); 
    return &i;
    
}

void g(void)
{
    int k=24;
    printf("&k=%p\n",&k); 
    printf("k=%d\n",k);
}

Tips
• Do not use global variables to pass parameters and results between function
• Try to avoid using global variables
• Toyota's case
• * function using global variables and static local variable is disturbed by a thread

C language learning global variables

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