C: Variable Scopes

/* LOCAL VARIABLES */

int main(void)
{  /* start of basic block */
    int a = 5; /* local to main() */

    if (a != 0) {
        int b = 10; /* local to if basic block */

        a = b; /* perfectly legal--both a and b are visible here */
    }

    int b = 12; /* ERROR -- b is not visible out here--only in the if */

    { /* notice I started a basic block with no statement--this is legal */
        int c = 12;
        int a; /* Hey!  Wait!  There was already an "a" out in main! */

        /* the a that is local to this block hides the a from main */
        a = c; /* this modified the a local to this block to be 12 */
    }

    /* but this a back in main is still 10 (since we set it in the if): */
    printf("%d\n", a);

    return 0;
}



/* GLOBAL VARIABLES */

#include <stdio.h>

/* this is a global variable.  We know it's global, because it's been declared in "global scope", and not in a basic block somewhere */
int g = 10;

void afunc(int x)
{
    g = x; /* this sets the global to whatever x is */
}

int main(void)
{
    g = 10;    /* global g is now 10 */
    afunc(20); /* but this function will set it to 20 */
    printf("%d\n", g); /* so this will print "20" */

    return 0;
}