optimize2.c

/* optimize2.c */
#include <stdio.h>
#include <stdlib.h>
#include <setjmp.h>

static void f1(int, int, int);
static void f2();

static jmp_buf jmpbuf;

int main()
{
    int a;
    register int b;
    volatile int c;

    a = 1; b = 2; c = 3;
    if (setjmp(jmpbuf) != 0) {
        printf("after longjmp: a = %d, b = %d, c = %d\n", a, b, c);
        exit(0);
    }
    a = 91; b = 92; c = 93;
    f1(a, b, c);

    return 0;
}

static void f1(int i, int j, int k)
{
    printf("in f1(): a = %d, b = %d, c = %d\n", i, j, k);
    f2();
}

static void f2()
{
    longjmp(jmpbuf, 1);
}

************
* OUTPUT 1 *
************
% gcc -W -Wall -O2 -ansi -pedantic -s optimize2.c
% ./a.out
in f1(): a = 91, b = 92, c = 93
after longjmp: a = 1, b = 2, c = 93
%
************
* OUTPUT 2 *
************
Keep optimized.c unchanged, but drops compiler optimization.
% gcc -W -Wall -ansi -pedantic -c optimize2.c
optimize2.c: In function 'main':
optimize2.c:14:15: warning: variable 'b' might be clobbered by 'longjmp' or 'vfork' [-Wclobbered]
  register int b;
               ^
% ./a.out
in f1(): a = 91, b = 92, c = 93
after longjmp: a = 91, b = 2, c = 93
%
************
* OUTPUT 3 *
************
In optimize2.c, line 15, drops volatile.
So c is declared as 'int c'.
And enable compiler optimization.
% gcc -W -Wall -O2 -ansi -pedantic -s optimize2.c
% ./a.out
in f1(): a = 91, b = 92, c = 93
after longjmp: a = 1, b = 2, c = 3
%