#define MAXVAL  100   /* maximum depth of val stack */

int sp = 0;           /* next free stack position */
double val[MAXVAL];   /* value stack */

/* push:  push f onto value stack */
void push(double f)
{
    if (sp < MAXVAL)
        val[sp++] = f;
    else
        printf("error: stack full, can't push %g\n", f);
}