#ifndef lint
static char *rcs = "$Header: stack.c,v 1.1 88/01/15 13:05:28 simpson Rel $";
#endif
/*
$Log:	stack.c,v $
 * Revision 1.1  88/01/15  13:05:28  simpson
 * initial release
 * 
 * Revision 0.1  87/12/11  18:31:20  simpson
 * beta test
 * 
*/

/* Generalized stack manipulation routines.  Routines provided are:
 *
 *	char *push(p)
 *	char *p;
 *
 * Pushes the stack with the data in pointer p.  Space is not allocated 
 * for the data; that is, the data is by reference.  Consequently, you should
 * not delete the data until you have popped it from the stack.  A pointer to 
 * the passed data is returned.  NULL is returned if memory could not be
 * allocated to push the data.  If the data passed is larger than
 * elementsize, then only elementsize bytes will be copied.  If it is
 * smaller, you will probably get a segmentation violation.
 *
 *	char *pop()
 * 
 * Pops the stack and returns the popped value.  If the stack is empty, NULL
 * is returned.
 *
 * Implementation rationale:  We don't copy data by value since we would have
 * trouble popping the data.  When we pop data, we want to free its space but
 * we also want to return the data popped.  We could store the data in a
 * temporary buffer of the element size but it would get written over with
 * every pop.  Also, copying by value slows things down.
 */
#include <local/standard.h>
#define NULL	    0

/* Represent the stack as a linked list of elements */
static struct element {
    char	    *data;	    /* Pointer to actual data */
    struct element  *next;	    /* Pointer to next element down */
} *Head;

/* Push the stack */
char *push(data)
char *data;
{
    struct element  *p;
    char	    *malloc();

    if (!data || !(p = (struct element *)malloc((unsigned)sizeof(struct
    element))))
	return NULL;
    p->next = Head, Head = p;
    p->data = data;
    return data;
}

/* Pop the stack */
char *pop()
{
    char	    *p;
    struct element  *ep;

    if (!Head)
	return NULL;
    p = Head->data;
    ep = Head;
    Head = Head->next;
    free((char *)ep);
    return p;
}