/*	alloca -- (mostly) portable public-domain implementation -- D A Gwyn	last edit:	86/05/30	rms	   include config.h, since on VMS it renames some symbols.	   Use xmalloc instead of malloc.	This implementation of the PWB library alloca() function,	which is used to allocate space off the run-time stack so	that it is automatically reclaimed upon procedure exit, 	was inspired by discussions with J. Q. Johnson of Cornell.	It should work under any C implementation that uses an	actual procedure stack (as opposed to a linked list of	frames).  There are some preprocessor constants that can	be defined when compiling for your specific system, for	improved efficiency; however, the defaults should be okay.	The general concept of this implementation is to keep	track of all alloca()-allocated blocks, and reclaim any	that are found to be deeper in the stack than the current	invocation.  This heuristic does not reclaim storage as	soon as it becomes invalid, but it will do so eventually.	As a special case, alloca(0) reclaims storage without	allocating any.  It is a good idea to use alloca(0) in	your main control loop, etc. to force garbage collection.*/#ifndef lintstatic char	SCCSid[] = "@(#)alloca.c	1.1";	/* for the "what" utility */#endif#ifdef emacs#include "config.h"#ifdef static/* actually, only want this if static is defined as ""   -- this is for usg, in which emacs must undefine static   in order to make unexec workable   */#ifndef STACK_DIRECTIONyoulose-- must know STACK_DIRECTION at compile-time#endif /* STACK_DIRECTION undefined */#endif static#endif emacs#ifdef X3J11typedef void	*pointer;		/* generic pointer type */#elsetypedef char	*pointer;		/* generic pointer type */#endif#define	NULL	0			/* null pointer constant */extern void	free();extern pointer	xmalloc();/*	Define STACK_DIRECTION if you know the direction of stack	growth for your system; otherwise it will be automatically	deduced at run-time.	STACK_DIRECTION > 0 => grows toward higher addresses	STACK_DIRECTION < 0 => grows toward lower addresses	STACK_DIRECTION = 0 => direction of growth unknown*/#ifndef STACK_DIRECTION#define	STACK_DIRECTION	0		/* direction unknown */#endif#if STACK_DIRECTION != 0#define	STACK_DIR	STACK_DIRECTION	/* known at compile-time */#else	/* STACK_DIRECTION == 0; need run-time code */static int	stack_dir;		/* 1 or -1 once known */#define	STACK_DIR	stack_dirstatic voidfind_stack_direction (/* void */){  static char	*addr = NULL;	/* address of first				   `dummy', once known */  auto char	dummy;		/* to get stack address */  if (addr == NULL)    {				/* initial entry */      addr = &dummy;      find_stack_direction ();	/* recurse once */    }  else				/* second entry */    if (&dummy > addr)      stack_dir = 1;		/* stack grew upward */    else      stack_dir = -1;		/* stack grew downward */}#endif	/* STACK_DIRECTION == 0 *//*	An "alloca header" is used to:	(a) chain together all alloca()ed blocks;	(b) keep track of stack depth.	It is very important that sizeof(header) agree with malloc()	alignment chunk size.  The following default should work okay.*/#ifndef	ALIGN_SIZE#define	ALIGN_SIZE	sizeof(double)#endiftypedef union hdr{  char	align[ALIGN_SIZE];	/* to force sizeof(header) */  struct    {      union hdr *next;		/* for chaining headers */      char *deep;		/* for stack depth measure */    } h;} header;/*	alloca( size ) returns a pointer to at least `size' bytes of	storage which will be automatically reclaimed upon exit from	the procedure that called alloca().  Originally, this space	was supposed to be taken from the current stack frame of the	caller, but that method cannot be made to work for some	implementations of C, for example under Gould's UTX/32.*/static header *last_alloca_header = NULL; /* -> last alloca header */pointeralloca (size)			/* returns pointer to storage */     unsigned	size;		/* # bytes to allocate */{  auto char	probe;		/* probes stack depth: */  register char	*depth = &probe;#if STACK_DIRECTION == 0  if (STACK_DIR == 0)		/* unknown growth direction */    find_stack_direction ();#endif				/* Reclaim garbage, defined as all alloca()ed storage that				   was allocated from deeper in the stack than currently. */  {    register header	*hp;	/* traverses linked list */    for (hp = last_alloca_header; hp != NULL;)      if (STACK_DIR > 0 && hp->h.deep > depth	  || STACK_DIR < 0 && hp->h.deep < depth)	{	  register header	*np = hp->h.next;	  free ((pointer) hp);	/* collect garbage */	  hp = np;		/* -> next header */	}      else	break;			/* rest are not deeper */    last_alloca_header = hp;	/* -> last valid storage */  }  if (size == 0)    return NULL;		/* no allocation required */  /* Allocate combined header + user data storage. */  {    register pointer	new = xmalloc (sizeof (header) + size);    /* address of header */    ((header *)new)->h.next = last_alloca_header;    ((header *)new)->h.deep = depth;    last_alloca_header = (header *)new;    /* User storage begins just after header. */    return (pointer)((char *)new + sizeof(header));  }}