www.pudn.com > flex_bingle.rar > alloca.c

/* 
	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 lint 
static 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_DIRECTION 
you 
lose 
-- must know STACK_DIRECTION at compile-time 
#endif /* STACK_DIRECTION undefined */ 
#endif /* static */ 
#endif /* emacs */ 
 
#ifndef alloca  /* If compiling with GCC, this file's not needed.  */ 
 
#ifdef __STDC__ 
typedef void	*pointer;		/* generic pointer type */ 
#else 
typedef 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_dir 
 
static void 
find_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) 
#endif 
 
typedef 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 */ 
 
pointer 
alloca (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)); 
  } 
} 
 
#endif /* no alloca */