mem.c

/*
**  This is a copyrighted work which is functionally identical to work
**  originally published in Micro Cornucopia magazine (issue #52, March-April,
**  1990) and is freely licensed by the author, Walter Bright, for any use.
*/

/*_ mem.c   Fri Jan 26 1990   Modified by: Walter Bright */
/* $Header: /proj/products/merlin/port/RCS/mem.c,v 1.19 89/10/20 14:36:02 bright Exp Locker: bright $ */
/* Memory management package				*/

#if defined(VAX11C)
#define  __FILE__  "mem.c"
#endif

#include	<stdio.h>
#include	<stdlib.h>
#include	<io.h>

#ifndef MEM_H
#include	"mem.h"
#endif

#ifndef assert
#include	<assert.h>
#endif

#if defined(_MSC_VER)
#include	<dos.h>
#endif

#if !defined(VAX11C)
#ifdef BSDUNIX
#include <strings.h>
#else
#include <string.h>
#endif
#else
extern char *strcpy(),*memcpy();
extern int strlen();
#endif  /* VAX11C */

int mem_inited = 0;		/* != 0 if initialized			*/

static int mem_behavior = MEM_ABORTMSG;
static int (*fp)() = NULL;	/* out-of-memory handler	*/
static int mem_count;		/* # of allocs that haven't been free'd	*/
static int mem_scount;		/* # of sallocs that haven't been free'd */
static int near mem_exception(); /* called when out of memory */

/* Determine where to send error messages	*/
#ifdef MSDOS
#define ferr	stdout	/* stderr can't be redirected with MS-DOS	*/
#else
#define ferr	stderr
#endif

/*******************************/

void mem_setexception(flag,handler_fp)
#if __cplusplus
enum MEM_E flag;
#else
int flag;
#endif
int (*handler_fp)();
{
    mem_behavior = flag;
    fp = (mem_behavior == MEM_CALLFP) ? handler_fp : 0;
#if MEM_DEBUG
    assert(0 <= flag && flag <= MEM_RETRY);
#endif
}

/*************************
 * This is called when we're out of memory.
 * Returns:
 *	1:	try again to allocate the memory
 *	0:	give up and return NULL
 */

static int near mem_exception()
{   int behavior;

    behavior = mem_behavior;
    while (1)
    {
	switch (behavior)
	{
	    case MEM_ABORTMSG:
#if defined(MSDOS) || defined(__OS2__)
		/* Avoid linking in buffered I/O */
	    {	static char msg[] = "Fatal error: out of memory\r\n";

		write(1,msg,sizeof(msg) - 1);
	    }
#else
		fputs("Fatal error: out of memory\n",ferr);
#endif
		/* FALL-THROUGH */
	    case MEM_ABORT:
		exit(EXIT_FAILURE);
		/* NOTREACHED */
	    case MEM_CALLFP:
		assert(fp);
		behavior = (*fp)();
		break;
	    case MEM_RETNULL:
		return 0;
	    case MEM_RETRY:
		return 1;
	    default:
		assert(0);
	}
    }
}

/****************************/

#if MEM_DEBUG

#undef mem_strdup

char *mem_strdup(s)
const char *s;
{
	return mem_strdup_debug(s,__FILE__,__LINE__);
}

char *mem_strdup_debug(s,file,line)
char *file;
const char *s;
int line;
{
	char *p;

	p = s
	    ? (char *) mem_malloc_debug((unsigned) strlen(s) + 1,file,line)
	    : NULL;
	return p ? strcpy(p,s) : p;
}
#else
char *mem_strdup(s)
const char *s;
{
	char *p;

	p = s ? (char *) mem_malloc((unsigned) strlen(s) + 1) : NULL;
	return p ? strcpy(p,s) : p;
}

#endif /* MEM_DEBUG */

#ifdef MEM_DEBUG

static long mem_maxalloc;	/* max # of bytes allocated		*/
static long mem_numalloc;	/* current # of bytes allocated		*/

#define BEFOREVAL	0x12345678	/* value to detect underrun	*/
#define AFTERVAL	0x87654321	/* value to detect overrun	*/

#if SUN || SUN386
static long afterval = AFTERVAL;	/* so we can do &afterval	*/
#endif

/* The following should be selected to give maximum probability that	*/
/* pointers loaded with these values will cause an obvious crash. On	*/
/* Unix machines, a large value will cause a segment fault.		*/
/* MALLOCVAL is the value to set malloc'd data to.			*/

#if MSDOS || __OS2__
#define BADVAL		0xFF
#define MALLOCVAL	0xEE
#else
#define BADVAL		0x7A
#define MALLOCVAL	0xEE
#endif

/* Disable mapping macros	*/
#undef	mem_malloc
#undef	mem_calloc
#undef	mem_realloc
#undef	mem_free

/* Create a list of all alloc'ed pointers, retaining info about where	*/
/* each alloc came from. This is a real memory and speed hog, but who	*/
/* cares when you've got obscure pointer bugs.				*/

static struct mem_debug
{	struct mh
	{ struct mem_debug *Mnext;	/* next in list			*/
	  struct mem_debug *Mprev;	/* previous value in list	*/
	  char *Mfile;		/* filename of where allocated		*/
	  int Mline;		/* line number of where allocated	*/
	  unsigned Mnbytes;	/* size of the allocation		*/
	  long Mbeforeval;	/* detect underrun of data		*/
	} m;
	char data[1];		/* the data actually allocated		*/
} mem_alloclist =
{
   {	(struct mem_debug *) NULL,
	(struct mem_debug *) NULL,
	"noname",
	11111,
	0,
	BEFOREVAL
   },
   AFTERVAL
};

/* Convert from a void *to a mem_debug struct.	*/
#define mem_ptrtodl(p)	((struct mem_debug *) ((char *)p - sizeof(struct mh)))

/* Convert from a mem_debug struct to a mem_ptr.	*/
#define mem_dltoptr(dl)	((void *) &((dl)->data[0]))

#define next		m.Mnext
#define prev		m.Mprev
#define file		m.Mfile
#define line		m.Mline
#define nbytes		m.Mnbytes
#define beforeval	m.Mbeforeval

/*****************************
 * Set new value of file,line
 */

void mem_setnewfileline(ptr,fil,lin)
void *ptr;
char *fil;
int lin;
{
    struct mem_debug *dl;

    dl = mem_ptrtodl(ptr);
    dl->file = fil;
    dl->line = lin;
}

/****************************
 * Print out struct mem_debug.
 */

static void near mem_printdl(dl)
struct mem_debug *dl;
{
#if LPTR
	fprintf(ferr,"alloc'd from file '%s' line %d nbytes %d ptr x%lx\n",
		dl->file,dl->line,dl->nbytes,mem_dltoptr(dl));
#else
	fprintf(ferr,"alloc'd from file '%s' line %d nbytes %d ptr x%x\n",
		dl->file,dl->line,dl->nbytes,mem_dltoptr(dl));
#endif
}

/****************************
 * Print out file and line number.
 */

static void near mem_fillin(fil,lin)
char *fil;
int lin;
{
	fprintf(ferr,"File '%s' line %d\n",fil,lin);
	fflush(ferr);
}

/****************************
 * If MEM_DEBUG is not on for some modules, these routines will get
 * called.
 */

void *mem_calloc(u)
unsigned u;
{
     	return mem_calloc_debug(u,__FILE__,__LINE__);
}

void *mem_malloc(u)
unsigned u;
{
     	return mem_malloc_debug(u,__FILE__,__LINE__);
}

void *mem_realloc(p,u)
void *p;
unsigned u;
{
     	return mem_realloc_debug(p,u,__FILE__,__LINE__);
}

void mem_free(p)
void *p;
{
	mem_free_debug(p,__FILE__,__LINE__);
}    


/**************************/

void mem_freefp(p)
void *p;
{
	mem_free(p);
}

/***********************
 * Debug versions of mem_calloc(), mem_free() and mem_realloc().
 */

void *mem_malloc_debug(n,fil,lin)
unsigned n;
char *fil;
int lin;
{   void *p;

    p = mem_calloc_debug(n,fil,lin);
    if (p)
	memset(p,MALLOCVAL,n);
    return p;
}

void *mem_calloc_debug(n,fil,lin)
unsigned n;
char *fil;
int lin;
{
    struct mem_debug *dl;

    do
	dl = (struct mem_debug *)
	    calloc(sizeof(*dl) + n + sizeof(AFTERVAL) - 1,1);
    while (dl == NULL && mem_exception());
    if (dl == NULL)
    {
#if 0
	printf("Insufficient memory for alloc of %d at ",n);
	mem_fillin(fil,lin);
	printf("Max allocated was: %ld\n",mem_maxalloc);
#endif
	return NULL;
    }
    dl->file = fil;
    dl->line = lin;
    dl->nbytes = n;
    dl->beforeval = BEFOREVAL;
#if SUN || SUN386 /* bus error if we store a long at an odd address */
    memcpy(&(dl->data[n]),&afterval,sizeof(AFTERVAL));
#else
    *(long *) &(dl->data[n]) = AFTERVAL;
#endif

    /* Add dl to start of allocation list	*/
    dl->next = mem_alloclist.next;
    dl->prev = &mem_alloclist;
    mem_alloclist.next = dl;
    if (dl->next != NULL)
	dl->next->prev = dl;

    mem_count++;
    mem_numalloc += n;
    if (mem_numalloc > mem_maxalloc)
	mem_maxalloc = mem_numalloc;
    return mem_dltoptr(dl);
}

void mem_free_debug(ptr,fil,lin)
void *ptr;
char *fil;
int lin;
{
	struct mem_debug *dl;

	if (ptr == NULL)
		return;
#if 0
	{	fprintf(ferr,"Freeing NULL pointer at ");
		goto err;
	}
#endif
	if (mem_count <= 0)
	{	fprintf(ferr,"More frees than allocs at ");
		goto err;
	}
	dl = mem_ptrtodl(ptr);
	if (dl->beforeval != BEFOREVAL)
	{
#if LPTR
		fprintf(ferr,"Pointer x%lx underrun\n",ptr);
#else
		fprintf(ferr,"Pointer x%x underrun\n",ptr);
#endif
		goto err2;
	}
#if SUN || SUN386 /* Bus error if we read a long from an odd address	*/
	if (memcmp(&dl->data[dl->nbytes],&afterval,sizeof(AFTERVAL)) != 0)
#else
	if (*(long *) &dl->data[dl->nbytes] != AFTERVAL)
#endif
	{
#if LPTR
		fprintf(ferr,"Pointer x%lx overrun\n",ptr);
#else
		fprintf(ferr,"Pointer x%x overrun\n",ptr);
#endif
		goto err2;
	}
	mem_numalloc -= dl->nbytes;
	if (mem_numalloc < 0)
	{	fprintf(ferr,"error: mem_numalloc = %ld, dl->nbytes = %d\n",
			mem_numalloc,dl->nbytes);
		goto err2;
	}

	/* Remove dl from linked list	*/
	if (dl->prev)
		dl->prev->next = dl->next;
	if (dl->next)
		dl->next->prev = dl->prev;

	/* Stomp on the freed storage to help detect references	*/
	/* after the storage was freed.				*/
	memset((void *) dl,BADVAL,sizeof(*dl) + dl->nbytes);
	mem_count--;

	/* Some compilers can detect errors in the heap.	*/
#if defined(DLC)
	{	int i;
		i = free(dl);
		assert(i == 0);
	}
#else
	free((void *) dl);
#endif
	return;

err2:
	mem_printdl(dl);
err:
	fprintf(ferr,"free'd from ");
	mem_fillin(fil,lin);
	assert(0);
	/* NOTREACHED */
}

/*******************
 * Debug version of mem_realloc().
 */

void *mem_realloc_debug(oldp,n,fil,lin)
void *oldp;
unsigned n;
char *fil;
int lin;
{   void *p;
    struct mem_debug *dl;

    if (n == 0)
    {	mem_free_debug(oldp,fil,lin);
	p = NULL;
    }
    else if (oldp == NULL)
	p = mem_malloc_debug(n,fil,lin);
    else
    {
	p = mem_malloc_debug(n,fil,lin);
	if (p != NULL)
	{
	    dl = mem_ptrtodl(oldp);
	    if (dl->nbytes < n)
		n = dl->nbytes;
	    memcpy(p,oldp,n);
	    mem_free_debug(oldp,fil,lin);
	}
    }
    return p;
}

/***************************/

void mem_check()
{   register struct mem_debug *dl;

    for (dl = mem_alloclist.next; dl != NULL; dl = dl->next)
	mem_checkptr(mem_dltoptr(dl));
}

/***************************/

void mem_checkptr(p)
register void *p;
{   register struct mem_debug *dl;

    for (dl = mem_alloclist.next; dl != NULL; dl = dl->next)
    {
	if (p >= (void *) &(dl->data[0]) &&
	    p < (void *)((char *)dl + sizeof(struct mem_debug)-1 + dl->nbytes))
	    goto L1;
    }
    assert(0);

L1:
    dl = mem_ptrtodl(p);
    if (dl->beforeval != BEFOREVAL)
    {
#if LPTR
	    fprintf(ferr,"Pointer x%lx underrun\n",p);
#else
	    fprintf(ferr,"Pointer x%x underrun\n",p);
#endif
	    goto err2;
    }
#if SUN || SUN386 /* Bus error if we read a long from an odd address	*/
    if (memcmp(&dl->data[dl->nbytes],&afterval,sizeof(AFTERVAL)) != 0)
#else
    if (*(long *) &dl->data[dl->nbytes] != AFTERVAL)
#endif
    {
#if LPTR
	    fprintf(ferr,"Pointer x%lx overrun\n",p);
#else
	    fprintf(ferr,"Pointer x%x overrun\n",p);
#endif
	    goto err2;
    }
    return;

err2:
    mem_printdl(dl);
    assert(0);
}

#else

/***************************/

void *mem_malloc(numbytes)
unsigned numbytes;
{	void *p;

	if (numbytes == 0)
		return NULL;
	while (1)
	{
		p = malloc(numbytes);
		if (p == NULL)
		{	if (mem_exception())
				continue;
		}
		else
			mem_count++;
		break;
	}
	/*printf("malloc(%d) = x%lx\n",numbytes,p);*/
	return p;
}

/***************************/

void *mem_calloc(numbytes)
unsigned numbytes;
{	void *p;

	if (numbytes == 0)
		return NULL;
	while (1)
	{
		p = calloc(numbytes,1);
		if (p == NULL)
		{	if (mem_exception())
				continue;
		}
		else
			mem_count++;
		break;
	}
	/*printf("calloc(%d) = x%lx\n",numbytes,p);*/
	return p;
}

/***************************/

void *mem_realloc(oldmem_ptr,newnumbytes)
void *oldmem_ptr;
unsigned newnumbytes;
{   void *p;

    if (oldmem_ptr == NULL)
	p = mem_malloc(newnumbytes);
    else if (newnumbytes == 0)
    {	mem_free(oldmem_ptr);
	p = NULL;
    }
    else
    {
	do
	    p = realloc(oldmem_ptr,newnumbytes);
	while (p == NULL && mem_exception());
    }
    /*printf("realloc(x%lx,%d) = x%lx\n",oldmem_ptr,newnumbytes,p);*/
    return p;
}

/***************************/

void mem_free(ptr)
void *ptr;
{
    /*printf("free(x%lx)\n",ptr);*/
    if (ptr != NULL)
    {	assert(mem_count > 0);
	mem_count--;
#if DLC
	{	int i;

		i = free(ptr);
		assert(i == 0);
	}
#else
	free(ptr);
#endif
    }
}

#endif /* MEM_DEBUG */

/***************************/

void mem_init()
{
	if (mem_inited == 0)
	{	mem_count = 0;
#if MEM_DEBUG
		mem_numalloc = 0;
		mem_maxalloc = 0;
		mem_alloclist.next = NULL;
#endif
#if defined(__ZTC__) || defined(__SC__)
		/* Necessary if mem_sfree() calls free() before any	*/
		/* calls to malloc().					*/
		free(malloc(1));	/* initialize storage allocator	*/
#endif
		mem_inited++;
	}
}

/***************************/

void mem_term()
{

	if (mem_inited)
	{
#if MEM_DEBUG
		register struct mem_debug *dl;

		for (dl = mem_alloclist.next; dl; dl = dl->next)
		{	fprintf(ferr,"Unfreed pointer: ");
			mem_printdl(dl);
		}
#if 0
		fprintf(ferr,"Max amount ever allocated == %ld bytes\n",
			mem_maxalloc);
#endif
#else
		if (mem_count)
			fprintf(ferr,"%d unfreed items\n",mem_count);
		if (mem_scount)
			fprintf(ferr,"%d unfreed s items\n",mem_scount);
#endif /* MEM_DEBUG */
		assert(mem_count == 0 && mem_scount == 0);
		mem_inited = 0;
	}
}

#undef next
#undef prev
#undef file
#undef line
#undef nbytes
#undef beforeval

mem.h

/*
**  This is a copyrighted work which is functionally identical to work
**  originally published in Micro Cornucopia magazine (issue #52, March-April,
**  1990) and is freely licensed by the author, Walter Bright, for any use.
*/

/*_ mem.h   Fri May 26 1989   Modified by: Walter Bright */
/* $Header: /proj/products/merlin/port/RCS/mem.h,v 1.11 89/10/23 11:39:00 bright Exp $ */
/* Copyright 1986-1988 by Northwest Software	*/
/* All Rights Reserved				*/
/* Written by Walter Bright			*/

#ifndef MEM_H
#define MEM_H	1

#ifndef TOOLKIT_H
#include	"toolkit.h"
#endif

/*
 * Memory management routines.
 *
 * Compiling:
 *
 *	#define MEM_DEBUG 1 when compiling to enable extended debugging
 *	features.
 *
 * Features always enabled:
 *
 *	o mem_init() is called at startup, and mem_term() at
 *	  close, which checks to see that the number of alloc's is
 *	  the same as the number of free's.
 *	o Behavior on out-of-memory conditions can be controlled
 *	  via mem_setexception().
 *
 * Extended debugging features:
 *
 *	o Enabled by #define MEM_DEBUG 1 when compiling.
 *	o Check values are inserted before and after the alloc'ed data
 *	  to detect pointer underruns and overruns.
 *	o Free'd pointers are checked against alloc'ed pointers.
 *	o Free'd storage is cleared to smoke out references to free'd data.
 *	o Realloc'd pointers are always changed, and the previous storage
 *	  is cleared, to detect erroneous dependencies on the previous
 *	  pointer.
 *	o The routine mem_checkptr() is provided to check an alloc'ed
 *	  pointer.
 */

/********************* GLOBAL VARIABLES *************************/

extern int mem_inited;		/* != 0 if mem package is initialized.	*/
				/* Test this if you have other packages	*/
				/* that depend on mem being initialized	*/

/********************* PUBLIC FUNCTIONS *************************/

/***********************************
 * Set behavior when mem runs out of memory.
 * Input:
 *	flag =	MEM_ABORTMSG:	Abort the program with the message
 *				'Fatal error: out of memory' sent
 *				to stdout. This is the default behavior.
 *		MEM_ABORT:	Abort the program with no message.
 *		MEM_RETNULL:	Return NULL back to caller.
 *		MEM_CALLFP:	Call application-specified function.
 *				fp must be supplied.
 *	fp			Optional function pointer. Supplied if
 *				(flag == MEM_CALLFP). This function returns
 *				MEM_XXXXX, indicating what mem should do next.
 *				The function could do things like swap
 *				data out to disk to free up more memory.
 *	fp could also return:
 *		MEM_RETRY:	Try again to allocate the space. Be
 *				careful not to go into an infinite loop.
 */

#if __cplusplus
enum MEM_E { MEM_ABORTMSG, MEM_ABORT, MEM_RETNULL, MEM_CALLFP, MEM_RETRY };
void mem_setexception P((enum MEM_E, int (*)()));
#else
#define MEM_ABORTMSG	0
#define MEM_ABORT	1
#define MEM_RETNULL	2
#define MEM_CALLFP	3
#define MEM_RETRY	4
void mem_setexception P((int, int(*)()));
#endif


/****************************
 * Allocate space for string, copy string into it, and
 * return pointer to the new string.
 * This routine doesn't really belong here, but it is used so often
 * that I gave up and put it here.
 * Use:
 *	char *mem_strdup(const char *s);
 * Returns:
 *	pointer to copied string if succussful.
 *	else returns NULL (if MEM_RETNULL)
 */

char *mem_strdup P((const char *));

/**************************
 * Function so we can have a pointer to function mem_free().
 * This is needed since mem_free is sometimes defined as a macro,
 * and then the preprocessor screws up.
 * The pointer to mem_free() is used frequently with the list package.
 * Use:
 *	void mem_freefp(void *p);
 */

/***************************
 * Check for errors. This routine does a consistency check on the
 * storage allocator, looking for corrupted data. It should be called
 * when the application has CPU cycles to burn.
 * Use:
 *	void mem_check(void);
 */

void mem_check P((void ));

/***************************
 * Check ptr to see if it is in the range of allocated data.
 * Cause assertion failure if it isn't.
 */

void mem_checkptr P((void *ptr));

/***************************
 * Allocate and return a pointer to numbytes of storage.
 * Use:
 *	void *mem_malloc(unsigned numbytes);
 *	void *mem_calloc(unsigned numbytes); allocated memory is cleared
 * Input:
 *	numbytes	Number of bytes to allocate
 * Returns:
 *	if (numbytes > 0)
 *		pointer to allocated data, NULL if out of memory
 *	else
 *		return NULL
 */

void *mem_malloc P((unsigned));
void *mem_calloc P((unsigned));

/*****************************
 * Reallocate memory.
 * Use:
 *	void *mem_realloc(void *ptr,unsigned numbytes);
 */

void *mem_realloc P((void *,unsigned));

/*****************************
 * Free memory allocated by mem_malloc(), mem_calloc() or mem_realloc().
 * Use:
 *	void mem_free(void *ptr);
 */

void mem_free P((void *));

/***************************
 * Initialize memory handler.
 * Use:
 *	void mem_init(void);
 * Output:
 *	mem_inited = 1
 */

void mem_init P((void ));

/***************************
 * Terminate memory handler. Useful for checking for errors.
 * Use:
 *	void mem_term(void);
 * Output:
 *	mem_inited = 0
 */

void mem_term P((void ));

/* The following stuff forms the implementation rather than the
 * definition, so ignore it.
 */

#if MEM_DEBUG		/* if creating debug version	*/
#define mem_strdup(p)	mem_strdup_debug((p),__FILE__,__LINE__)
#define mem_malloc(u)	mem_malloc_debug((u),__FILE__,__LINE__)
#define mem_calloc(u)	mem_calloc_debug((u),__FILE__,__LINE__)
#define mem_realloc(p,u)	mem_realloc_debug((p),(u),__FILE__,__LINE__)
#define mem_free(p)	mem_free_debug((p),__FILE__,__LINE__)

char *mem_strdup_debug	P((const char *,char *,int));
void *mem_calloc_debug	P((unsigned,char *,int));
void *mem_malloc_debug	P((unsigned,char *,int));
void *mem_realloc_debug P((void *,unsigned,char *,int));
void  mem_free_debug	P((void *,char *,int));
void  mem_freefp	P((void *));

void mem_setnewfileline P((void *,char *,int));

#else

#define mem_freefp	mem_free
#define mem_check()
#define mem_checkptr(p)

#endif /* MEM_DEBUG */

#endif /* MEM_H */

MEM.txt

                        Walter Bright's MEM Package
                        ---------------------------


PREFACE:
--------

The files, MEM.H and MEM.C which constitute the MEM package were originally
published in the March-April 1990 edition of Micro Cornucopia magazine, now
sadly out of print.  The files as they appear in SNIPPETS have been edited
somewhat to remove compiler dependencies and correct minor discrepancies.

For those who don't already know, Walter Bright is the author of Datalight
Optimum-C, the original optimizing C compiler for PC's.  Through a succession
of sales and acquisitions plus continual improvement by Walter,, his compiler
became Zortech C++ and is now sold as Symantec C++.  As such, it is the only
major PC compiler which can claim single authorship.  It also compiles faster
than most other compilers and is still a market leader in its optimization
technology.

Like many other library and ancillary functions unique to Walter's compilers,
the MEM package was originally something he wrote for his own use.  As noted
above, he published it only once but it has been included as an unheralded
"freebie" in Walter's compilers for the past several years.  Walter was kind
enough to grant permission for its inclusion in SNIPPETS beginning with the
April, '94 release.


WHAT IS MEM?:
-------------

MEM is a set of functions used for debugging C pointers and memory allocation
problems.  Quoting Walter, "Symptoms of pointer bugs include: hung machines,
scrambled disks, failures that occur once-in-10,000 iterations, irreprodu-
cible results, and male pattern baldness." After writing MEM for use in
developing his own compiler and tools, he reported that its use reduced
pointer bugs by as much as 75%.  MEM is simple to add to existing programs
and adds little or no overhead.


USING MEM:
----------

Included in the MEM package is TOOLKIT.H, which isolates compiler and
environmental dependencies.  It should work as-is for most PC compilers and
the Microsoft compiler for SCO Unix.  Other environments may be customized by
writing your own HOST.H file, using the existing definitions in TOOLKIT.H as
examples and modifying the values to match your system.  Using these
techniques, the MEM package has been used successfully on Amigas, Macs,
VAXes, and many other non-DOS systems.

The MEM functions exactly parallel the standard library (plus 1 non-standard)
memory allocation functions.  To implement MEM in your program, simply do a
global search-and-replace of the following functions:

      malloc()    ->    mem_malloc()
      calloc()    ->    mem_calloc()
      realloc()   ->    mem_realloc()
      free()      ->    mem_free()
      strdup()    ->    mem_strdup()

At the beginning of main(), add the following lines:

      mem_init();
      atexit(mem_term);

In the header section of each of your C files, add...

#include "mem.h"

...to every .C file which calls any of the above functions.

The final step is to compile and link MEM.C into all programs using the MEM
package.  It really is a pretty simple procedure!

MEM has 2 modes of operation, debugging and non-debugging.  Use debugging
mode during program development and then turn debugging off for final
production code.  Control of debugging is by defining the MEM_DEBUG macro.
If the macro is defined, debugging is on; if undefined, debugging is off.
The default is non-debugging, in which case the MEM functions become trivial
wrappers for the standard functions, incurring virtually no overhead.


WHAT MEM DOES:
--------------

1.    ISO/ANSI verification:

When Walter wrote MEM, compiler compliance with ANSI standards was still
quite low.  MEM verifies ISO/ANSI compliance for situations such as passing
NULL or size 0 to allocation/reallocation functions.

2.    Logging of all allocations and frees:

All MEM's functions pass the __FILE__ and __LINE__ arguments.  During alloca-
tion, MEM makes an entry into a linked list and stores the file and line
information in the list for whichever allocation or free function is called.

This linked list is the backbone of MEM.  When MEM detects a bug, it tells
you where to look in which file to begin tracking the problem.

3.    Verification of frees:

Since MEM knows about all allocations, when a pointer is freed, MEM can
verify that the pointer was allocated originally.  Additionally, MEM will
only allow a pointer to be freed once.

Freed data is overwritten with a non-zero known value, flushing such problems
as continuing to reference data after it's been freed.  The value written
over the data is selected to maximize the probability of a segment fault or
assertion failure if your application references it after it's been freed.

MEM obviously can't directly detect "if" instances such as...

      mem_free(p);
      if (p) ...

...but by guaranteeing that `p' points to garbage after being freed, code
like this will hopefully never work and will thus be easier to find.

4.    Detection of pointer over- and under-run:

Pointer overrun occurs when a program stores data past the end of a buffer,
e.g.

      p = malloc(strlen(s));        /* No space for terminating NUL      */
      strcpy(p,s);                  /* Terminating NUL clobber memory   */

Pointer underrun occurs when a program stores data before the beginning of a
buffer.  This error occurs less often than overruns, but MEM detects it
anyway.  MEM does this by allocating a little extra at each end of every
buffer, which is filled with a known value, called a sentinel. MEM detects
overruns and underruns by verifying the sentinel value when the buffer is
freed.

5.    Dependence on values in buffer obtained from malloc():

When obtaining a buffer from malloc(), a program may develop erroneous and
creeping dependencies on whatever random (and sometimes repeatable) values
the buffer may contain.  The mem_malloc() function prevents this by always
setting the data in a buffer to a known non-zero value before returning its
pointer.  This also prevents another common error when running under MS-DOS
which doesn't clear unused memory when loading a program.  These bugs are
particularly nasty to find since correct program operation may depend on what
was last run!

6.    Realloc problems:

Common problems when using realloc() are: 1) depending on realloc() *not*
shifting the location of the buffer in memory, and 2) depending on finding
certain values in the uninitialized region of the realloc'ed buffer.

MEM flushes these out by *always* moving the buffer and stomping on values
past the initialized area.

7.    Memory leak detection:

Memory "leaks" are areas that are allocated but never freed.  This can become
a major problem in programs that must run for long periods without interrup-
tion (e.g. BBS's).  If there are leaks, eventually the program will run out
of memory and fail.

Another form of memory leak occurs when a piece of allocated memory should
have been added to some central data structure, but wasn't.

MEM find memory leaks by keeping track of all allocations and frees.  When
mem_term() is called, a list of all unfreed allocations is printed along with
the files and line numbers where the allocations occurred.

8.    Pointer checking:

Sometimes it's useful to be able to verify that a pointer is actually
pointing into free store. MEM provides a function...

      mem_checkptr(void *p);

...to do this.

9.    Consistency checking:

Occasionally, even MEM's internal data structures get clobbered by a wild
pointer.  When this happens, you can track it down by sprinkling your code
temporarily with calls to mem_check(), which performs a consistency check on
the free store.

10.   Out of memory handling:

MEM can be set using mem_setexception() (see MEM.H) to handle out-of-memory
conditions in any one of several predefined ways:

      1.    Present an "Out of memory" message and terminate the program.
      2.    Abort the program with no message.
      3.    Mimic ISO/ANSI and return NULL.
      4.    Call a user-specified function, perhaps involving virtual memory
            or some other "emergency reserve".
      5.    Retry (be careful to avoid infinite loops!)

11.   Companion techniques:

Since MEM presets allocated and stomps on freed memory, this facilitates
adding your own code to add tags to your data structures when debugging.  If
the structures are invalid, you'll know it because MEM will have clobbered
your verification tags.


SUMMARY:
--------

Since it is, in the final analysis, a software solution, MEM is fallible.  As
the saying goes, "Nothing is foolproof because fools are so ingenious."
Walter himself readily acknowledges that there are circumstances where your
code can do sufficient damage to MEM's internal data structures to render it
useless.  The good news is such circumstances are few and far between.  For
most memory debugging, MEM is a highly reliable and valuable addition to your
C programming toolchest.