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MALLOC(3)		 BSD Library Functions Manual		     MALLOC(3)

NAME
     malloc, calloc, realloc, free, reallocf --	general	purpose	memory alloca-
     tion functions

SYNOPSIS
     #include <stdlib.h>

     void *
     malloc(size_t size);

     void *
     calloc(size_t number, size_t size);

     void *
     realloc(void *ptr,	size_t size);

     void *
     reallocf(void *ptr, size_t	size);

     void
     free(void *ptr);

     char * malloc_options;

DESCRIPTION
     The malloc() function allocates size bytes	of memory.  The	allocated
     space is suitably aligned (after possible pointer coercion) for storage
     of	any type of object.  If	the space is at	least pagesize bytes in	length
     (see getpagesize((3)),) the returned memory will be page boundary aligned
     as	well.  If malloc() fails, a NULL pointer is returned.

     The calloc() function allocates space for number objects, each size bytes
     in	length.	 The result is identical to calling malloc() with an argument
     of	"number	* size", with the exception that the allocated memory is ini-
     tialized to nul bytes.

     The realloc() function changes the	size of	the previously allocated mem-
     ory referenced by ptr to size bytes.  The contents	of the memory are un-
     changed up	to the lesser of the new and old sizes.	 If the	new size is
     larger, the value of the newly allocated portion of the memory is unde-
     fined.  If	the requested memory cannot be allocated, NULL is returned and
     the memory	referenced by ptr is valid and unchanged.  If ptr is NULL, the
     realloc() function	behaves	identically to malloc()	for the	specified
     size.

     The reallocf() function call is identical to the realloc function call,
     except that it will free the passed pointer when the requested memory
     cannot be allocated.  This	is a FreeBSD specific API designed to ease the
     problems with traditional coding styles for realloc causing memory	leaks
     in	libraries.

     The free()	function causes	the allocated memory referenced	by ptr to be
     made available for	future allocations.  If	ptr is NULL, no	action occurs.

TUNING
     Once, when	the first call is made to one of these memory allocation rou-
     tines, various flags will be set or reset,	which affect the workings of
     this allocation implementation.

     The ``name'' of the file referenced by the	symbolic link named
     /etc/malloc.conf, the value of the	environment variable MALLOC_OPTIONS,
     and the string pointed to by the global variable malloc_options will be
     interpreted, in that order, character by character	as flags.

     Most flags	are single letters, where uppercase indicates that the behav-
     ior is set, or on,	and lowercase means that the behavior is not set, or
     off.

     A	     All warnings (except for the warning about	unknown	flags being
	     set), and failure to allocate memory become fatal.	 The process
	     will call abort(3)	in these cases.

     J	     Each byte of new memory allocated by malloc(), realloc() or
	     reallocf()	as well	as all memory returned by free(), realloc() or
	     reallocf()	will be	initialized to 0xd0.  This options also	sets
	     the "R" option.  This is intended for debugging and will impact
	     performance negatively.

     H	     Pass a hint to the	kernel about pages unused by the allocation
	     functions.	 This will help	performance if the system is paging
	     excessively.  This	option is off by default.

     R	     Causes the	realloc() and reallocf() functions to always reallo-
	     cate memory even if the initial allocation	was sufficiently
	     large.  This can substantially aid	in compacting memory.

     U	     Generate "utrace" entries for ktrace(1), for all operations.
	     Consult the source	for details on this option.

     V	     Attempting	to allocate zero bytes will return a NULL pointer in-
	     stead of a	valid pointer.	(The default behavior is to make a
	     minimal allocation	and return a pointer to	it.)  This option is
	     provided for System V compatibility.  This	option is incompatible
	     with the "X" option.

     X	     Rather than return	failure	for any	allocation function, display a
	     diagnostic	message	on stderr and cause the	program	to drop	core
	     (using abort(3, ).)  This option should be	set at compile time by
	     including the following in	the source code:

		   extern char *malloc_options;
		   malloc_options = "X";

     Z	     This option implicitly sets the "J" and "R" options, and then ze-
	     ros out the bytes that were requested.  This is intended for de-
	     bugging and will impact performance negatively.

     <	     Reduce the	size of	the cache by a factor of two.  The default
	     cache size	is 16 pages.  This option can be specified multiple
	     times.

     >	     Double the	size of	the cache by a factor of two.  The default
	     cache size	is 16 pages.  This option can be specified multiple
	     times.

     The "J" and "Z" options are intended for testing and debugging.  An ap-
     plication which changes its behavior when these options are used is
     flawed.

EXAMPLES
     To	set a systemwide reduction of cache size, and to dump core whenever a
     problem occurs:

	   ln -s 'A<' /etc/malloc.conf

     To	specify	in the source that a program does no return value checking on
     calls to these functions:

	   extern char *malloc_options;
	   malloc_options = "X";

ENVIRONMENT
     The following environment variables affect	the execution of the alloca-
     tion functions:

     MALLOC_OPTIONS
	  If the environment variable MALLOC_OPTIONS is	set, the characters it
	  contains will	be interpreted as flags	to the allocation functions.

RETURN VALUES
     The malloc() and calloc() functions return	a pointer to the allocated
     memory if successful; otherwise a NULL pointer is returned.

     The realloc() and reallocf() functions return a pointer, possibly identi-
     cal to ptr, to the	allocated memory if successful;	otherwise a NULL
     pointer is	returned, in which case	the memory referenced by ptr is	still
     available and intact.

     The free()	function returns no value.

DEBUGGING MALLOC PROBLEMS
     The major difference between this implementation and other	allocation im-
     plementations is that the free pages are not accessed unless allocated,
     and are aggressively returned to the kernel for reuse.

	   Most	allocation implementations will	store a	 data  structure  con-
	   taining a linked list in the	free chunks of memory, used to tie all
	   the free memory together.  That can be suboptimal,  as  every  time
	   the	free-list is traversed,	the otherwise unused, and likely paged
	   out,	pages are faulted into primary memory.	On systems  which  are
	   paging,  this can result in a factor	of five	increase in the	number
	   of page-faults done by a process.

     A side effect of this architecture	is that	many minor transgressions on
     the interface which would traditionally not be detected are in fact de-
     tected.  As a result, programs that have been running happily for years
     may suddenly start	to complain loudly, when linked	with this allocation
     implementation.

     The first and most	important thing	to do is to set	the "A"	option.	 This
     option forces a coredump (if possible) at the first sign of trouble,
     rather than the normal policy of trying to	continue if at all possible.

     It	is probably also a good	idea to	recompile the program with suitable
     options and symbols for debugger support.

     If	the program starts to give unusual results, coredump or	generally be-
     have differently without emitting any of the messages listed in the next
     section, it is likely because it depends on the storage being filled with
     nul bytes.	 Try running it	with "Z" option	set; if	that improves the sit-
     uation, this diagnosis has	been confirmed.	 If the	program	still misbe-
     haves, the	likely problem is accessing memory outside the allocated area,
     more likely after than before the allocated area.

     Alternatively, if the symptoms are	not easy to reproduce, setting the "J"
     option may	help provoke the problem.

     In	truly difficult	cases, the "U" option, if supported by the kernel, can
     provide a detailed	trace of all calls made	to these functions.

     Unfortunately this	implementation does not	provide	much detail about the
     problems it detects, the performance impact for storing such information
     would be prohibitive.  There are a	number of allocation implementations
     available on the 'Net which focus on detecting and	pinpointing problems
     by	trading	performance for	extra sanity checks and	detailed diagnostics.

DIAGNOSTIC MESSAGES
     If	malloc(), calloc(), realloc() or free()	detect an error	or warning
     condition,	a message will be printed to file descriptor STDERR_FILENO.
     Errors will result	in the process dumping core.  If the "A" option	is
     set, all warnings are treated as errors.

     The following is a	brief description of possible error messages and their
     meanings:

     (ES): mumble mumble mumble
	     The allocation functions were compiled with "EXTRA_SANITY"	de-
	     fined, and	an error was found during the additional error check-
	     ing.  Consult the source code for further information.

     allocation	failed
	     If	the "A"	option is specified it is a fatal error	for an alloca-
	     tion function to fail.

     mmap(2) failed, check limits
	     This most likely means that the system is dangerously overloaded
	     or	that the process' limits are incorrectly specified.

     freelist is destroyed
	     The internal free-list has	been corrupted.

     The following is a	brief description of possible warning messages and
     their meanings:

     chunk/page	is already free
	     The process attempted to free() memory which had already been
	     freed.

     junk pointer ...
	     A pointer specified to one	of the allocation functions points
	     outside the bounds	of the memory of which they are	aware.

     malloc() has never	been called
	     No	memory has been	allocated, yet something is being freed	or re-
	     alloc'ed.

     modified (chunk-/page-) pointer
	     The pointer passed	to free() or realloc() has been	modified.

     pointer to	wrong page
	     The pointer that malloc() or calloc() is trying to	free does not
	     reference a possible page.

     recursive call
	     A process has attempted to	call an	allocation function recur-
	     sively.  This is not permitted.  In particular, signal handlers
	     should not	attempt	to allocate memory.

     out of memory
	     The "X" option was	specified and an allocation of memory failed.

     unknown char in MALLOC_OPTIONS
	     An	unknown	option was specified.  Even with the "A" option	set,
	     this warning is still only	a warning.

SEE ALSO
     brk(2), alloca(3),	getpagesize(3),	memory(3)
     /usr/share/doc/papers/malloc.ascii.gz

STANDARDS
     The malloc(), calloc(), realloc() and free() functions conform to ANSI
     X3.159-1989 ("ANSI	C89").

BUGS
     The messages printed in case of problems provide no detail	about the ac-
     tual values.

     It	can be argued that returning a null pointer when asked to allocate
     zero bytes	is a silly response to a silly question.

     This implementation was authored by Poul-Henning Kamp.  Please report any
     problems to him at	<phk@FreeBSD.org>.

HISTORY
     The present allocation implementation started out as a filesystem for a
     drum attached to a	20bit binary challenged	computer which was built with
     discrete germanium	transistors.  It has since graduated to	handle primary
     storage rather than secondary.  It	first appeared in its new shape	and
     ability in	FreeBSD	2.2.

FreeBSD				August 27, 1996			       FreeBSD

NAME | SYNOPSIS | DESCRIPTION | TUNING | EXAMPLES | ENVIRONMENT | RETURN VALUES | DEBUGGING MALLOC PROBLEMS | DIAGNOSTIC MESSAGES | SEE ALSO | STANDARDS | BUGS | HISTORY

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