MALLOC(3)

NAME
malloc, calloc, realloc, free - general purpose memory allocation 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
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 initialized to nul bytes.

The realloc() function changes the size of the previously allocated memory referenced by ptr to size bytes. The contents of the memory are unchanged 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 undefined. 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 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 routines, 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 behavior 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() or realloc() as well as all memory returned by free() or realloc() 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 may help performance if the system is paging excessively.

R   Cause the realloc() function to always reallocate 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 instead 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 zeros out the bytes that were requested. This is intended for debugging 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 application 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 allocation functions:

MALLOC_OPTIONS
    If the environmental 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() function returns a pointer, possibly identical 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 implementations 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 containing 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 detected. 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 behave 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 situation, this diagnosis has been confirmed. If the program still misbehaves, 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” defined, and an error was found during the additional error checking. Consult the source code for further information.

allocation failed
    If the “A” option is specified it is a fatal error for an allocation 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 realloc'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 recursively. 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 ansiC.

BUGS
The messages printed in case of problems provide no detail about the actual 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 release 2.2.