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Freedom
WARNING - COMMUNIST PROPAGANDA DETECTED. MATERIAL THAT GENERATED THE OFFENSE:
This is what we all strive for. We want to be big strong independent basement dwellers who still live in their parents' home. Achieving total freedom in his use of technology is the greatest accomplishment a gentoomen can attain. There are rumors that when this happens, golden light starts to emanate from your computer screen and Richard Stallman materializes in your room. He has written the GPL to help you reach this state of nirvana. In order to have true freedom, you must have: * The freedom to run the program, for any purpose (freedom 0). * The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this. * The freedom to redistribute copies so you can help your neighbor (freedom 2). * The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
ADMINISTERING SANE AND RATIONAL COUNTERMEASURES FOR THOSE SEEKING THE /g/ WIKI PAGE "Free":
MALLOC(3) OpenBSD Programmer's Manual MALLOC(3)
NAME
malloc, calloc, realloc, free, cfree - memory allocation and deallocation
SYNOPSIS
#include <stdlib.h>
void *
malloc(size_t size);
void *
calloc(size_t nmemb, size_t size);
void *
realloc(void *ptr, size_t size);
void
free(void *ptr);
void
cfree(void *ptr);
char * malloc_options;
DESCRIPTION
The malloc() function allocates uninitialized space for an object whose
size is specified by size. The malloc() function maintains multiple
lists of free blocks according to size, allocating space from the
appropriate list.
The allocated space is suitably aligned (after possible pointer coercion)
for storage of any type of object. If the space is of pagesize or
larger, the memory returned will be page-aligned.
Allocation of a zero size object returns a pointer to a zero size object.
This zero size object is access protected, so any access to it will
generate an exception (SIGSEGV). Many zero-sized objects can be placed
consecutively in shared protected pages. The minimum size of the
protection on each object is suitably aligned and sized as previously
stated, but the protection may extend further depending on where in a
protected zone the object lands.
When using malloc() be careful to avoid the following idiom:
if ((p = malloc(num * size)) == NULL)
err(1, "malloc");
The multiplication may lead to an integer overflow. To avoid this,
calloc() is recommended.
If malloc() must be used, be sure to test for overflow:
if (size && num > SIZE_MAX / size) {
errno = ENOMEM;
err(1, "overflow");
}
The calloc() function allocates space for an array of nmemb objects, each
of whose size is size. The space is initialized to zero. The use of
calloc() is strongly encouraged when allocating multiple sized objects in
order to avoid possible integer overflows.
The free() function causes the space pointed to by ptr to be either
placed on a list of free pages to make it available for future allocation
or, if required, to be returned to the kernel using munmap(2). If ptr is
a null pointer, no action occurs.
A cfree() function is also provided for compatibility with old systems
and other malloc libraries; it is simply an alias for free().
The realloc() function changes the size of the object pointed to by ptr
to size bytes and returns a pointer to the (possibly moved) object. The
contents of the object 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 object is indeterminate and uninitialized. If ptr is a
null pointer, the realloc() function behaves like the malloc() function
for the specified size. If the space cannot be allocated, the object
pointed to by ptr is unchanged. If size is zero and ptr is not a null
pointer, the object it points to is freed and a new zero size object is
returned.
When using realloc() be careful to avoid the following idiom:
size += 50;
if ((p = realloc(p, size)) == NULL)
return (NULL);
Do not adjust the variable describing how much memory has been allocated
until the allocation has been successful. This can cause aberrant
program behavior if the incorrect size value is used. In most cases, the
above sample will also result in a leak of memory. As stated earlier, a
return value of NULL indicates that the old object still remains
allocated. Better code looks like this:
newsize = size + 50;
if ((newp = realloc(p, newsize)) == NULL) {
free(p);
p = NULL;
size = 0;
return (NULL);
}
p = newp;
size = newsize;
As with malloc() it is important to ensure the new size value will not
overflow; i.e. avoid allocations like the following:
if ((newp = realloc(p, num * size)) == NULL) {
...
MALLOC_OPTIONS
Malloc will first look for a symbolic link called /etc/malloc.conf and
next check the environment for a variable called MALLOC_OPTIONS and
finally for the global variable malloc_options and scan them for flags in
that order. Flags are single letters, uppercase means on, lowercase
means off.
A ``Abort. malloc() will coredump the process, rather than
tolerate internal inconsistencies or incorrect usage. This is
the default and a very handy debugging aid, since the core file
represents the time of failure, rather than when the bogus
pointer was used.
D ``Dump. malloc() will dump statistics to the file
./malloc.out, if it already exists, at exit. This option
requires the library to have been compiled with -DMALLOC_STATS in
order to have any effect.
F ``Freeguard. Enable use after free detection. Unused pages on
the freelist are read and write protected to cause a segmentation
fault upon access. This will also switch off the delayed freeing
of chunks, reducing random behaviour but detecting double free()
calls as early as possible. This option is intended for
debugging rather than improved security (use the U option for
security).
G ``Guard. Enable guard pages. Each page size or larger
allocation is followed by a guard page that will cause a
segmentation fault upon any access.
H ``Hint. Pass a hint to the kernel about pages we don't use.
If the machine is paging a lot this may help a bit.
J ``Junk. Fill some junk into the area allocated. Currently
junk is bytes of 0xd0 when allocating; this is pronounced
``Duh. :-) Freed chunks are filled with 0xdf.
P ``Move allocations within a page. Allocations larger than half
a page but smaller than a page are aligned to the end of a page
to catch buffer overruns in more cases. This is the default.
R ``realloc. Always reallocate when realloc() is called, even if
the initial allocation was big enough. This can substantially
aid in compacting memory.
S Enable all options suitable for security auditing.
U ``Free unmap. Enable use after free protection for larger
allocations. Unused pages on the freelist are read and write
protected to cause a segmentation fault upon access.
X ``xmalloc. Rather than return failure, abort(3) the program
with a diagnostic message on stderr. It is the intention that
this option be set at compile time by including in the source:
extern char *malloc_options;
malloc_options = "X";
Note that this will cause code that is supposed to handle out-of-
memory conditions gracefully to abort instead.
Z ``Zero. Fill some junk into the area allocated (see J), except
for the exact length the user asked for, which is zeroed.
< ``Half the cache size. Decrease the size of the free page
cache by a factor of two.
> ``Double the cache size. Increase the size of the free page
cache by a factor of two.
So to set a systemwide reduction of the cache to a quarter of the default
size and use guard pages:
# ln -s 'G<<' /etc/malloc.conf
The flags are mostly for testing and debugging. If a program changes
behavior if any of these options (except X) are used, it is buggy.
The default number of free pages cached is 64.
RETURN VALUES
The malloc() and calloc() functions return a pointer to the allocated
space if successful; otherwise, a null pointer is returned and errno is
set to ENOMEM.
The free() and cfree() functions return no value.
The realloc() function returns a pointer to the (possibly moved)
allocated space if successful; otherwise, a null pointer is returned and
errno is set to ENOMEM.
ENVIRONMENT
MALLOC_OPTIONS See above.
FILES
/etc/malloc.conf symbolic link to filename containing option flags
DIAGNOSTICS
If malloc(), calloc(), realloc(), or free() detect an error condition, a
message will be printed to file descriptor 2 (not using stdio). Errors
will result in the process being aborted, unless the a option has been
specified.
Here is a brief description of the error messages and what they mean:
``out of memory
If the X option is specified it is an error for malloc(),
calloc(), or realloc() to return NULL.
``malloc init mmap failed
This is a rather weird condition that is most likely to indicate
a seriously overloaded system or a ulimit restriction.
``bogus pointer (double free?)
An attempt to free() or realloc() an unallocated pointer was
made.
``chunk is already free
There was an attempt to free a chunk that had already been freed.
``modified chunk-pointer
The pointer passed to free() or realloc() has been modified.
``recursive call
An attempt was made to call recursively into these functions,
i.e., from a signal handler. This behavior is not supported. In
particular, signal handlers should not use any of the malloc()
functions nor utilize any other functions which may call malloc()
(e.g., stdio(3) routines).
``unknown char in MALLOC_OPTIONS
We found something we didn't understand.
``malloc cache overflow/underflow
The internal malloc page cache has been corrupted.
``malloc free slot lost
The internal malloc page cache has been corrupted.
``guard size
An inconsistent guard size was detected.
any other error
malloc() detected an internal error; consult sources and/or
wizards.
SEE ALSO
brk(2), mmap(2), munmap(2), alloca(3), getpagesize(3), posix_memalign(3)
STANDARDS
The malloc() function conforms to ANSI X3.159-1989 (``ANSI C89).
HISTORY
A free() internal kernel function and a predecessor to malloc(), alloc(),
first appeared in Version 1 AT&T UNIX. C library functions alloc() and
free() appeared in Version 6 AT&T UNIX. The functions malloc(),
calloc(), and realloc() first appeared in Version 7 AT&T UNIX.
A new implementation by Chris Kingsley was introduced in 4.2BSD, followed
by a complete rewrite by Poul-Henning Kamp which appeared in FreeBSD 2.2
and was included in OpenBSD 2.0. These implementations were all sbrk(2)
based. In OpenBSD 3.8, Thierry Deval rewrote malloc to use the mmap(2)
system call, making the page addresses returned by malloc random. A
rewrite by Otto Moerbeek introducing a new central data structure and
more randomization appeared in OpenBSD 4.4.
OpenBSD 5.4 July 18, 2013 OpenBSD 5.4
