freebsd-dev/lib/libc/stdlib/malloc.3

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.\"     @(#)malloc.3	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt MALLOC 3
.Os BSD 4
.Sh NAME
.Nm malloc ,
.Nd general memory allocation function
.Pp
.Nm free
.Nd free up memory allocated with malloc, calloc or realloc
.Pp
.Nm realloc
.Nd reallocation of memory function
.Sh SYNOPSIS
.Fd #include <stdlib.h>
.Ft void *
.Fn malloc "size_t size"
.Ft void
.Fn free "void *ptr"
.Ft void *
.Fn realloc "void *ptr" "size_t size"
.Sh DESCRIPTION
The
.Fn malloc
function allocates uninitialized space for an object whose
size is specified by
.Fa size .
The
.Fn malloc
function maintains multiple lists of free blocks according to size, allocating
space from the appropriate list.
.Pp
The allocated space is
suitably aligned (after possible pointer
coercion) for storage of any type of object. If the space is of
.Em pagesize
or larger, the memory returned will be page-aligned.
.Pp
The
.Fn free
function causes the space pointed to by
.Fa ptr
to be deallocated, that is, at least made available for further allocation,
but if possible, it will passed back to the kernel with
.Xr sbrk 2 .
If
.Fa ptr
is a null pointer, no action occurs.
.Pp
The
.Fn realloc
function changes the size of the object pointed to by
.Fa ptr
to the size specified by
.Fa size .
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.
If
.Fa ptr
is a null pointer, the
.Fn realloc
function behaves like the
.Fn malloc 
function for the specified size.
If the space cannot be allocated, the object 
pointed to by
.Fa ptr
is unchanged.
If
.Fa size
is zero and
.Fa ptr
is not a null pointer, the object it points to is freed.
.Pp

.Sh ENVIRONMENT
This malloc will check the environment for a variable called
.Em MALLOC_OPTIONS
and scan it for flags.
Flags are single letters, uppercase means on, lowercase means off.
.Bl -tag -width indent
.It A
``abort'' malloc will coredump the process, rather than tolerate failure.
This is a very handy debugging aid, since the core file will represent the
time of failure,
rather than when the NULL pointer was accessed.

.It D
``dump'' malloc will dump statistics in a file called ``malloc.out'' at exit.

.It J
``junk'' fill some junk into the area allocated.
Currently junk is bytes of 0xd0, this is pronounced ``Duh'' :-)

.It R
``realloc'' always reallocate when
.Fn realloc
is called, even if the initial allocation was big enough.
This can substantially aid in compacting memory.

.It Z
``zero'' fill some junk into the area allocated (see ``J''),
except for the exact length the user asked for, which is zeroed.

.El
.Pp
The ``J'' and ``Z'' is mostly for testing and debugging,
if a program changes behavior if either of these options are used,
it is buggy.
.Sh RETURN VALUES
The
.Fn malloc
function returns
a pointer to the allocated space if successful; otherwise
a null pointer is returned.
.Pp
The
.Fn free
function returns no value.
.Pp
The
.Fn realloc
function returns either a null pointer or a pointer
to the possibly moved allocated space.
.Sh SEE ALSO
.Xr brk 2 ,
.Xr alloca 3 ,
.Xr calloc 3 ,
.Xr getpagesize 3 ,
.Xr memory 3
.Sh STANDARDS
The
.Fn malloc
function conforms to
.St -ansiC .
.Sh HISTORY
The present implementation of malloc started out as a filesystem on a drum
attached to a 20bit binary challenged computer built with discrete germanium
transistors, and it has since graduated to handle primary storage rather than
secondary.
.Pp
The main difference from other malloc implementations are believed to be that
the free pages are not accessed until allocated.
Most malloc implementations will store a data structure containing a, 
possibly double-, linked list in the free chunks of memory, used to tie
all the free memory together.
That is a quite suboptimal thing to do.
Every time the free-list is traversed, all the otherwise unused, and very
likely paged out, pages get faulted into primary memory, just to see what
lies after them in the list.
.Pp
On systems which are paging, this can make a factor five in difference on the
page-faults of a process.