4.1cBSD/usr/man/man3/malloc.3

.TH MALLOC 3  "19 January 1983"
.UC 4
.SH NAME
malloc, free, realloc, calloc \- main memory allocator
.SH SYNOPSIS
.nf
.B char *malloc(size)
.B unsigned size;
.PP
.B free(ptr)
.B char *ptr;
.PP
.B char *realloc(ptr, size)
.B char *ptr;
.B unsigned size;
.PP
.B char *calloc(nelem, elsize)
.B unsigned nelem, elsize;
.fi
.SH DESCRIPTION
.I Malloc
and
.I free
provide a simple general-purpose memory allocation package.
.I Malloc
returns a pointer to a block of at least
.I size
bytes beginning on a word boundary.
.PP
The argument to
.I free
is a pointer to a block previously allocated by
.IR malloc ;
this space is made available for further allocation,
but its contents are left undisturbed.
.PP
Needless to say, grave disorder will result if the space assigned by
.I malloc
is overrun or if some random number is handed to
.IR free .
.PP
.I Malloc
allocates the first big enough contiguous reach of free space
found in a circular search from the last block allocated or freed,
coalescing adjacent free blocks as it searches.
It calls
.I sbrk
(see
.IR brk (2))
to get more memory from the system when there is no
suitable space already free.
.PP
.I Realloc
changes the size of the block pointed to by
.I ptr
to
.I size
bytes and returns a pointer to the (possibly moved) block.
The contents will be unchanged up to the lesser of the new and old sizes.
.PP
.I Realloc
also works if
.I ptr
points to a block freed since the last call of
.I malloc, realloc
or
.IR calloc ;
thus sequences of
.I free, malloc
and
.I realloc
can exploit the search strategy of
.I malloc
to do storage compaction.
.PP
.I Calloc
allocates space for an array of
.I nelem
elements of size
.I elsize.
The space is initialized to zeros.
.PP
Each of the allocation routines returns a pointer
to space suitably aligned (after possible pointer coercion)
for storage of any type of object.
.SH DIAGNOSTICS
.I Malloc, realloc
and
.I calloc
return a null pointer (0) if there is no available memory or if the arena
has been detectably corrupted by storing outside the bounds of a block.
.I Malloc
may be recompiled to check the arena very stringently on every transaction;
those sites with a source code license may check the source code to see
how this can be done.
.SH BUGS
When
.I realloc
returns 0, the block pointed to by
.I ptr
may be destroyed.
.PP
The current incarnation of the allocator is unsuitable for direct use in
a large virtual environment where many small blocks are to be kept, since
it keeps all allocated and freed blocks on a single circular list.  Just
before more memory is allocated, all allocated and freed blocks are
referenced; this can cause a huge number of page faults.