V7/usr/man/man3/malloc.3

.TH MALLOC 3 
.SH NAME
malloc, free, realloc, calloc \- main memory allocator
.SH SYNOPSIS
.B char *malloc(size)
.br
.B unsigned size;
.PP
.B free(ptr)
.br
.B char *ptr;
.PP
.B char *realloc(ptr, size)
.br
.B char *ptr;
.br
.B unsigned size;
.PP
.B char *calloc(nelem, elsize)
.br
.B unsigned nelem, elsize;
.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 break (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;
see the source code.
.SH BUGS
When
.I realloc
returns 0,
the block
pointed to by
.I ptr
may be destroyed.