4.2BSD/usr/man/man3/malloc.3

.TH MALLOC 3  "19 January 1983"
.UC 4
.SH NAME
malloc, free, realloc, calloc, alloca \- 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;
.PP
.B char *alloca(size)
.B int size;
.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
maintains multiple lists of free blocks according to size,
allocating space from the appropriate list.
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
In order to be compatible with older versions,
.I realloc
also works if
.I ptr
points to a block freed since the last call of
.I malloc, realloc
or
.IR calloc ;
sequences of
.I free, malloc
and
.I realloc
were previously used to attempt storage compaction.
This procedure is no longer recommended.
.PP
.I Calloc
allocates space for an array of
.I nelem
elements of size
.I elsize.
The space is initialized to zeros.
.PP
.I Alloca
allocates 
.I size
bytes of space in the stack frame of the caller.
This temporary space is automatically freed on
return.
.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
.I Alloca
is machine dependent; it's use is discouraged.