4.3BSD/usr/man/man3/malloc.3

.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)malloc.3	6.3 (Berkeley) 5/14/86
.\"
.TH MALLOC 3  "May 14, 1986"
.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 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.
If the space is of
.I pagesize
or larger, the memory returned will be page-aligned.
.SH SEE ALSO
brk(2),
pagesize(2)
.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 implementation of
.I malloc
does not always fail gracefully when system
memory limits are approached.
It may fail to allocate memory when larger free blocks could be broken
up, or when limits are exceeded because the size is rounded up.
It is optimized for sizes that are powers of two.
.PP
.I Alloca
is machine dependent; its use is discouraged.