MALLOC(2) MALLOC(2)
NAME
malloc, mallocalign, mallocz, free, realloc, calloc, msize,
setmalloctag, setrealloctag, getmalloctag, getrealloctag,
malloctopoolblock - memory allocator
SYNOPSIS
#include <u.h>
#include <libc.h>
void* malloc(ulong size)
void* mallocalign(ulong size, ulong align, long offset,
ulong span)
void* mallocz(ulong size, int clr)
void free(void *ptr)
void* realloc(void *ptr, ulong size)
void* calloc(ulong nelem, ulong elsize)
ulong msize(void *ptr)
void setmalloctag(void *ptr, uintptr tag)
uintptrgetmalloctag(void *ptr)
void setrealloctag(void *ptr, uintptr tag)
uintptrgetrealloctag(void *ptr)
void* malloctopoolblock(void*)
DESCRIPTION
Malloc and free provide a simple memory allocation package.
Malloc returns a pointer to a new block of at least size
bytes. The block is suitably aligned for storage of any
type of object. No two active pointers from malloc will
have the same value. The call malloc(0) returns a valid
pointer rather than null.
The argument to free is a pointer to a block previously
allocated by malloc; this space is made available for fur-
ther allocation. It is legal to free a null pointer; the
effect is a no-op. The contents of the space returned by
malloc are undefined. Mallocz behaves as malloc, except
that if clr is non-zero, the memory returned will be zeroed.
Mallocalign allocates a block of at least n bytes of memory
Page 1 Plan 9 (printed 11/21/24)
MALLOC(2) MALLOC(2)
respecting alignment contraints. If align is non-zero, the
returned pointer is aligned to be equal to offset modulo
align. If span is non-zero, the n byte block allocated will
not span a span-byte boundary.
Realloc changes the size of the block pointed to by ptr to
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. Realloc takes on special meanings
when one or both arguments are zero:
realloc(0, size)
means `malloc(size)'; returns a pointer to the newly-
allocated memory
realloc(ptr, 0)
means `free(ptr)'; returns null
realloc(0, 0)
no-op; returns null
Calloc allocates space for an array of nelem elements of
size elsize. The space is initialized to zeros. Free frees
such a block.
When a block is allocated, sometimes there is some extra
unused space at the end. Msize grows the block to encompass
this unused space and returns the new number of bytes that
may be used.
The memory allocator maintains two word-sized fields associ-
ated with each block, the ``malloc tag'' and the ``realloc
tag''. By convention, the malloc tag is the PC that allo-
cated the block, and the realloc tag the PC that last real-
located the block. These may be set or examined with
setmalloctag, getmalloctag, setrealloctag, and
getrealloctag. When allocating blocks directly with malloc
and realloc, these tags will be set properly. If a custom
allocator wrapper is used, the allocator wrapper can set the
tags itself (usually by passing the result of getcallerpc(2)
to setmalloctag) to provide more useful information about
the source of allocation.
Malloctopoolblock takes the address of a block returned by
malloc and returns the address of the corresponding block
allocated by the pool(2) routines.
SOURCE
/sys/src/libc/port/malloc.c
SEE ALSO
leak(1), trump (in acid(1)), brk(2), getcallerpc(2), pool(2)
Page 2 Plan 9 (printed 11/21/24)
MALLOC(2) MALLOC(2)
DIAGNOSTICS
Malloc, realloc and calloc return 0 if there is no available
memory. Errstr is likely to be set. If the allocated
blocks have no malloc or realloc tags, getmalloctag and
getrealloctag return ~0.
After including pool.h, the call poolcheck(mainmem) can be
used to scan the storage arena for inconsistencies such as
data written beyond the bounds of allocated blocks. It is
often useful to combine this with setting
mainmem->flags |= POOL_NOREUSE;
at the beginning of your program. This will cause malloc
not to reallocate blocks even once they are freed;
poolcheck(mainmem) will then detect writes to freed blocks.
The trump library for acid can be used to obtain traces of
malloc execution; see acid(1).
BUGS
The different specification of calloc is bizarre.
User errors can corrupt the storage arena. The most common
gaffes are (1) freeing an already freed block, (2) storing
beyond the bounds of an allocated block, and (3) freeing
data that was not obtained from the allocator. When malloc
and free detect such corruption, they abort.
Page 3 Plan 9 (printed 11/21/24)