plan9fox/sys/man/2/malloc

.TH MALLOC 2
.SH NAME
malloc, mallocalign, mallocz, free, realloc, calloc, msize, setmalloctag, setrealloctag, getmalloctag, getrealloctag, malloctopoolblock \- memory allocator
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.PP
.ta \w'\fLvoid* 'u
.B
void*	malloc(ulong size)
.PP
.B
void*	mallocalign(ulong size, ulong align, long offset, ulong span)
.PP
.B
void*	mallocz(ulong size, int clr)
.PP
.B
void	free(void *ptr)
.PP
.B
void*	realloc(void *ptr, ulong size)
.PP
.B
void*	calloc(ulong nelem, ulong elsize)
.PP
.B
ulong	msize(void *ptr)
.PP
.B
void	setmalloctag(void *ptr, uintptr tag)
.PP
.B
uintptr	getmalloctag(void *ptr)
.PP
.B
void	setrealloctag(void *ptr, uintptr tag)
.PP
.B
uintptr	getrealloctag(void *ptr)
.PP
.B
void*	malloctopoolblock(void*)
.PP
.SH DESCRIPTION
.I Malloc
and
.I free
provide a simple memory allocation package.
.I Malloc
returns a pointer to a new block of at least
.I size
bytes.
The block is suitably aligned for storage of any type of object.
No two active pointers from
.I malloc
will have the same value.
The call
.B malloc(0)
returns a valid pointer rather than null.
.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.
It is legal to free a null pointer; the effect is a no-op.
The contents of the space returned by
.I malloc
are undefined.
.I Mallocz
behaves as 
.IR malloc ,
except that if 
.I clr
is non-zero, the memory returned will be zeroed.
.PP
.I Mallocalign
allocates a block of at least 
.I n
bytes of memory respecting alignment contraints.
If
.I align
is non-zero,
the returned pointer is aligned to be equal to
.I offset
modulo
.IR align .
If
.I span
is non-zero,
the
.I n
byte block allocated will not span a
.IR span -byte
boundary.
.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.
.I Realloc
takes on special meanings when one or both arguments are zero:
.TP
.B "realloc(0,\ size)
means
.LR malloc(size) ;
returns a pointer to the newly-allocated memory
.TP
.B "realloc(ptr,\ 0)
means
.LR free(ptr) ;
returns null
.TP
.B "realloc(0,\ 0)
no-op; returns null
.PD
.PP
.I Calloc
allocates space for
an array of
.I nelem
elements of size
.IR elsize .
The space is initialized to zeros.
.I Free
frees such a block.
.PP
When a block is allocated, sometimes there is some extra unused space at the end.
.I Msize
grows the block to encompass this unused space and returns the new number
of bytes that may be used.
.PP
The memory allocator maintains two word-sized fields
associated with each block, the ``malloc tag'' and the ``realloc tag''.
By convention, the malloc tag is the PC that allocated the block,
and the realloc tag the PC that last reallocated the block.
These may be set or examined with 
.IR setmalloctag ,
.IR getmalloctag ,
.IR setrealloctag ,
and
.IR getrealloctag .
When allocating blocks directly with
.I malloc
and
.IR 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
.IR getcallerpc (2) 
to 
.IR setmalloctag )
to provide more useful information about
the source of allocation.
.PP
.I Malloctopoolblock
takes the address of a block returned by
.I malloc
and returns the address of the corresponding
block allocated by the
.IR pool (2)
routines.
.SH SOURCE
.B /sys/src/libc/port/malloc.c
.SH SEE ALSO
.IR leak (1),
.I trump
(in
.IR acid (1)),
.IR brk (2),
.IR getcallerpc (2),
.IR pool (2)
.SH DIAGNOSTICS
.I Malloc, realloc
and
.I calloc
return 0 if there is no available memory.
.I Errstr
is likely to be set.
If the allocated blocks have no malloc or realloc tags,
.I getmalloctag
and
.I getrealloctag
return
.BR ~0 .
.PP
After including
.BR pool.h ,
the call
.B 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
.EX
    mainmem->flags |= POOL_NOREUSE;
.EE
at the beginning of your program.
This will cause malloc not to reallocate blocks even
once they are freed;
.B poolcheck(mainmem)
will then detect writes to freed blocks.
.PP
The 
.I trump
library for
.I acid
can be used to obtain traces of malloc execution; see
.IR acid (1).
.SH BUGS
The different specification of
.I calloc
is bizarre.
.PP
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
.I malloc
and
.I free
detect such corruption, they abort.