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malloc


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.\" $NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
.\" $FreeBSD: src/share/man/man9/malloc.9,v 1.13 2000/03/03 14:05:06 sheldonh Exp $
.\"
.Dd June 16, 1996
.Dt MALLOC 9
.Os FreeBSD
.Sh NAME
.Nm malloc ,
.Nm MALLOC ,
.Nm free ,
.Nm FREE
.Nd kernel memory management routines
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/malloc.h>
.Ft void *
.Fn malloc "unsigned long size" "struct malloc_type *type" "int flags"
.Fn MALLOC "space" "cast" "unsigned long size" "struct malloc_type  *type" "int flags"
.Ft void
.Fn free "void *addr" "struct malloc_type *type"
.Fn FREE "void *addr" "struct malloc_type *type"
.Sh DESCRIPTION
The
.Fn malloc
.Ed
.Pp
and the
.Fn FREE
macro variant is equivalent to
.Bd -literal -offset indent
free((addr), type)
.Ed
.Pp
Unlike its standard C library counterpart
.Pq Xr malloc 3 ,
the kernel version takes two more arguments.  The
.Fa flags
argument further qualifies
.Fn malloc No Ns 's
operational characteristics as follows:
.Bl -tag -width indent
.It Dv M_NOWAIT
Causes
.Fn malloc
to return
.Dv NULL
if the request cannot be immediately fulfilled due to resource shortage.
Otherwise,
.Fn malloc
may call sleep to wait for resources to be released by other processes.
If this flag is set,
.Fn malloc
will return 
.Dv NULL
rather then block.  Note that
.Dv M_WAITOK
is defined to be 0, meaning that blocking operation is the default.
.It Dv M_ASLEEP
Causes
.Fn malloc
to call 
.Fn asleep
if the request cannot be immediately fulfilled due to a resource shortage.
M_ASLEEP is not useful alone and should always be or'd with M_NOWAIT to allow
malloc to call
.Fn asleep
and return
.Dv NULL
immediately.  It is expected that the caller will at some point call
.Fn await
and then retry the allocation.  Depending on the routine in question, the
caller may decide to propagate the temporary failure up the call chain
and actually have some other higher level routine block on the async wait
that
.Fn malloc
queued.
renamed to something more obvious.  This option has been depreciated and is
slowly being removed from the kernel, and so should not be used with any new
programming.
.El
.Pp
The
.Fa type
argument is used to perform statistics on memory usage, and for
basic sanity checks.
The statistics can be examined by
.Sq vmstat -m .
.Pp
A
.Fa type
is defined using the
.Va malloc_type_t
typedef via the
.Fn MALLOC_DECLARE
and
.Fn MALLOC_DEFINE
macros.
.Bd -literal -offset indent
/* sys/something/foo_extern.h */

MALLOC_DECLARE(M_FOOBUF);

/* sys/something/foo_main.c */

MALLOC_DEFINE(M_FOOBUF, "foobuffers", "Buffers to foo data into the ether");

/* sys/something/foo_subr.c */

...
MALLOC(buf, struct foo_buf *, sizeof *buf, M_FOOBUF, M_NOWAIT);

.Ed
.Sh RETURN VALUES
.Fn malloc
returns a kernel virtual address that is suitably aligned for storage of
any type of object, or
.Dv NULL
if the request could not be satisfied and 
.Dv M_NOWAIT 
was set.  If 
.Dv M_ASLEEP
was set and
.Fn malloc
returns
.Dv NULL ,
it will call
.Fn asleep
as a side effect.
.Pp
.It
panic:
.Dq malloc: bogus type
.It
panic:
.Dq malloc: allocation too large
.It
panic:
.Dq malloc: wrong bucket
.It
panic:
.Dq malloc: lost data
.It
panic:
.Dq free: address 0x%x out of range
.It
panic:
.Dq free: type %d out of range
.It
panic:
.Dq free: unaligned addr Aq description of object
.It
panic:
.Dq free: item modified
.It
panic:
.Dq free: multiple free[s]
.It
.Dq Data modified on freelist: Aq description of object
.El
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