Copyright (c) 1996 The NetBSD Foundation, Inc.
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This code is derived from software contributed to The NetBSD Foundation
by Paul Kranenburg.

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$NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
$FreeBSD: head/share/man/man9/malloc.9 72123 2001-02-07 10:11:57Z ru $

.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 function allocates uninitialized memory in kernel address space for an object whose size is specified by .Fa size . .Fn free releases memory at address .Fa addr that was previously allocated by .Fn malloc for re-use. The memory is not zeroed. The .Fn MALLOC macro variant is functionally equivalent to d -literal -offset indent (space) = (cast)malloc((u_long)(size), type, flags) .Ed

p and the .Fn FREE macro variant is equivalent to d -literal -offset indent free((addr), type) .Ed

p Unlike its standard C library counterpart

q Xr malloc 3 , the kernel version takes two more arguments. The .Fa flags argument further qualifies .Fn malloc Ns 's operational characteristics as follows: l -tag -width indent t Dv M_ZERO Causes the allocated memory to be set to all zeros. t 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. t 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. t Dv M_WAITOK Indicates that it is Ok to wait for resources. It is unconveniently defined as 0 so care should be taken never to compare against this value directly or try to AND it as a flag. The default operation is to block until the memory allocation succeeds. .Fn malloc can only return .Dv NULL if .Dv M_NOWAIT is specified. t Dv M_USE_RESERVE Indicates that the system can dig into its reserve in order to obtain the requested memory. This option used to be called M_KERNEL but has been 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

p 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 .

p A .Fa type is defined using the .Va malloc_type_t typedef via the .Fn MALLOC_DECLARE and .Fn MALLOC_DEFINE macros. d -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. .Sh SEE ALSO .Xr vmstat 8 .Sh DIAGNOSTICS A kernel compiled with the .Dv DIAGNOSTIC configuration option attempts to detect memory corruption caused by such things as writing outside the allocated area and imbalanced calls to the .Fn malloc and .Fn free functions. Failing consistency checks will cause a panic or a system console message: l -bullet -offset indent -compact

p t panic: .Dq malloc: bogus type t panic: .Dq malloc: allocation too large t panic: .Dq malloc: wrong bucket t panic: .Dq malloc: lost data t panic: .Dq free: address 0x%x out of range t panic: .Dq free: type %d out of range t panic: .Dq free: unaligned addr Aq description of object t panic: .Dq free: item modified t panic: .Dq free: multiple free[s] t .Dq Data modified on freelist: Aq description of object .El