Copyright (c) 1996 The NetBSD Foundation, Inc.
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.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/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 .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 No Ns 's operational characteristics as follows: l -tag -offset indent 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 not set, .Fn malloc will never return .Dv NULL . Note that .Dv M_WAITOK is conveniently defined to be 0, and hence maybe or'ed into the .Fa flags argument to indicate that it's Ok to wait for resources. .El
p Currently, only one flag is defined.
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 like this: d -literal -offset indent /* sys/something/foo_extern.h */ extern malloc_type_t M_FOOBUF; /* sys/something/foo_main.c */ malloc_type_t M_FOOBUF = { "Foo Buffers", "Buffers for foo data in transit to the InfImpDrive" }; /* sys/something/foo_subr.c */ ... MALLOC(buf, sizeof *buf, struct foo_buf *, M_FOOBUF, M_NOWAIT); e .Sh RETURN VALUES .Fn malloc returns a kernel virtual address that is suitably aligned for storage of any type of object. .Sh SEE ALSO .Xr vmstat 8 .Sh DIAGNOSTICS A kernel compiled with the .Dv DIAGNOSTIC configuration option attempts to detect 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