xref: /src/share/mk/bsd.README

#	$NetBSD: bsd.README,v 1.95 2002/04/10 15:05:45 lukem Exp $
#	@(#)bsd.README	8.2 (Berkeley) 4/2/94

This is the README file for the new make "include" files for the BSD
source tree.  The files are installed in /usr/share/mk, and are, by
convention, named with the suffix ".mk".

Note, this file is not intended to replace reading through the .mk
files for anything tricky.

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

RANDOM THINGS WORTH KNOWING:

The files are simply C-style #include files, and pretty much behave like
you'd expect.  The syntax is slightly different in that a single '.' is
used instead of the hash mark, i.e. ".include <bsd.prog.mk>".

One difference that will save you lots of debugging time is that inclusion
of the file is normally done at the *end* of the Makefile.  The reason for
this is because .mk files often modify variables and behavior based on the
values of variables set in the Makefile.  To make this work, remember that
the FIRST target found is the target that is used, i.e. if the Makefile has:

	a:
		echo a
	a:
		echo a number two

the command "make a" will echo "a".  To make things confusing, the SECOND
variable assignment is the overriding one, i.e. if the Makefile has:

	a=	foo
	a=	bar

	b:
		echo ${a}

the command "make b" will echo "bar".  This is for compatibility with the
way the V7 make behaved.

It's fairly difficult to make the BSD .mk files work when you're building
multiple programs in a single directory.  It's a lot easier to split up the
programs than to deal with the problem.  Most of the agony comes from making
the "obj" directory stuff work right, not because we switched to a new version
of make.  So, don't get mad at us, figure out a better way to handle multiple
architectures so we can quit using the symbolic link stuff.  (Imake doesn't
count.)

The file .depend in the source directory is expected to contain dependencies
for the source files.  This file is read automatically by make after reading
the Makefile.

The variable DESTDIR works as before.  It's not set anywhere but will change
the tree where the file gets installed.

The profiled libraries are no longer built in a different directory than
the regular libraries.  A new suffix, ".po", is used to denote a profiled
object, and ".so" denotes a shared (position-independent) object.

The following variables that control how things are made/installed that
are not set by default. These should not be set by Makefiles; they're for
the user to define in MAKECONF (see bsd.own.mk, below) or on the make(1)
command line:

BUILD 		If defined, 'make install' checks that the targets in the
		source directories are up-to-date and remakes them if they
                are out of date, instead of blindly trying to install
                out of date or non-existent targets.

UPDATE 		If defined, 'make install' only installs targets that are
		more recently modified in the source directories that their
		installed counterparts.

UNPRIVED	If defined, don't set the owner/group/mode when installing
		files or directories, and keep a metadata log of what
		the owner/group/mode should be.  This allows a
		non-root "make install".

MKBFD		If "no", don't build libbfd, libiberty, or any of
		the things that depend on them (binutils/gas/ld,
		gdb, dbsym, mdsetimage).  Only meaningful if
		USE_NEW_TOOLCHAIN is set.

MKCATPAGES	If "no", don't build or install the catman pages.

MKDOC		If "no", don't build or install the documentation.

MKGDB		If "no", don't build gdb.  Only meaningful if
		USE_NEW_TOOLCHAIN is set.

MKGCC		If "no", don't build gcc or any of the gcc-related
		libraries (libg2c, libgcc, libobjc, libstdc++).
		Only meaningful if USE_NEW_TOOLCHAIN is set.

MKIEEEFP	If "no", don't add code for IEEE754/IEC60559 conformance.
		Has no effect on most platforms.

MKINFO		If "no", don't build or install Info documentation from
		Texinfo source files.

MKLINT		If "no", don't build or install the lint libraries.

MKMAN		If "no", don't build or install the man or catman pages.
		Also acts as "MKCATPAGES=no"

MKNLS		If "no", don't build or install the NLS files and locale
		definition files.

MKOBJ		If "no", don't enable the rule which creates objdirs.
		"yes" by default.

MKOBJDIRS	If "no", don't create objdirs during a "make build".
		"no" by default.

MKPIC		If "no", don't build or install shared libraries.

MKPICINSTALL	If "no", don't install the *_pic.a libraries.

MKPROFILE	If "no", don't build or install the profiling libraries.

MKSHARE		If "no", act as "MKCATPAGES=no MKDOC=no MKINFO=no MKMAN=no
		MKNLS=no".  I.e, don't build catman pages, documentation,
		Info documentation, man pages, NLS files, ...

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

The include file <sys.mk> has the default rules for all makes, in the BSD
environment or otherwise.  You probably don't want to touch this file.
If you intend to run a cross build, you will need to supply the following
host tools, and configure the following variables properly:

OBJCOPY		objcopy - copy and translate object files

STRIP		strip - Discard symbols from object files

CONFIG		config - build kernel compilation directories

RPCGEN		rpcgen - Remote Procedure Call (RPC) protocol compiler

MKLOCALE	mklocale - make LC_CTYPE locale files

MTREE		mtree - build directory tree from a spec file

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The include file <bsd.man.mk> handles installing manual pages and their
links.

It has a two targets:

	maninstall:
		Install the manual page sources and their links.
	catinstall:
		Install the preformatted manual pages and their links.

It sets/uses the following variables:

MANDIR		Base path for manual installation.

MANGRP		Manual group.

MANOWN		Manual owner.

MANMODE		Manual mode.

MANSUBDIR	Subdirectory under the manual page section, i.e. "/vax"
		or "/tahoe" for machine specific manual pages.

MAN		The manual pages to be installed (use a .1 - .9 suffix).

MLINKS		List of manual page links (using a .1 - .9 suffix).  The
		linked-to file must come first, the linked file second,
		and there may be multiple pairs.  The files are soft-linked.

The include file <bsd.man.mk> includes a file named "../Makefile.inc" if
it exists.

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The include file <bsd.own.mk> contains source tree configuration parameters,
such as the owners, groups, etc. for both manual pages and binaries, and
a few global "feature configuration" parameters.

It has no targets.

To get system-specific configuration parameters, bsd.own.mk will try to
include the file specified by the "MAKECONF" variable.  If MAKECONF is not
set, or no such file exists, the system make configuration file, /etc/mk.conf
is included.  These files may define any of the variables described below.

bsd.own.mk sets the following variables, if they are not already defined
(defaults are in brackets):

_SRC_TOP_	Top of the system source tree, as determined by <bsd.own.mk>
		based on the presence of build.sh and tools/ .

BSDSRCDIR	The real path to the system sources, so that 'make obj'
		will work correctly. [/usr/src]

BSDOBJDIR	The real path to the system 'obj' tree, so that 'make obj'
		will work correctly. [/usr/obj]

BINGRP		Binary group. [wheel]

BINOWN		Binary owner. [root]

BINMODE		Binary mode. [555]

NONBINMODE	Mode for non-executable files. [444]

MANDIR		Base path for manual installation. [/usr/share/man/cat]

MANGRP		Manual group. [wheel]

MANOWN		Manual owner. [root]

MANMODE		Manual mode. [${NONBINMODE}]

MANINSTALL	Manual installation type: maninstall, catinstall, or both

LDSTATIC	Control program linking; if set blank, link everything
		dynamically. If set to "-static", link everything statically.
		If not set, programs link according to their makefile.

LIBDIR		Base path for library installation. [/usr/lib]

SHLIBDIR	If ${USE_SHLIBDIR} is "yes", use ${SHLIBDIR} instead of
		${LIBDIR} as the base path for shared library installation.
		[/usr/lib]

_LIBSODIR	Set to ${SHLIBDIR} if ${USE_SHLIBDIR} is "yes",
		otherwise set to ${LIBDIR}

SHLINKDIR	Base path for shared linker. [/usr/libexec]

LINTLIBDIR	Base path for lint(1) library installation. [/usr/libdata/lint]

LIBGRP		Library group. [${BINGRP}]

LIBOWN		Library owner. [${BINOWN}]

LIBMODE		Library mode. [${NONBINMODE}]

DOCDIR		Base path for system documentation (e.g. PSD, USD, etc.)
	        installation. [/usr/share/doc]

HTMLDOCDIR	Base path for html system documentation installation.
		[/usr/share/doc/html]

DOCGRP		Documentation group. [wheel]

DOCOWN		Documentation owner. [root]

DOCMODE		Documentation mode. [${NONBINMODE}]

NLSDIR		Base path for Native Language Support files installation.
		[/usr/share/nls]

NLSGRP		Native Language Support files group. [wheel]

NLSOWN		Native Language Support files owner. [root]

NLSMODE		Native Language Support files mode. [${NONBINMODE}]

STRIPFLAG	The flag passed to the install program to cause the binary
		to be stripped.  This is to be used when building your
		own install script so that the entire system can be made
		stripped/not-stripped using a single knob. [-s]

COPY		The flag passed to the install program to cause the binary
		to be copied rather than moved.  This is to be used when
		building our own install script so that the entire system
		can either be installed with copies, or with moves using
		a single knob. [-c]

Additionally, the following variables may be set by bsd.own.mk or in a
make configuration file to modify the behaviour of the system build
process (default values are in brackets along with comments, if set by
bsd.own.mk):

MKCRYPTO	If set to "no", no cryptography support will be built
		into the system.  Defaults to "yes".

NOCRYPTO	If set, it is equivalent to setting MKCRYPTO to "no".

MKCRYPTO_IDEA	If set to "yes", IDEA support will be built into
		libcrypto_idea.a.  Defaults to "no".

MKCRYPTO_RC5	If set to "yes", RC5 support will be built into
		libcrypto_rc5.a.  Defaults to "no".

MKHESIOD	If set to "no", disables building of Hesiod infrastructure
		(libraries and support programs).

MKKERBEROS	If set to "no", disables building of Kerberos (v4 or v5)
		infrastructure (libraries and support programs).

MKSKEY		If set to "no", disables building of S/key authentication
		infrastructure (libraries and support programs).

MKYP		If set to "no", disables building of YP (NIS)
		infrastructure (libraries and support programs).

USE_HESIOD	If set to "no", disables building Hesiod support into
		various system utilities/libraries that support it.
		If MKHESIOD is set to "no", USE_HESIOD will also be
		forced to "no".

USE_KERBEROS	If set to "no", disables building Kerberos (v4 or v5)
		support into various system utilities/libraries that
		support it.  If MKKERBEROS is set to "no", USE_KERBEROS
		will also be forced to "no".

USE_SKEY	If set to "no", disables building S/key authentication
		support into various system utilities/libraries that
		support it.  If MKSKEY is set to "no", USE_SKEY will
		also be forced to "no".

USE_YP		If set to "no", disables building YP (NIS) support into
		various system utilities/libraries that support it.  If
		MKYP is set to "no", USE_YP will also be forced to "no".

MANZ		Compress manual pages at installation time.

SYS_INCLUDE	Copy or symlink kernel include files into /usr/include.
		Possible values are "symlinks" or "copies" (which is
		the same as the variable being unset).

NOPROFILE	Do not build profiled versions of system libraries

NOPIC		Do not build PIC versions of system libraries, and
		do not build shared libraries.  [set if ${MACHINE_ARCH}
		is "sh3" and ${OBJECT_FMT} is "COFF", unset otherwise.]

NOLINT		Do not build lint libraries.

OBJECT_FMT	Object file format. [set to "ELF" on architectures that
		use ELF -- currently if ${MACHINE_ARCH} is "alpha",
		"mipsel", "mipseb", "powerpc", "sparc", "sparc64",
		"i386" and some m68k machines, or set to "a.out" on
		other architectures].

MKSOFTFLOAT	If "yes", build with options to enable the compiler to
		generate output containing library calls for floating
		point and possibly soft-float library support.  Defaults
		to "no".

bsd.own.mk is generally useful when building your own Makefiles so that
they use the same default owners etc. as the rest of the tree.

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The include file <bsd.prog.mk> handles building programs from one or
more source files, along with their manual pages.  It has a limited number
of suffixes, consistent with the current needs of the BSD tree.

It has eight targets:

	all:
		build the program and its manual page
	clean:
		remove the program, any object files and the files a.out,
		Errs, errs, mklog, and ${PROG}.core.
	cleandir:
		remove all of the files removed by the target clean, as
		well as .depend, tags, and any manual pages.
		`distclean' is a synonym for `cleandir'.
	depend:
		make the dependencies for the source files, and store
		them in the file .depend.
	includes:
		install any header files.
	install:
		install the program and its manual pages; if the Makefile
		does not itself define the target install, the targets
		beforeinstall and afterinstall may also be used to cause
		actions immediately before and after the install target
		is executed.
	lint:
		run lint on the source files
	tags:
		create a tags file for the source files.

It sets/uses the following variables:

BINGRP		Binary group.

BINOWN		Binary owner.

BINMODE		Binary mode.

CLEANFILES	Additional files to remove for the clean and cleandir targets.

COPTS		Additional flags to the compiler when creating C objects.

CPPFLAGS	Additional flags to the C pre-processor

LDADD		Additional loader objects.  Usually used for libraries.
		For example, to load with the compatibility and utility
		libraries, use:

			LDADD+=-lutil -lcompat

LDFLAGS		Additional loader flags.

LINKS		The list of binary links; should be full pathnames, the
		linked-to file coming first, followed by the linked
		file.  The files are hard-linked.  For example, to link
		/bin/test and /bin/[, use:

			LINKS=	${DESTDIR}/bin/test ${DESTDIR}/bin/[

SYMLINKS	The list of symbolic links; should be full pathnames.
                Syntax is identical to LINKS. Note that DESTDIR is not
		automatically included in the link.

MAN		Manual pages (should end in .1 - .9).  If no MAN variable is
		defined, "MAN=${PROG}.1" is assumed.

PROG		The name of the program to build.  If not supplied, nothing
		is built.

PROG_CXX	If defined, the name of the program to build.  Also
		causes <bsd.prog.mk> to link the program with the C++
		compiler rather than the C compiler.  PROG_CXX overrides
		the value of PROG if PROG is also set.

PROGNAME	The name that the above program will be installed as, if
		different from ${PROG}.

SRCS		List of source files to build the program.  If SRCS is not
		defined, it's assumed to be ${PROG}.c.

DPADD		Additional dependencies for the program.  Usually used for
		libraries.  For example, to depend on the compatibility and
		utility libraries use:

			DPADD+=${LIBCOMPAT} ${LIBUTIL}

		The following libraries are predefined for DPADD:

		LIBCRT0?=	${DESTDIR}/usr/lib/crt0.o
		LIBC?=		${DESTDIR}/usr/lib/libc.a
		LIBC_PIC?=	${DESTDIR}/usr/lib/libc_pic.a
		LIBCOMPAT?=	${DESTDIR}/usr/lib/libcompat.a
		LIBCRYPT?=	${DESTDIR}/usr/lib/libcrypt.a
		LIBCURSES?=	${DESTDIR}/usr/lib/libcurses.a
		LIBDBM?=	${DESTDIR}/usr/lib/libdbm.a
		LIBDES?=	${DESTDIR}/usr/lib/libdes.a
		LIBEDIT?=	${DESTDIR}/usr/lib/libedit.a
		LIBFORM?=	${DESTDIR}/usr/lib/libform.a
		LIBGCC?=	${DESTDIR}/usr/lib/libgcc.a
		LIBGNUMALLOC?=	${DESTDIR}/usr/lib/libgnumalloc.a
		LIBINTL?=	${DESTDIR}/usr/lib/libintl.a
		LIBIPSEC?=	${DESTDIR}/usr/lib/libipsec.a
		LIBKDB?=	${DESTDIR}/usr/lib/libkdb.a
		LIBKRB?=	${DESTDIR}/usr/lib/libkrb.a
		LIBKVM?=	${DESTDIR}/usr/lib/libkvm.a
		LIBL?=		${DESTDIR}/usr/lib/libl.a
		LIBM?=		${DESTDIR}/usr/lib/libm.a
		LIBMENU?=	${DESTDIR}/usr/lib/libmenu.a
		LIBMP?=		${DESTDIR}/usr/lib/libmp.a
		LIBNTP?=	${DESTDIR}/usr/lib/libntp.a
		LIBPC?=		${DESTDIR}/usr/lib/libpc.a
		LIBPCAP?=	${DESTDIR}/usr/lib/libpcap.a
		LIBPLOT?=	${DESTDIR}/usr/lib/libplot.a
		LIBPOSIX?=	${DESTDIR}/usr/lib/libposix.a
		LIBRESOLV?=	${DESTDIR}/usr/lib/libresolv.a
		LIBRPCSVC?=	${DESTDIR}/usr/lib/librpcsvc.a
		LIBSKEY?=	${DESTDIR}/usr/lib/libskey.a
		LIBTERMCAP?=	${DESTDIR}/usr/lib/libtermcap.a
		LIBTELNET?=	${DESTDIR}/usr/lib/libtelnet.a
		LIBUTIL?=	${DESTDIR}/usr/lib/libutil.a
		LIBWRAP?=	${DESTDIR}/usr/lib/libwrap.a
		LIBY?=		${DESTDIR}/usr/lib/liby.a
		LIBZ?=		${DESTDIR}/usr/lib/libz.a

SHAREDSTRINGS	If defined, a new .c.o rule is used that results in shared
		strings, using xstr(1). Note that this will not work with
		parallel makes.

STRIPFLAG	The flag passed to the install program to cause the binary
		to be stripped.

SUBDIR		A list of subdirectories that should be built as well.
		Each of the targets will execute the same target in the
		subdirectories.

SCRIPTS		A list of interpreter scripts [file.{sh,csh,pl,awk,...}].
		These are installed exactly like programs.

SCRIPTSNAME	The name that the above program will be installed as, if
		different from ${SCRIPTS}. These can be further specialized
		by setting SCRIPTSNAME_<script>.

FILES		A list of files to install. The installation is controlled
		by the FILESNAME, FILESOWN, FILESGRP, FILESMODE, FILESDIR
		variables that can be further specialized by FILES<VAR>_<file>

SHLINKDIR	Target directory for shared linker.

The include file <bsd.prog.mk> includes the file named "../Makefile.inc"
if it exists, as well as the include file <bsd.man.mk>.

Some simple examples:

To build foo from foo.c with a manual page foo.1, use:

	PROG=	foo

	.include <bsd.prog.mk>

To build foo from foo.c with a manual page foo.2, add the line:

	MAN=	foo.2

If foo does not have a manual page at all, add the line:

	MKMAN=	no

If foo has multiple source files, add the line:

	SRCS=	a.c b.c c.c d.c

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The include file <bsd.subdir.mk> contains the default targets for building
subdirectories.  It has the same eight targets as <bsd.prog.mk>: all, 
clean, cleandir, depend, includes, install, lint, and tags.  For all of
the directories listed in the variable SUBDIR, the specified directory 
will be visited and the target made.  There is also a default target which
allows the command "make subdir" where subdir is any directory listed in
the variable SUBDIR.

As a special case, the use of a token .WAIT as an entry in SUBDIR acts
as a synchronization barrier when multiple make jobs are run; subdirs
before the .WAIT must complete before any subdirs after .WAIT are
started.  See make(1) for some caveats on use of .WAIT and other
special sources.

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The include file <bsd.links.mk> handles the LINKS and SYMLINKS variables
and is included from from bsd.lib.mk and bsd.prog.mk.

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The include file <bsd.files.mk> handles the FILES variables and is included
from bsd.lib.mk and bsd.prog.mk.

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The include file <bsd.inc.mk> defines the includes target and uses two
variables:

INCS		The list of include files

INCSDIR		The location to install the include files.

INCSNAME	Target name of the include file, if only one; same as
		FILESNAME, but for include files.

INCSNAME_<file>	The name file <file> should be installed as, if not <file>,
		same as FILESNAME_<file>, but for include files.

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The include file <bsd.kinc.mk> defines the many targets (includes,
subdirectories, etc.), and is used by kernel makefiles to handle
include file installation.  It is intended to be included alone, by
kernel Makefiles.  Please see bsd.kinc.mk for more details, and keep
the documentation in that file up to date.

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

The include file <bsd.info.mk> is used to generate and install GNU Info
documentation from respective Texinfo source files.  It defines three
implicit targets (.txi.info, .texi.info, and .texinfo.info), and uses the
following variables:

TEXINFO		List of Texinfo source files.  Info documentation will
		consist of single files with the extension replaced by
		.info.

INFOFLAGS	Flags to pass to makeinfo.  []

INSTALL_INFO	Name of install-info program.  [install-info]

MAKEINFO	Name of makeinfo program.  [makeinfo]

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The include file <bsd.sys.mk> is used by <bsd.prog.mk> and
<bsd.lib.mk>.  It contains overrides that are used when building
the NetBSD source tree.  For instance, if "PARALLEL" is defined by
the program/library Makefile, it includes a set of rules for lex and
yacc that allow multiple lex and yacc targets to be built in parallel.

Other variables of note (incomplete list):

WARNS		Crank up gcc warning options; the distinct levels are:
			WARNS=1
			WARNS=2
			WARNS=3

FORMAT_AUDIT	If FORMAT_AUDIT is set, and WFORMAT is set and > 1, turn on
WFORMAT 	-Wnetbsd-format-audit for extra-stringent format checking.
		WFORMAT belongs in individual makefiles and/or
		Makefile.inc files.  (set WFORMAT=1 in individual
		makefiles if a program is not security critical and is
		doing bizarre things with format strings which would
		be even uglier if rewritten) FORMAT_AUDIT should go in
		mk.conf if you're doing format-string auditing. 
		FORMAT_AUDIT may go away in time.

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The include file <bsd.lib.mk> has support for building libraries.  It has
the same eight targets as <bsd.prog.mk>: all, clean, cleandir, depend,
includes, install, lint, and tags.  Additionally, it has a checkver target
which checks for installed shared object libraries whose version is greater
that the version of the source. It has a limited number of suffixes,
consistent with the current needs of the BSD tree.

It sets/uses the following variables:

LIB		The name of the library to build.

LIBDIR		Target directory for libraries.

SHLIBDIR	Target directory for shared libraries if ${USE_SHLIBDIR}
		is "yes".

USE_SHLIBDIR	If "yes", use ${SHLIBDIR} instead of ${LIBDIR}
		as the path to install shared libraries to.

LINTLIBDIR	Target directory for lint libraries.

LIBGRP		Library group.

LIBOWN		Library owner.

LIBMODE		Library mode.

LDADD		Additional loader objects.

MAN		The manual pages to be installed (use a .1 - .9 suffix).

MKLINKLIB	If "no", act as "MKPICINSTALL=no MKPROFILE=no".
		Also:
			- don't install the .a libraries
			- don't install _pic.a libraries on PIC systems
			- don't build .a libraries on PIC systems
			- don't install the .so symlink on ELF systems
		I.e, only install the shared library (and the .so.major
		symlink on ELF).

MKPICLIB	If "no", don't build _pic.a libraries, and build the
		shared object libraries from the .a libraries.  A
		symlink is installed in ${DESTDIR}/usr/lib for the
		_pic.a library pointing to the .a library.

NOCHECKVER_<library>
NOCHECKVER	If set, disables checking for installed shared object
		libraries with versions greater than the source.  A
		particular library name, without the "lib" prefix, may
		be appended to the variable name to disable the check for
		only that library.

SRCS		List of source files to build the library.  Suffix types
		.s, .c, and .f are supported.  Note, .s files are preferred
		to .c files of the same name.  (This is not the default for
		versions of make.)

The include file <bsd.lib.mk> includes the file named "../Makefile.inc"
if it exists, as well as the include file <bsd.man.mk>.

It has rules for building profiled objects; profiled libraries are
built by default.

Libraries are ranlib'd when made.

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The include file <bsd.obj.mk> defines targets related to the creation
and use of separated object and source directories.

If an environment variable named MAKEOBJDIRPREFIX is set, make(1) uses
${MAKEOBJDIRPREFIX}${.CURDIR} as the name of the object directory if
it exists.  Otherwise make(1) looks for the existence of a
subdirectory (or a symlink to a directory) of the source directory
into which built targets should be placed.  If an environment variable
named MAKEOBJDIR is set, make(1) uses its value as the name of the
object directory; failing that, make first looks for a subdirectory
named "obj.${MACHINE}", and if that doesn't exist, it looks for "obj".

Object directories are not created automatically by make(1) if they
don't exist; you need to run a separate "make obj".  (This will happen
during a top-level build if "MKOBJDIRS" is set to a value other than
"no").  When the source directory is a subdirectory of ${BSDSRCDIR} --
and this is determined by a simple string prefix comparison -- object
directories are created in a separate object directory tree, and a
symlink to the object directory in that tree is created in the source
directory; otherwise, "make obj" assumes that you're not in the main
source tree and that it's not safe to use a separate object tree.

Several variables used by <bsd.obj.mk> control exactly what
directories and links get created during a "make obj":

MAKEOBJDIR	If set, this is the component name of the object
		directory.

OBJMACHINE	If this is set but MAKEOBJDIR is not set, creates
		object directories or links named "obj.${MACHINE}";
		otherwise, just creates ones named "obj".

USR_OBJMACHINE  If set, and the current directory is a subdirectory of
		${BSDSRCDIR}, create object directory in the
		corresponding subdirectory of ${BSDOBJDIR}.${MACHINE};
		otherwise, create it in the corresponding subdirectory
		of ${BSDOBJDIR}

BUILDID		If set, the contents of this variable are appended
		to the object directory name.  If OBJMACHINE is also
		set, ".${BUILDID}" is added after ".${MACHINE}".

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The include file <bsd.kernobj.mk> defines variables related to the
location of kernel sources and object directories.

KERNSRCDIR	Is the location of the top of the kernel src.
		It defaults to ${_SRC_TOP_}/sys

KERNARCHDIR	Is the location of the machine dependent kernel
		sources.  It defaults to arch/${MACHINE}
		
KERNCONFDIR	Is where the configuration files for kernels are
		found; default is ${KERNSRCDIR}/${KERNARCHDIR}/conf.

KERNOBJDIR	Is the kernel build directory.  The kernel GENERIC for
		instance will be compiled in ${KERNOBJDIR}/GENERIC.
		The default value is
		${MAKEOBJDIRPREFIX}${KERNSRCDIR}/${KERNARCHDIR}/compile
		if it exists or the target 'obj' is being made.
		Otherwise the default is
		${KERNSRCDIR}/${KERNARCHDIR}/compile.

It is important that Makefiles (such as those under src/distrib) that
wish to find compiled kernels use bsd.kernobj.mk and ${KERNOBJDIR}
rather than make assumptions about the location of the compiled kernel.

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