Target makefile fragments can set these Makefile variables.
Compiler flags to use when compiling libgcc2.c.
A list of source file names to be compiled or assembled and inserted into libgcc.a.
Special flags used when compiling crtstuff.c. See Initialization.
Special flags used when compiling crtstuff.c for shared
linking. Used if you use crtbeginS.o and crtendS.o
For some targets, invoking GCC in different ways produces objects that cannot be linked together. For example, for some targets GCC produces both big and little endian code. For these targets, you must arrange for multiple versions of libgcc.a to be compiled, one for each set of incompatible options. When GCC invokes the linker, it arranges to link in the right version of libgcc.a, based on the command line options used.
MULTILIB_OPTIONS macro lists the set of options for which
special versions of libgcc.a must be built. Write options that
are mutually incompatible side by side, separated by a slash. Write
options that may be used together separated by a space. The build
procedure will build all combinations of compatible options.
For example, if you set
MULTILIB_OPTIONS to ‘m68000/m68020
msoft-float’, Makefile will build special versions of
libgcc.a using the following sets of options: -m68000,
-m68020, -msoft-float, ‘-m68000 -msoft-float’, and
MULTILIB_OPTIONS is used, this variable specifies the
directory names that should be used to hold the various libraries.
Write one element in
MULTILIB_DIRNAMES for each element in
MULTILIB_DIRNAMES is not used, the
default value will be
MULTILIB_OPTIONS, with all slashes treated
MULTILIB_DIRNAMES describes the multilib directories using GCC
conventions and is applied to directories that are part of the GCC
installation. When multilib-enabled, the compiler will add a
subdirectory of the form prefix/multilib before each
directory in the search path for libraries and crt files.
For example, if
MULTILIB_OPTIONS is set to ‘m68000/m68020
msoft-float’, then the default value of
‘m68000 m68020 msoft-float’. You may specify a different value if
you desire a different set of directory names.
Sometimes the same option may be written in two different ways. If an
option is listed in
MULTILIB_OPTIONS, GCC needs to know about
any synonyms. In that case, set
MULTILIB_MATCHES to a list of
items of the form ‘option=option’ to describe all relevant
synonyms. For example, ‘m68000=mc68000 m68020=mc68020’.
Sometimes when there are multiple sets of
specified, there are combinations that should not be built. In that
MULTILIB_EXCEPTIONS to be all of the switch exceptions
in shell case syntax that should not be built.
For example the ARM processor cannot execute both hardware floating
point instructions and the reduced size THUMB instructions at the same
time, so there is no need to build libraries with both of these
options enabled. Therefore
MULTILIB_EXCEPTIONS is set to:
Sometimes when there are only a few combinations are required, it would
be a big effort to come up with a
MULTILIB_EXCEPTIONS list to
cover all undesired ones. In such a case, just listing all the required
MULTILIB_REQUIRED would be more straightforward.
The way to specify the entries in
MULTILIB_REQUIRED is same with
the way used for
MULTILIB_EXCEPTIONS, only this time what are
required will be specified. Suppose there are multiple sets of
MULTILIB_OPTIONS and only two combinations are required, one
for ARMv7-M and one for ARMv7-R with hard floating-point ABI and FPU, the
MULTILIB_REQUIRED can be set to:
MULTILIB_REQUIRED can be used together with
MULTILIB_EXCEPTIONS. The option combinations generated from
MULTILIB_OPTIONS will be filtered by
and then by
Sometimes it is desirable to reuse one existing multilib for different
sets of options. Such kind of reuse can minimize the number of multilib
variants. And for some targets it is better to reuse an existing multilib
than to fall back to default multilib when there is no corresponding multilib.
This can be done by adding reuse rules to
A reuse rule is comprised of two parts connected by equality sign. The left
part is the option set used to build multilib and the right part is the option
set that will reuse this multilib. Both parts should only use options
MULTILIB_OPTIONS and the equality signs found in options
name should be replaced with periods. An explicit period in the rule can be
escaped by preceding it with a backslash. The order of options in the left
part matters and should be same with those specified in
MULTILIB_REQUIRED or aligned with the order in
There is no such limitation for options in the right part as we don’t build
multilib from them.
MULTILIB_REUSE is different from
MULTILIB_MATCHES in that it
sets up relations between two option sets rather than two options. Here is an
example to demo how we reuse libraries built in Thumb mode for applications built
in ARM mode:
Before the advent of
MULTILIB_REUSE, GCC select multilib by comparing command
line options with options used to build multilib. The
complementary to that way. Only when the original comparison matches nothing it will
work to see if it is OK to reuse some existing multilib.
Sometimes it is desirable that when building multiple versions of
libgcc.a certain options should always be passed on to the
compiler. In that case, set
MULTILIB_EXTRA_OPTS to be the list
of options to be used for all builds. If you set this, you should
CRTSTUFF_T_CFLAGS to a dash followed by it.
MULTILIB_OPTIONS is used, this variable specifies
a list of subdirectory names, that are used to modify the search
path depending on the chosen multilib. Unlike
MULTILIB_OSDIRNAMES describes the multilib directories using
operating systems conventions, and is applied to the directories such as
lib or those in the
LIBRARY_PATH environment variable.
The format is either the same as of
MULTILIB_DIRNAMES, or a set of mappings. When it is the same
MULTILIB_DIRNAMES, it describes the multilib directories
using operating system conventions, rather than GCC conventions. When it is a set
of mappings of the form gccdir=osdir, the left side gives
the GCC convention and the right gives the equivalent OS defined
location. If the osdir part begins with a ‘!’,
GCC will not search in the non-multilib directory and use
exclusively the multilib directory. Otherwise, the compiler will
examine the search path for libraries and crt files twice; the first
time it will add multilib to each directory in the search path,
the second it will not.
For configurations that support both multilib and multiarch,
MULTILIB_OSDIRNAMES also encodes the multiarch name, thus
MULTIARCH_DIRNAME. The multiarch name is appended to
each directory name, separated by a colon (e.g.
Each multiarch subdirectory will be searched before the corresponding OS
multilib directory, for example ‘/lib/i386-linux-gnu’ before
‘/lib/../lib32’. The multiarch name will also be used to modify the
system header search path, as explained for
This variable specifies the multiarch name for configurations that are multiarch-enabled but not multilibbed configurations.
The multiarch name is used to augment the search path for libraries, crt
files and system header files with additional locations. The compiler
will add a multiarch subdirectory of the form
prefix/multiarch before each directory in the library and
crt search path. It will also add two directories
NATIVE_SYSTEM_HEADER_DIR/multiarch) to the system header
search path, respectively before
MULTIARCH_DIRNAME is not used for configurations that support
both multilib and multiarch. In that case, multiarch names are encoded
More documentation about multiarch can be found at https://wiki.debian.org/Multiarch.
MULTILIB_EXTRA_OPTS is not enough, since
it does not affect the build of target libraries, at least not the
build of the default multilib. One possible work-around is to use
DRIVER_SELF_SPECS to bring options from the specs file
as if they had been passed in the compiler driver command line.
However, you don’t want to be adding these options after the toolchain
is installed, so you can instead tweak the specs file that will
be used during the toolchain build, while you still install the
original, built-in specs. The trick is to set
some other filename (say specs.install), that will then be
created out of the built-in specs, and introduce a Makefile
rule to generate the specs file that’s going to be used at
build time out of your specs.install.
These are extra flags to pass to the C compiler. They are used both when building GCC, and when compiling things with the just-built GCC. This variable is deprecated and should not be used.