# Installing GCC: Building

Now that GCC is configured, you are ready to build the compiler and runtime libraries.

We highly recommend that GCC be built using GNU make; other versions may work, then again they might not.

(For example, many broken versions of make will fail if you use the recommended setup where objdir is different from srcdir. Other broken versions may recompile parts of the compiler when installing the compiler.)

Some commands executed when making the compiler may fail (return a nonzero status) and be ignored by `make`. These failures, which are often due to files that were not found, are expected, and can safely be ignored.

It is normal to have compiler warnings when compiling certain files. Unless you are a GCC developer, you can generally ignore these warnings unless they cause compilation to fail.

On certain old systems, defining certain environment variables such as `CC` can interfere with the functioning of `make`.

If you encounter seemingly strange errors when trying to build the compiler in a directory other than the source directory, it could be because you have previously configured the compiler in the source directory. Make sure you have done all the necessary preparations.

If you build GCC on a BSD system using a directory stored in an old System V file system, problems may occur in running `fixincludes` if the System V file system doesn't support symbolic links. These problems result in a failure to fix the declaration of `size_t` in `sys/types.h`. If you find that `size_t` is a signed type and that type mismatches occur, this could be the cause.

The solution is not to use such a directory for building GCC.

When building from CVS or snapshots, or if you modify parser sources, you need the Bison parser generator installed. Any version 1.25 or later should work; older versions may also work. If you do not modify parser sources, releases contain the Bison-generated files and you do not need Bison installed to build them.

When building from CVS or snapshots, or if you modify Texinfo documentation, you need version 4.0 or later of Texinfo installed if you want Info documentation to be regenerated. Releases contain Info documentation pre-built for the unmodified documentation in the release.

## Building a native compiler

For a native build issue the command `make bootstrap`. This will build the entire GCC system, which includes the following steps:

- Build host tools necessary to build the compiler such as texinfo, bison, gperf.
- Build target tools for use by the compiler such as binutils (bfd, binutils, gas, gprof, ld, and opcodes) if they have been individually linked or moved into the top level GCC source tree before configuring.
- Perform a 3-stage bootstrap of the compiler.
- Perform a comparison test of the stage2 and stage3 compilers.
- Build runtime libraries using the stage3 compiler from the previous step.

If you are short on disk space you might consider `make bootstrap-lean` instead. This is identical to `make bootstrap` except that object files from the stage1 and stage2 of the 3-stage bootstrap of the compiler are deleted as soon as they are no longer needed.

If you want to save additional space during the bootstrap and in the final installation as well, you can build the compiler binaries without debugging information with `make CFLAGS='-O' LIBCFLAGS='-g -O2' LIBCXXFLAGS='-g -O2 -fno-implicit-templates' bootstrap`. This will save roughly 40% of disk space both for the bootstrap and the final installation. (Libraries will still contain debugging information.)

If you wish to use non-default GCC flags when compiling the stage2 and stage3 compilers, set `BOOT_CFLAGS` on the command line when doing `make bootstrap`. Non-default optimization flags are less well tested here than the default of `-g -O2`, but should still work. In a few cases, you may find that you need to specify special flags such as `-msoft-float` here to complete the bootstrap; or, if the native compiler miscompiles the stage1 compiler, you may need to work around this, by choosing `BOOT_CFLAGS` to avoid the parts of the stage1 compiler that were miscompiled, or by using `make bootstrap4` to increase the number of stages of bootstrap.

If you used the flag `--enable-languages=...` to restrict the compilers to be built, only those you've actually enabled will be built. This will of course only build those runtime libraries, for which the particular compiler has been built. Please note, that re-defining `LANGUAGES` when calling `make bootstrap` does not work anymore!

If the comparison of stage2 and stage3 fails, this normally indicates that the stage2 compiler has compiled GCC incorrectly, and is therefore a potentially serious bug which you should investigate and report. (On a few systems, meaningful comparison of object files is impossible; they always appear "different". If you encounter this problem, you will need to disable comparison in the `Makefile`.)

## Building a cross compiler

We recommend reading the crossgcc FAQ for information about building cross compilers.

When building a cross compiler, it is not generally possible to do a 3-stage bootstrap of the compiler. This makes for an interesting problem as parts of GCC can only be built with GCC.

To build a cross compiler, we first recommend building and installing a native compiler. You can then use the native GCC compiler to build the cross compiler.

Assuming you have already installed a native copy of GCC and configured your cross compiler, issue the command `make`, which performs the following steps:

- Build host tools necessary to build the compiler such as texinfo, bison, gperf.
- Build target tools for use by the compiler such as binutils (bfd, binutils, gas, gprof, ld, and opcodes) if they have been individually linked or moved into the top level GCC source tree before configuring.
- Build the compiler (single stage only).
- Build runtime libraries using the compiler from the previous step.

Note that if an error occurs in any step the make process will exit.

## Building in parallel

If you have a multiprocessor system you can use `make bootstrap MAKE="make -j 2" -j 2` or just `make -j 2 bootstrap` for GNU Make 3.79 and above instead of just `make bootstrap` when building GCC. You can use a bigger number instead of two if you like. In most cases, it won't help to use a number bigger than the number of processors in your machine.

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