5.3. RTEMS Kernel — RTEMS User Manual 5.4-rc2 (21st August 2026) documentation

# 5.3. RTEMS Kernel

RTEMS is an open source real-time operating system. As a user you have access to all the source code. The `RTEMS Kernel` section will show you how you build the RTEMS kernel on your host.

## 5.3.1. Development Sources

Create a new location to build the RTEMS kernel:

    $ cd
    $ cd development/rtems
    $ mkdir kernel
    $ cd kernel

Clone the RTEMS respository:

    $ git clone git://git.rtems.org/rtems.git rtems
    Cloning into 'rtems'...
    remote: Counting objects: 483342, done.
    remote: Compressing objects: 100% (88974/88974), done.
    remote: Total 483342 (delta 390053), reused 475669 (delta 383809)
    Receiving objects: 100% (483342/483342), 69.88 MiB | 1.37 MiB/s, done.
    Resolving deltas: 100% (390053/390053), done.
    Checking connectivity... done.

## 5.3.2. Tools Path Set Up

We need to set our path to include the RTEMS tools we built in the previous section. The RTEMS tools needs to be first in your path because RTEMS provides specific versions of the `autoconf` and `automake` tools. We want to use the RTEMS version and not your host’s versions:

    $ export PATH=$HOME/development/rtems/5/bin:$PATH

## 5.3.3. Bootstrapping

The developers version of the code from git requires we `bootstrap` the source code. This is an `autoconf` and `automake` bootstrap to create the various files generated by `autoconf` and `automake`. RTEMS does not keep these generated files under version control. The bootstrap process is slow so to speed it up the RSB provides a command that can perform the bootstrap in parallel using your available cores. We need to enter the cloned source directory then run the bootstrap commands:

    $ cd rtems
    $ ./bootstrap -c && $HOME/development/rtems/rsb/source-builder/sb-bootstrap
    removing automake generated Makefile.in files
    removing configure files
    removing aclocal.m4 files
    RTEMS Source Builder - RTEMS Bootstrap, 5 (089327b5dcf9)
      1/139: autoreconf: configure.ac
      2/139: autoreconf: cpukit/configure.ac
      3/139: autoreconf: tools/cpu/configure.ac
      4/139: autoreconf: tools/cpu/generic/configure.ac
      5/139: autoreconf: tools/cpu/sh/configure.ac
      6/139: autoreconf: tools/cpu/nios2/configure.ac
      7/139: autoreconf: tools/build/configure.ac
      8/139: autoreconf: doc/configure.ac
     ......
    124/139: autoreconf: c/src/make/configure.ac
    125/139: autoreconf: c/src/librtems++/configure.ac
    126/139: autoreconf: c/src/ada-tests/configure.ac
    127/139: autoreconf: testsuites/configure.ac
    128/139: autoreconf: testsuites/libtests/configure.ac
    129/139: autoreconf: testsuites/mptests/configure.ac
    130/139: autoreconf: testsuites/fstests/configure.ac
    131/139: autoreconf: testsuites/sptests/configure.ac
    132/139: autoreconf: testsuites/tmtests/configure.ac
    133/139: autoreconf: testsuites/smptests/configure.ac
    134/139: autoreconf: testsuites/tools/configure.ac
    135/139: autoreconf: testsuites/tools/generic/configure.ac
    136/139: autoreconf: testsuites/psxtests/configure.ac
    137/139: autoreconf: testsuites/psxtmtests/configure.ac
    138/139: autoreconf: testsuites/rhealstone/configure.ac
    139/139: autoreconf: testsuites/samples/configure.ac
    Bootstrap time: 0:02:47.398824

## 5.3.4. Building a BSP

We build RTEMS in a directory outside of the source tree we have just cloned and `bootstrapped`. You cannot build RTEMS while in the source tree. Lets create a suitable directory using the name of the BSP we are going to build:

    $ cd ..
    $ mkdir erc32
    $ cd erc32

Configure RTEMS using the `configure` command. We use a full path to `configure` so the object files built contain the absolute path of the source files. If you are source level debugging you will be able to access the source code to RTEMS from the debugger. We will build for the `erc32` BSP with POSIX enabled and the networking stack disabled:

    $ $HOME/development/rtems/kernel/rtems/configure --prefix=$HOME/development/rtems/5 \
                       --target=sparc-rtems5 --enable-rtemsbsp=erc32 --enable-posix \
                       --disable-networking
    checking for gmake... no
    checking for make... make
    checking for RTEMS Version... 4.11.99.0
    checking build system type... x86_64-pc-linux-gnu
    checking host system type... x86_64-pc-linux-gnu
    checking target system type... sparc-unknown-rtems5
    checking for a BSD-compatible install... /usr/bin/install -c
    checking whether build environment is sane... yes
    checking for a thread-safe mkdir -p... /bin/mkdir -p
    checking for gawk... no
    checking for mawk... mawk
    checking whether make sets $(MAKE)... yes
    checking whether to enable maintainer-specific portions of Makefiles... no
    checking that generated files are newer than configure... done
     ......
    checking target system type... sparc-unknown-rtems5
    checking rtems target cpu... sparc
    checking for a BSD-compatible install... /usr/bin/install -c
    checking whether build environment is sane... yes
    checking for sparc-rtems5-strip... sparc-rtems5-strip
    checking for a thread-safe mkdir -p... /bin/mkdir -p
    checking for gawk... no
    checking for mawk... mawk
    checking whether make sets $(MAKE)... yes
    checking whether to enable maintainer-specific portions of Makefiles... no
    checking that generated files are newer than configure... done
    configure: creating ./config.status
    config.status: creating Makefile

    target architecture: sparc.
    available BSPs: erc32.
    'make all' will build the following BSPs: erc32.
    other BSPs can be built with 'make RTEMS_BSP="bsp1 bsp2 ..."'

    config.status: creating Makefile

Build RTEMS using two cores:

    $ make -j 2
    Making all in tools/build
    make[1]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/build'
    make  all-am
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/build'
    gcc -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/tools/build     -g -O2 -MT cklength.o -MD -MP -MF .deps/cklength.Tpo -c -o cklength.o /home/chris/development/rtems/kernel/rtems/tools/build/cklength.c
    gcc -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/tools/build     -g -O2 -MT eolstrip.o -MD -MP -MF .deps/eolstrip.Tpo -c -o eolstrip.o /home/chris/development/rtems/kernel/rtems/tools/build/eolstrip.c
    mv -f .deps/cklength.Tpo .deps/cklength.Po
    mv -f .deps/eolstrip.Tpo .deps/eolstrip.Po
    gcc -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/tools/build     -g -O2 -MT compat.o -MD -MP -MF .deps/compat.Tpo -c -o compat.o /home/chris/development/rtems/kernel/rtems/tools/build/compat.c
    gcc -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/tools/build     -g -O2 -MT packhex.o -MD -MP -MF .deps/packhex.Tpo -c -o packhex.o /home/chris/development/rtems/kernel/rtems/tools/build/packhex.c
    mv -f .deps/compat.Tpo .deps/compat.Po
    gcc -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/tools/build     -g -O2 -MT unhex.o -MD -MP -MF .deps/unhex.Tpo -c -o unhex.o /home/chris/development/rtems/kernel/rtems/tools/build/unhex.c
    mv -f .deps/packhex.Tpo .deps/packhex.Po
    gcc -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/tools/build     -g -O2 -MT rtems-bin2c.o -MD -MP -MF .deps/rtems-bin2c.Tpo -c -o rtems-bin2c.o /home/chris/development/rtems/kernel/rtems/tools/build/rtems-bin2c.c
    mv -f .deps/unhex.Tpo .deps/unhex.Po
    gcc -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/tools/build     -g -O2 -MT binpatch.o -MD -MP -MF .deps/binpatch.Tpo -c -o binpatch.o /home/chris/development/rtems/kernel/rtems/tools/build/binpatch.c
    mv -f .deps/rtems-bin2c.Tpo .deps/rtems-bin2c.Po
    gcc  -g -O2   -o cklength cklength.o
    mv -f .deps/binpatch.Tpo .deps/binpatch.Po
    gcc  -g -O2   -o eolstrip eolstrip.o compat.o
    gcc  -g -O2   -o packhex packhex.o
    gcc  -g -O2   -o rtems-bin2c rtems-bin2c.o compat.o
    gcc  -g -O2   -o unhex unhex.o compat.o
    gcc  -g -O2   -o binpatch binpatch.o
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/build'
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/build'
    Making all in tools/cpu
    make[1]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    Making all in generic
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/cpu/generic'
    make[2]: Nothing to be done for 'all'.
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/cpu/generic'
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    make[2]: Nothing to be done for 'all-am'.
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    Making all in testsuites/tools
    make[1]: Entering directory '/home/chris/development/rtems/kernel/erc32/testsuites/tools'
    Making all in generic
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/testsuites/tools/generic'
    make[2]: Nothing to be done for 'all'.
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/testsuites/tools/generic'
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/testsuites/tools'
    make[2]: Nothing to be done for 'all-am'.
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/testsuites/tools'
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32/testsuites/tools'
    Making all in sparc-rtems5/c
    make[1]: Entering directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/c'
    Making all in .
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/c'
    Configuring RTEMS_BSP=erc32
    checking for gmake... no
    checking for make... make
    checking build system type... x86_64-pc-linux-gnu
    checking host system type... sparc-unknown-rtems5
     ......
    sparc-rtems5-gcc -B../../../../../erc32/lib/ -specs bsp_specs -qrtems -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/c/src/../../testsuites/samples/nsecs -I.. -I/home/chris/development/rtems/kernel/rtems/c/src/../../testsuites/samples/../support/include   -mcpu=cypress -O2 -g -ffunction-sections -fdata-sections -Wall -Wmissing-prototypes -Wimplicit-function-declaration -Wstrict-prototypes -Wnested-externs -MT init.o -MD -MP -MF .deps/init.Tpo -c -o init.o /home/chris/development/rtems/kernel/rtems/c/src/../../testsuites/samples/nsecs/init.c
    sparc-rtems5-gcc -B../../../../../erc32/lib/ -specs bsp_specs -qrtems -DHAVE_CONFIG_H -I. -I/home/chris/development/rtems/kernel/rtems/c/src/../../testsuites/samples/nsecs -I.. -I/home/chris/development/rtems/kernel/rtems/c/src/../../testsuites/samples/../support/include   -mcpu=cypress -O2 -g -ffunction-sections -fdata-sections -Wall -Wmissing-prototypes -Wimplicit-function-declaration -Wstrict-prototypes -Wnested-externs -MT empty.o -MD -MP -MF .deps/empty.Tpo -c -o empty.o /home/chris/development/rtems/kernel/rtems/c/src/../../testsuites/samples/nsecs/empty.c
    mv -f .deps/empty.Tpo .deps/empty.Po
    mv -f .deps/init.Tpo .deps/init.Po
    sparc-rtems5-gcc -B../../../../../erc32/lib/ -specs bsp_specs -qrtems -mcpu=cypress -O2 -g -ffunction-sections -fdata-sections -Wall -Wmissing-prototypes -Wimplicit-function-declaration -Wstrict-prototypes -Wnested-externs -Wl,--gc-sections  -mcpu=cypress   -o nsecs.exe init.o empty.o
    sparc-rtems5-nm -g -n nsecs.exe > nsecs.num
    sparc-rtems5-size nsecs.exe
       text    data     bss     dec     hex filename
     121392    1888    6624  129904   1fb70 nsecs.exe
    cp nsecs.exe nsecs.ralf
    make[6]: Leaving directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/ c/erc32/testsuites/samples/nsecs'
    make[5]: Leaving directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/ c/erc32/testsuites/samples'
    make[4]: Leaving directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/ c/erc32/testsuites/samples'
    make[4]: Entering directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/ c/erc32/testsuites'
    make[4]: Nothing to be done for 'all-am'.
    make[4]: Leaving directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/ c/erc32/testsuites'
    make[3]: Leaving directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/ c/erc32/testsuites'
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/ c/erc32'
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/c'
    make[1]: Entering directory '/home/chris/development/rtems/kernel/erc32'
    make[1]: Nothing to be done for 'all-am'.
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32'

## 5.3.5. Installing A BSP

All that remains to be done is to install the kernel. Installing RTEMS copies the API headers and architecture specific libraries to a locaiton under the prefix you provide. You can install any number of BSPs under the same prefix. We recommend you have a separate prefix for different versions of RTEMS. Do not mix versions of RTEMS under the same prefix. Make installs RTEMS with the following command:

    $ make install
    Making install in tools/build
    make[1]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/build'
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/build'
    /bin/mkdir -p '/home/chris/development/rtems/5/bin'
    /usr/bin/install -c cklength eolstrip packhex unhex rtems-bin2c '/home/chris/development/rtems/5/bin'
    /bin/mkdir -p '/home/chris/development/rtems/5/bin'
    /usr/bin/install -c install-if-change '/home/chris/development/rtems/5/bin'
    make[2]: Nothing to be done for 'install-data-am'.
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/build'
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/build'
    Making install in tools/cpu
    make[1]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    Making install in generic
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/cpu/generic'
    make[3]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/cpu/generic'
    make[3]: Nothing to be done for 'install-exec-am'.
    make[3]: Nothing to be done for 'install-data-am'.
    make[3]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/cpu/generic'
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/cpu/generic'
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    make[3]: Entering directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    make[3]: Nothing to be done for 'install-exec-am'.
    make[3]: Nothing to be done for 'install-data-am'.
    make[3]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32/tools/cpu'
      ....
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32/sparc-rtems5/c'
    make[1]: Entering directory '/home/chris/development/rtems/kernel/erc32'
    make[2]: Entering directory '/home/chris/development/rtems/kernel/erc32'
    make[2]: Nothing to be done for 'install-exec-am'.
    /bin/mkdir -p '/home/chris/development/rtems/5/make'
    /usr/bin/install -c -m 644 /home/chris/development/rtems/kernel/rtems/make/main.cfg /home/chris/development/rtems/kernel/rtems/make/leaf.cfg '/home/chris/development/rtems/5/make'
    /bin/mkdir -p '/home/chris/development/rtems/5/share/rtems5/make/Templates'
    /usr/bin/install -c -m 644 /home/chris/development/rtems/kernel/rtems/make/Templates/Makefile.dir /home/chris/development/rtems/kernel/rtems/make/Templates/Makefile.leaf /home/chris/development/rtems/kernel/rtems/make/Templates/Makefile.lib '/home/chris/development/rtems/5/share/rtems5/make/Templates'
    /bin/mkdir -p '/home/chris/development/rtems/5/make/custom'
    /usr/bin/install -c -m 644 /home/chris/development/rtems/kernel/rtems/make/custom/default.cfg '/home/chris/development/rtems/5/make/custom'
    make[2]: Leaving directory '/home/chris/development/rtems/kernel/erc32'
    make[1]: Leaving directory '/home/chris/development/rtems/kernel/erc32'

## 5.3.6. Contributing Patches

RTEMS welcomes fixes to bugs and new features. The RTEMS Project likes to have bugs fixed against a ticket created on our Developer Site. Please raise a ticket if you have a bug. Any changes that are made can be tracked against the ticket. If you want to add a new a feature please post a message to Developers Mailing List describing what you would like to implement. The RTEMS maintainer will help decide if the feature is in the best interest of the project. Not everything is and the maintainers need to evalulate how much effort it is to maintain the feature. Once accepted into the source tree it becomes the responsibility of the maintainers to keep the feature updated and working.

Changes to the source tree are tracked using git. If you have not made changes and enter the source tree and enter a git status command you will see nothing has changed:

    $ cd ../rtems
    $ git status
    On branch master
    Your branch is up-to-date with 'origin/master'.
    nothing to commit, working directory clean

We will make a change to the source code. In this example I change the help message to the RTEMS shell’s `halt` command. Running the same git status command reports:

    $ git status
    On branch master
    Your branch is up-to-date with 'origin/master'.
    Changes not staged for commit:
      (use "git add <file>..." to update what will be committed)
      (use "git checkout -- <file>..." to discard changes in working directory)

            modified:   cpukit/libmisc/shell/main_halt.c

    no changes added to commit (use "git add" and/or "git commit -a")

As an example I have a ticket open and the ticket number is 9876. I commit the change with the follow git command:

    $ git commit cpukit/libmisc/shell/main_halt.c

An editor is opened and I enter my commit message. The first line is a title and the following lines form a body. My message is:

    shell: Add more help detail to the halt command.

    Closes #9876.

    # Please enter the commit message for your changes. Lines starting
    # with '#' will be ignored, and an empty message aborts the commit.
    # Explicit paths specified without -i or -o; assuming --only paths...
    #
    # Committer: Chris Johns <chrisj@rtems.org>
    #
    # On branch master
    # Your branch is up-to-date with 'origin/master'.
    #
    # Changes to be committed:
    #       modified:   cpukit/libmisc/shell/main_halt.c

When you save and exit the editor git will report the commit’s status:

    $ git commit cpukit/libmisc/shell/main_halt.c
    [master 9f44dc9] shell: Add more help detail to the halt command.
     1 file changed, 1 insertion(+), 1 deletion(-)

You can either email the patch to Developers Mailing List with the following git command, and it is minus one on the command line:

    $ git send-email --to=devel@rtems.org -1
     <add output here>

Or you can ask git to create a patch file using:

    $ git format-patch -1
    0001-shell-Add-more-help-detail-to-the-halt-command.patch

This patch can be attached to a ticket.