Building the toolchain

To compile mlibc and any userspace programs which link against it, you'll need a suitable compiler. For that, you will need to build a full OS Specific Toolchain. Using your host's compiler or a generic toolchain could lead into strange build failures and is not recommended nor supported.

For mlibc-demo-os, the riscv64-demo-gcc toolchain is used. The patches (with comments) to add support for it in Binutils and GCC are available here.

The first step is downloading the sources for Binutils and GCC, then applying your patches. The versions used in this guide are Binutils 2.45.1 and GCC 15.2.0.

Building Binutils

As GCC depends on a binutils for the target, we will build binutils first:

cd binutils-2.45.1
mkdir build
cd build

# --with-sysroot will tell the linker where to search for libraries.
# --enable-default-execstack=no will tell the linker to 
#   not use an executable stack by default.
../configure \
    --target=riscv64-demo \
    --prefix=/usr \
    --with-sysroot="${SYSROOT_DIR}" \
    --disable-werror \
    --enable-default-execstack=no

make -j$(nproc)

DESTDIR="${TOOLCHAIN_DIR}" make install

Make sure you have ${TOOLCHAIN_DIR}/usr/bin in your $PATH. If you don't, you WILL run into issues when building GCC.

Building GCC

Now, build GCC:

cd gcc-15.2.0
mkdir build
cd build

# --with-sysroot will tell the compiler where to search for the libc headers 
#   during GCC compilation, as well as include dirs and libraries when compiling.
# --enable-languages=c,c++ will tell it to only build the C and C++ compilers
# --enable-threads=posix enables pthread support
# --disable-multilib disables building a multilib gcc
    CFLAGS_FOR_TARGET="-march=rv64gc -mabi-lp64d" \
    CXXFLAGS_FOR_TARGET="-march=rv64gc -mabi-lp64d" \
../configure \
    --target=riscv64-demo \
    --prefix=/usr \
    --with-sysroot="${SYSROOT_DIR}" \
    --enable-languages=c,c++ \
    --enable-threads=posix \
    --disable-multilib \
    --enable-shared \
    --enable-host-shared

make -j$(nproc) all-gcc all-target-libgcc
DESTDIR="${TOOLCHAIN_DIR}" make install-gcc install-target-libgcc

After that, you'll have a compiler that targets your OS and implicitly links against mlibc! However, if you try to compile a simple program using it, you will run into errors:

$ riscv64-demo-gcc -march=rv64gc -mabi=lp64d helloworld.c
ld: cannot find crt1.o: No such file or directory
ld: cannot find -lc: No such file or directory

This is because we still have not built mlibc, and so it cannot find needed libc files.