==== Installing musl ==== musl may be installed either as an alternate C library alongside the existing libraries on a system, or as the primary C library for a new or existing musl-based system. This document covers the prerequisites and procedures for compiling and installation. ==== Build Prerequisites ==== The only build-time prerequisites for musl are GNU Make and a freestanding C99 compiler toolchain targeting the desired instruction set architecture and ABI, with support for gcc-style inline assembly, weak aliases, and stand-alone assembly source files. The system used to build musl does not need to be Linux-based, nor do the Linux kernel headers need to be available. If support for dynamic linking is desired, some further requirements are placed on the compiler and linker. In particular, the linker must support the -Bsymbolic-functions option. At present, GCC 4.6 or later is the recommended compiler for building musl. Any earlier version of GCC with full C99 support should also work, but may be subject to minor floating point conformance issues on i386 targets. Sufficiently recent versions of PCC and LLVM/clang are also believed to work, but have not been tested as heavily; prior to Fall 2012, both had known bugs that affected musl. === Supported Targets ==== musl can be built for the following CPU instruction set architecture and ABI combinations: - i386 (requires 387 math and 486 cmpxchg instructions) - x86_64 - arm (EABI) - mips (o32 ABI, requires fpu or float emulation in kernel) - microblaze (requires a cpu with lwx/swx instructions) - powerpc (32-bit, must use "secure plt" mode for dynamic linking) For architectures with both little- and big-endian options, both are supported unless otherwise noted. In general, musl assumes the availability of all Linux syscall interfaces available in Linux 2.6.0. Some programs that do not use threads or other modern functionality may be able to run on 2.4.x kernels. Other kernels (such as BSD) that provide a Linux-compatible syscall ABI should also work but have not been extensively tested. ==== Option 1: Installing musl as an alternate C library ==== In this setup, musl and any third-party libraries linked to musl will reside under an alternate prefix such as /usr/local/musl or /opt/musl. A wrapper script for gcc, called musl-gcc, can be used in place of gcc to compile and link programs and libraries against musl. (Note: There are not yet corresponding wrapper scripts for other compilers, so if you wish to compile and link against musl using another compiler, you are responsible for providing the correct options to override the default include and library search paths.) To install musl as an alternate libc, follow these steps: 1. Configure musl's build with a command similar to: ./configure --prefix=/usr/local/musl --exec-prefix=/usr/local Refer to ./configure --help for details on other options. You may change the install prefix if you like, but DO NOT set it to a location that contains your existing libraries based on another libc such as glibc or uClibc. If you do not intend to use dynamic linking, you may disable it at this point via --disable-shared and cut the build time in half. If you wish to use dynamic linking but do not have permissions to write to /lib, you will need to set an alternate dynamic linker location via --syslibdir. 2. Run "make". Parallel build is fully supported, so you can instead use "make -j3" or so on SMP systems if you like. 3. Run "make install" as a user sufficient privileges to write to the destination. 4. Create a file named /etc/ld-musl-$ARCH.path (where $ARCH is replaced by i386, x86_64, etc. as appropriate) containing the correct colon-delimited search path for where you intend to install musl-linked shared library files. If this file is missing, musl will search the standard path, and you will encounter problems when it attempts to load libraries linked against your host libc. Note that this step can be skipped if you disabled dynamic linking. After installing, you can use musl via the musl-gcc wrapper. For example: cat > hello.c < int main() { printf("hello, world!\n"); return 0; } EOF musl-gcc hello.c ./a.out To configure autoconf-based program to compile and link against musl, set the CC variable to musl-gcc when running configure, as in: CC=musl-gcc ./configure ... You will probably also want to use --prefix when building libraries to ensure that they are installed under the musl prefix and not in the main host system library directories. Finally, it's worth noting that musl's include and lib directories in the build tree are setup to be usable without installation, if necessary. Just modify the paths in the spec file used by musl-gcc (it's located at $prefix/lib/musl-gcc.specs) to point to the source/build tree. ==== Option 2: Installing musl as the primary C library ==== In this setup, you will need an existing compiler/toolchain. It shouldn't matter whether it was configured for glibc, uClibc, musl, or something else entirely, but sometimes gcc can be uncooperative, especially if the system distributor has built gcc with strange options. It probably makes the most sense to perform the following steps inside a chroot setup or on a virtualized machine with the filesystem containing just a minimal toolchain. WARNING: DO NOT DO THIS ON AN EXISTING SYSTEM UNLESS YOU REALLY WANT TO CONVERT IT TO BE A MUSL-BASED SYSTEM!! 1. If you are just upgrading an existing version of musl, you can skip step 1 entirely. Otherwise, move the existing include and lib directories on your system out of the way. Unless all the binaries you will need are static-linked, you should edit /etc/ld.so.conf (or equivalent) and put the new locations of your old libraries in the search path before you move them, or your system will break badly and you will not be able to continue. 2. Configure musl's build with a command similar to: ./configure --prefix=/usr --disable-gcc-wrapper Refer to ./configure --help for details on other options. 3. Run "make" to compile musl. 4. Run "make install" with appropriate privileges. 5. If you are using gcc and wish to use dynamic linking, find the gcc directory containing libgcc.a (it should be something like /usr/lib/gcc/i486-linux-gnu/4.3.5, with the arch and version possibly different) and look for a specs file there. If none exists, use "gcc -dumpspecs > specs" to generate a specs file. Find the dynamic linker (/lib/ld-linux.so.2 or similar) and change it to "/lib/ld-musl-$ARCH.so.1" (with $ARCH replaced by your CPU arch). At this point, musl should be the default libc. Compile a small test program with gcc and verify (using readelf -a or objdump -x) that the dynamic linker (program interpreter) is /lib/ld-musl-$ARCH.so.1. If you're using static linking only, you might instead check the symbols and look for anything suspicious that would indicate your old glibc or uClibc was used.