New INSTALL guide, request for feedback

[Rich Felker <dalias@aerifal.cx>]

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I've spent some time this afternoon rewriting the INSTALL file to
address both "musl as alternate libc" and "musl as primary libc"
installations.

Feedback would be appreciated!

Rich

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==== 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.

First, some prerequisites:

- A C99 compiler with gcc-style inline assembly support, support for
  weak aliases, and support for building stand-alone assembly files.
  gcc 3.x and 4.x are known to work. pcc and LLVM/clang may work but
  are untested, and pcc is known to have some bugs.

- GNU make

- Linux, preferably 2.6.22 or later. Older versions are known to have
  serious bugs that will make some interfaces non-conformant, but if
  you don't need threads or POSIX 2008 features, even 2.4 is probably
  okay.

- A supported CPU architecture (currently i386, x86_64, or arm).

- If you want to use dynamic linking, it's recommended that you have
  permissions to write to /lib and /etc. Otherwise your binaries will
  have to use a nonstandard dynamic linker path.



== 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.

To install musl as an alternate libc, follow these steps:

1. Edit config.mak to select your system's CPU architecture (i386,
   x86_64, or arm), installation prefix, location for the dynamic
   linker, and other build preferences.

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. Ensure that /etc/ld-musl-$ARCH.path (where $ARCH is replaced by
   i386, x86_64, etc. as appropriate) contains the correct search path
   for where you intend to install musl-linked shared library files.
   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 <<EOF
#include <stdio.h>
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,
you may wish to use:

CC="musl-gcc -D_GNU_SOURCE" ./configure ...

Correctly-written build systems should not need -D_GNU_SOURCE as part
of $CC, but many programs do not use feature-test macros correctly and
simply assume the compiler will automatically give them the kitchen
sink, so the above command is an easy workaround.

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.



== Option 2: Installing musl as the primary C library ==

In this setup, you will need an existing compiler/toolchain. It
shouldnt 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.

!! 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. Edit musl's config.mak and set the installation prefix to the
   prefix your compiler toolchain is configured to search, probably
   /usr. Set ARCH to match your CPU architecture, and change any other
   options as you see fit.

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.

When building programs against musl, you may still want to ensure the
appropriate feature test macros get defined, as in:

CC="gcc -D_GNU_SOURCE" ./configure ...