Actually detecting musl library

detect-musl.sh

#! /bin/sh

# SPDX-License-Identifier: WTFPL

# Actually detecting musl libc.

# How it works
# -------------
#
# WARNING - This is a hack and the concept will likely change in future.
#
# We write a simple C file and have it compiled with the target compiler. The
# target compiler automatically compiles by dynamically linking against given
# libc. Now, there are 2 ways to do it. One, if host and target are the same
# (native compilation), then its as simple as executing `ldd` and getting the
# name of libc.so from it. The other way, is to run `readelf` with `-d` and grep
# out the names of all libraries its linked to. The last one is always C
# runtime.
#
# If its native compilation, we execute the libc.so (yes, by doing exec()
# because it's literally the `_start()`). And read out its contents. In case
# of cross-compiling, this won't be the case. So we run `strings` and grep out
# for 'musl libc' from it.
#
# In case of cross-compiling, this script will not generate the version numbers
# yet, but that is possible too if we look more intrusively.

# Idea
# -----
# I had asked a question on SO about how to detect various libc's in cross-
# compiling environments and another person asked similar question along those
# lines. See,
#
# Me -> https://stackoverflow.com/questions/50443164/how-to-detect-libc-name-and-version-in-cross-compiler-environments
# Other guy -> https://stackoverflow.com/questions/58177815/how-to-actually-detect-musl-libc
#
# So I decided to release this script in the hopes someone else finds a better
# way to do it. It seems SO has some strong political views that forces its
# users to see only the superficial aspects of the question instead of truly
# understanding the point of view of the question nor the research that has gone
# into it.

# Make sure to set `CROSS_COMPILE` environment variable to the right compiler
# prefix before invoking the script. See how it's used and massage the value
# accordingly.

# CROSS_COMPILE=/opt/buildroot-2020.02-rc1/output/host/bin/aarch64-buildroot-linux-musl

cat > /tmp/test.c <<- EOM
#include <stdio.h>

int main(void)
{
	printf("Hello, world!\n");
	return 0;
}
EOM

if [ ! -z "${CROSS_COMPILE}" ]; then
	CC=${CROSS_COMPILE}-gcc
	READELF=${CROSS_COMPILE}-readelf

	${CC} -o /tmp/test /tmp/test.c

	need_lib=$(${READELF} -d /tmp/test | grep 'NEEDED' | tail -1 | cut -d "[" -f2 | cut -d "]" -f1)
	rm -f /tmp/test.c

	CC_ROOT=$(dirname ${CC})/../
	CC_ROOT=$(realpath ${CC_ROOT})

	libc=$(find ${CC_ROOT} -name "$need_lib" -type f | head -1)

	is_musl=$(strings ${libc} | grep 'musl libc')

	if [ ! -z "${is_musl}" ]; then
		echo '-D__MUSL__'
	fi
	rm -f /tmp/test /tmp/test.c
else
	CC=$(which gcc)
	tmpf=/tmp/musl.log

	${CC} -o /tmp/test /tmp/test.c

	# Detect Musl C library.
	libc=$(ldd /tmp/test | grep 'musl' | head -1 | cut -d ' ' -f1)
	if [ -z $libc ]; then
		# This is not Musl.
		rm -f ${tmpf} /tmp/test /tmp/test.c
		exit 0
	fi
	$libc >${tmpf} 2>&1
	vstr=$(cat ${tmpf} | grep "Version" | cut -d ' ' -f2)

	v_major=$(echo $vstr | cut -d '.' -f1)
	v_minor=$(echo $vstr | cut -d '.' -f2)
	v_patch=$(echo $vstr | cut -d '.' -f3)

	rm -f ${tmpf} /tmp/test /tmp/test.c

	echo "-D__MUSL__ -D__MUSL_VER_MAJOR__=${v_major} -D__MUSL_VER_MINOR__=${v_minor} -D__MUSL_VER_PATCH__=${v_patch}"
fi