[CMake] RPATH cross-compile issue with CHECK_*_EXISTS

[Jörg Krause]

Hi,

when using CHECK_{SYMBOL,FUNCTION}_EXISTS in a cross-compilation
environment, CMake passes the host rpath to the linker:

""" CMakeOutput.log

/home/joerg/host/usr/bin/i586-linux-gcc  
--sysroot=/home/joerg/host/usr/i586-buildroot-linux-musl/sysroot
-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
-Os   -DNDEBUG    CMakeFiles/cmTC_cb8f6.dir/CheckSymbolExists.c.o  -o
cmTC_cb8f6 -Wl,-rpath,/usr/lib32 -rdynamic /home/joerg/host/usr/i586-
buildroot-linux-musl/sysroot/usr/lib32/libmbedtls.so
/home/joerg/host/usr/i586-buildroot-linux-
musl/sysroot/usr/lib32/libmbedx509.so /home/joerg/host/usr/i586-
buildroot-linux-musl/sysroot/usr/lib32/libmbedcrypto.so 

"""

This leads to a linker error if mbedtls is linked with zlib, as the
linker tries to link with the host zlib and does not find the host
libc:

""" CMakeError.txt

/home/joerg/host/usr/i586-buildroot-linux-musl/bin/ld: warning:
libc.so.6, needed by /usr/lib32/libz.so.1, not found (try using -rpath
or -rpath-link)
/usr/lib32/libz.so.1: undefined reference to `strcpy at GLIBC_2.0'
/usr/lib32/libz.so.1: undefined reference to `free at GLIBC_2.0'
/usr/lib32/libz.so.1: undefined reference to `fseeko64 at GLIBC_2.1
[..]

"""

I did not find any solution which allows me to remove the rpath from
the check. Setting CMAKE_SKIP_RPATH does not change the build behaviour of the check_symbol_exists macro.

>From my understanding, the rpath flag should not be used when the
sysroot flag is passed to the linker, right?

For reference, I added a minimal example which uses Buildroot for
cross-compilation.

""" CMakeLists.txt

cmake_minimum_required(VERSION 2.8.12)

project(test)

list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")

find_package(MbedTLS)
if(MBEDTLS_FOUND)
	message(STATUS "Using mbedTLS")
endif()

"""

""" FindMbedTLS.cmake

include(CheckSymbolExists)

find_path(MBEDTLS_INCLUDE_DIRS
	NAMES mbedtls/ssl.h
	PATH_SUFFIXES include
)

find_library(MBEDTLS_LIBRARY NAMES mbedtls)
find_library(MBEDX509_LIBRARY NAMES mbedx509)
find_library(MBEDCRYPTO_LIBRARY NAMES mbedcrypto)

if(MBEDTLS_LIBRARY AND MBEDX509_LIBRARY AND MBEDCRYPTO_LIBRARY)
	set(CMAKE_REQUIRED_INCLUDES ${MBEDTLS_INCLUDE_DIRS})
	set(CMAKE_REQUIRED_LIBRARIES ${MBEDTLS_LIBRARY}
${MBEDX509_LIBRARY} ${MBEDCRYPTO_LIBRARY})
	check_symbol_exists(mbedtls_ssl_init "mbedtls/ssl.h"
MBEDTLS_V2)
endif()

"""

""" toolchainfile.cmake

string(REPLACE "/usr/share/buildroot" "" RELOCATED_HOST_DIR
${CMAKE_CURRENT_LIST_DIR})

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR i586)

set(CMAKE_C_FLAGS_DEBUG "" CACHE STRING "Debug CFLAGS")
set(CMAKE_CXX_FLAGS_DEBUG "" CACHE STRING "Debug CXXFLAGS")
set(CMAKE_C_FLAGS_RELEASE " -DNDEBUG" CACHE STRING "Release CFLAGS")
set(CMAKE_CXX_FLAGS_RELEASE " -DNDEBUG" CACHE STRING "Release
CXXFLAGS")

# Build type from the Buildroot configuration
set(CMAKE_BUILD_TYPE Release CACHE STRING "Buildroot build
configuration")

# Buildroot defaults flags.
set(CMAKE_C_FLAGS "-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE
-D_FILE_OFFSET_BITS=64 -Os" CACHE STRING "Buildroot CFLAGS")
set(CMAKE_CXX_FLAGS "-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE
-D_FILE_OFFSET_BITS=64 -Os" CACHE STRING "Buildroot CXXFLAGS")
set(CMAKE_EXE_LINKER_FLAGS "" CACHE STRING "Buildroot LDFLAGS for
executables")

set(CMAKE_INSTALL_SO_NO_EXE 0)

set(CMAKE_PROGRAM_PATH "${RELOCATED_HOST_DIR}/usr/bin")
set(CMAKE_SYSROOT "${RELOCATED_HOST_DIR}/usr/i586-buildroot-linux-
musl/sysroot")
set(CMAKE_FIND_ROOT_PATH "${RELOCATED_HOST_DIR}/usr/i586-buildroot-
linux-musl/sysroot")
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(ENV{PKG_CONFIG_SYSROOT_DIR} "${RELOCATED_HOST_DIR}/usr/i586-
buildroot-linux-musl/sysroot")

# This toolchain file can be used both inside and outside Buildroot.
set(CMAKE_C_COMPILER "${RELOCATED_HOST_DIR}/usr/bin/i586-linux-gcc")
set(CMAKE_CXX_COMPILER "${RELOCATED_HOST_DIR}/usr/bin/i586-linux-g++")

"""

What do I miss?

Best regards,
Jörg Krause