I am developing a Net-SNMP subagent whose final target will be an ARM board, so I'm using CMake to make it easier to manage building the native and cross-compiled versions. I started testing it on my host platform (x86_64) and it builds and works fine with this CMakeLists.txt file:
cmake_minimum_required (VERSION 2.6)
project (snmp_agent C)
set(snmp_agent_VERSION_MAJOR 1)
set(snmp_agent_VERSION_MINOR 0)
# Defines path to the net-snmp-config script
set(NETSNMPCONFIG "${CMAKE_FIND_ROOT_PATH}/usr/bin/net-snmp-config")
# Gets compiling flags and libs linked to Net-SNMP
execute_process(COMMAND "${NETSNMPCONFIG}" "--base-cflags" OUTPUT_VARIABLE NETSNMPCFLAGS)
execute_process(COMMAND "${NETSNMPCONFIG}" "--agent-libs" OUTPUT_VARIABLE NETSNMPLIBS)
# Removes leading/trailing spaces from net-snmp-config output
string(STRIP ${NETSNMPCFLAGS} NETSNMPCFLAGS)
string(STRIP ${NETSNMPLIBS} NETSNMPLIBS)
# Prints compilation and linker flags used in Net-SNMP package
message("Net-SNMP package CFLAGS: ${NETSNMPCFLAGS}")
message("Net-SNMP package LIBS: ${NETSNMPLIBS}")
# Setting libs and compilation flags variables
set(LIBS "${NETSNMPLIBS}")
set(STRICT_FLAGS "-Wall -Wstrict-prototypes")
set(CFLAGS "-I. ${STRICT_FLAGS} ${NETSNMPCFLAGS}")
# Sets prefix for files created by 'mib2c' for the wanted MIB
set(ENT_PHYSICAL_ENTRY "scalars/entPhysicalEntry")
# Source files created by 'mib2c' and then user customized
set(USER_SRCS
${ENT_PHYSICAL_ENTRY}.c
)
# Setting subagent sources
set(SRCS ${USER_SRCS}
${CMAKE_PROJECT_NAME}.c
)
# Finds the required Net-SNMP lib paths and assigns them to variables
find_library(NETSNMPAGENT "netsnmpagent")
message("Found ${NETSNMPAGENT}")
find_library(NETSNMPMIBS "netsnmpmibs")
message("Found ${NETSNMPMIBS}")
find_library(NETSNMP "netsnmp")
message("Found ${NETSNMP}")
# Sets the flags to be used for compiling and linking the executable
set_source_files_properties(${SRCS} COMPILE_FLAGS ${CFLAGS})
add_executable(${CMAKE_PROJECT_NAME} ${SRCS})
target_link_libraries(${CMAKE_PROJECT_NAME} ${NETSNMPAGENT} ${NETSNMPMIBS} ${NETSNMP})
I use a separate build dir so I don't mix build and source files, which stay along with the CMakeLists.txt file, so the the output of these commands...
cd ~/git/snmp_agent # CMakeLists.txt is in here along with source files
mkdir build
cd build
cmake ..
...is as follows:
claudio@slackdev:~/git/snmp_agent/build$ cmake ..
-- The C compiler identification is GNU 5.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
Net-SNMP package CFLAGS: -DNETSNMP_ENABLE_IPV6 -fno-strict-aliasing -O2 -fPIC -Ulinux -Dlinux=linux -D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -I/usr/lib64/perl5/CORE -I/usr/include/libnl3 -I/usr/include
Net-SNMP package LIBS: -L/usr/lib64 -lnetsnmpmibs -lsensors -lpci -ldl -lnetsnmpagent -lwrap -lnsl -Wl,-E -Wl,-rpath,/usr/lib64/perl5/CORE -lnetsnmp -lcrypto -lnl-3 -lm
Found /usr/lib64/libnetsnmpagent.so
Found /usr/lib64/libnetsnmpmibs.so
Found /usr/lib64/libnetsnmp.so
-- Configuring done
-- Generating done
-- Build files have been written to: /home/claudio/git/snmp_agent/build
Running make, it builds just fine:
claudio@slackdev:~/git/snmp_agent/build$ make
Scanning dependencies of target snmp_agent
[ 25%] Building C object CMakeFiles/snmp_agent.dir/scalars/entPhysicalEntry.c.o
[ 50%] Building C object CMakeFiles/snmp_agent.dir/snmp_agent.c.o
[100%] Linking C executable snmp_agent
[100%] Built target snmp_agent
This snmp_agent runs as an agentX subagent, connecting to the main snmpd master agent and properly answering SNMP requests against entPhysicalEntry scalar objects.
Ok, so far so good. The problem is when I try to cross-compile this so it does the same thing on my final target, which is an ARMv7 Allwinner A20 board. I already have a cross-toolchain available for it - which I created using Crosstool-NG. It is in the search PATH and its tools begin with the prefix armv7-a20_allwinner-linux-gnueabihf. In order to use it, I set up the following toolchain file, which was named armv7-a20_allwinner-linux-gnueabihf.cmake:
# the name of the target operating system
SET(CMAKE_SYSTEM_NAME Linux)
# which C and C++ compiler to use
SET(CMAKE_C_COMPILER armv7-a20_allwinner-linux-gnueabihf-gcc)
SET(CMAKE_CXX_COMPILER armv7-a20_allwinner-linux-gnueabihf-g++)
# here is the target environment located
SET(CMAKE_FIND_ROOT_PATH "$ENV{HOME}/arm_rootfs")
# adjust the default behaviour of the FIND_XXX() commands:
# search headers and libraries in the target environment, search
# programs in the host environment
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
Using a different build dir for the ARM version, CMake should pick up the right paths pointing to the ARM root filesystem, which is a copy of what will be on the target board's SDcard:
cd ~/git/snmp_agent # CMakeLists.txt and armv7-a20_allwinner-linux-gnueabihf.cmake are here
mkdir build-arm
cd build-arm
cmake -DCMAKE_TOOLCHAIN_FILE=../armv7-a20_allwinner-linux-gnueabihf.cmake ..
Note that apparently CMake runs fine, as the paths found are all relative to the ARM root filesystem:
claudio@slackdev:~/git/snmp_agent/build-arm$ cmake -DCMAKE_TOOLCHAIN_FILE=../cmake_defs/armv7-a20_allwinner-linux-gnueabihf.cmake ..
-- The C compiler identification is GNU 6.3.0
-- Check for working C compiler: /home/claudio/x-tools/armv7-a20_allwinner-linux-gnueabihf/bin/armv7-a20_allwinner-linux-gnueabihf-gcc
-- Check for working C compiler: /home/claudio/x-tools/armv7-a20_allwinner-linux-gnueabihf/bin/armv7-a20_allwinner-linux-gnueabihf-gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
Net-SNMP package CFLAGS: -DNETSNMP_ENABLE_IPV6 -fno-strict-aliasing -O2 -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=hard -Ulinux -Dlinux=linux -D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -I/usr/lib/perl5/CORE -I/usr/include/libnl3 -I/usr/include
Net-SNMP package LIBS: -L/usr/lib -lnetsnmpmibs -lsensors -lpci -ldl -lnetsnmpagent -lwrap -lnsl -Wl,-E -Wl,-rpath,/usr/lib/perl5/CORE -lnetsnmp -lcrypto -lnl-3 -lm
Found /home/claudio/arm_rootfs/usr/lib/libnetsnmpagent.so
Found /home/claudio/arm_rootfs/usr/lib/libnetsnmpmibs.so
Found /home/claudio/arm_rootfs/usr/lib/libnetsnmp.so
-- Configuring done
-- Generating done
-- Build files have been written to: /home/claudio/git/snmp_agent/build-arm
But when running make it falls apart while looking for /usr/include/gnu/stubs.h. If we try to repeat make with VERBOSE=1, we can see the gcc invocation is using paths relative to the host machine:
claudio@slackdev:~/git/snmp_agent/build-arm$ make VERBOSE=1
/usr/bin/cmake -H/home/claudio/git/snmp_agent -B/home/claudio/git/snmp_agent/build-arm --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /home/claudio/git/snmp_agent/build-arm/CMakeFiles /home/claudio/git/snmp_agent/build-arm/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory 'snmp_agent/build-arm'
make -f CMakeFiles/snmp_agent.dir/build.make CMakeFiles/snmp_agent.dir/depend
make[2]: Entering directory 'snmp_agent/build-arm'
cd /home/claudio/git/snmp_agent/build-arm && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /home/claudio/git/snmp_agent /home/claudio/git/snmp_agent /home/claudio/git/snmp_agent/build-arm /home/claudio/git/snmp_agent/build-arm /home/claudio/git/snmp_agent/build-arm/CMakeFiles/snmp_agent.dir/DependInfo.cmake --color=
make[2]: Leaving directory 'snmp_agent/build-arm'
make -f CMakeFiles/snmp_agent.dir/build.make CMakeFiles/snmp_agent.dir/build
make[2]: Entering directory 'snmp_agent/build-arm'
[ 25%] Building C object CMakeFiles/snmp_agent.dir/scalars/entPhysicalEntry.c.o
/home/claudio/x-tools/armv7-a20_allwinner-linux-gnueabihf/bin/armv7-a20_allwinner-linux-gnueabihf-gcc -I. -Wall -Wstrict-prototypes -DNETSNMP_ENABLE_IPV6 -fno-strict-aliasing -O2 -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=hard -Ulinux -Dlinux=linux -D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -I/usr/lib/perl5/CORE -I/usr/include/libnl3 -I/usr/include -o CMakeFiles/snmp_agent.dir/scalars/entPhysicalEntry.c.o -c /home/claudio/git/snmp_agent/scalars/entPhysicalEntry.c
In file included from /usr/include/features.h:392:0,
from /usr/include/stdio.h:27,
from /usr/include/net-snmp/net-snmp-includes.h:14,
from /home/claudio/git/snmp_agent/scalars/entPhysicalEntry.c:7:
/usr/include/gnu/stubs.h:7:27: fatal error: gnu/stubs-32.h: No such file or directory
# include <gnu/stubs-32.h>
^
compilation terminated.
CMakeFiles/snmp_agent.dir/build.make:62: recipe for target 'CMakeFiles/snmp_agent.dir/scalars/entPhysicalEntry.c.o' failed
make[2]: *** [CMakeFiles/snmp_agent.dir/scalars/entPhysicalEntry.c.o] Error 1
make[2]: Leaving directory 'snmp_agent/build-arm'
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/snmp_agent.dir/all' failed
make[1]: *** [CMakeFiles/snmp_agent.dir/all] Error 2
make[1]: Leaving directory 'snmp_agent/build-arm'
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
The strange thing here is the /usr/include/gnu/stubs.h from the ARM root filesystem has nothing to do with stubs-32.h:
/* This file is automatically generated.
This file selects the right generated file of `__stub_FUNCTION' macros
based on the architecture being compiled for. */
#if !defined __ARM_PCS_VFP
# include <gnu/stubs-soft.h>
#endif
#if defined __ARM_PCS_VFP
# include <gnu/stubs-hard.h>
#endif
But if you look at the same file from the host system - which is a x86_64 machine - we can guess why it tried to find stubs-32.h (remember we are trying to compile for ARM, so it won't find the __x86_64__ symbol definition):
/* This file is automatically generated.
This file selects the right generated file of `__stub_FUNCTION' macros
based on the architecture being compiled for. */
#if !defined __x86_64__
# include <gnu/stubs-32.h>
#endif
#if defined __x86_64__ && defined __LP64__
# include <gnu/stubs-64.h>
#endif
#if defined __x86_64__ && defined __ILP32__
# include <gnu/stubs-x32.h>
#endif
Why is that happening, since the toolchain file clearly specifies that libraries and includes are to be searched ONLY in the path set by CMAKE_FIND_ROOT_PATH?
UPDATE (problem not totally solved yet!):
After I had accepted the answer from @Tsyvarev I was double checking my CMakeLists.txt file and I found while trying to make it work I had made a hack by manually setting CMAKE_FIND_ROOT_PATH as prefix of each compiler include switches (-I) returned by net-snmp-config, which obviously is not the ideal thing to do. That, along with CMAKE_SYSROOT, makes it work, but CMAKE_SYSROOT alone doesn't prefix the include paths by itself:
# This is the manually hacked line:
set(NETSNMPCFLAGS "-DNETSNMP_ENABLE_IPV6 -fno-strict-aliasing -O2 -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=hard -Ulinux -Dlinux=linux -D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I${CMAKE_FIND_ROOT_PATH}/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -I${CMAKE_FIND_ROOT_PATH}/usr/lib/perl5/CORE -I${CMAKE_FIND_ROOT_PATH}/usr/include/libnl3 -I${CMAKE_FIND_ROOT_PATH}/usr/include")
set(LIBS "${NETSNMPLIBS}")
set(STRICT_FLAGS "-Wall -Wstrict-prototypes")
set(CFLAGS "-I. ${STRICT_FLAGS} ${NETSNMPCFLAGS}")
You need to set CMAKE_SYSROOT variable for refer to "here is the target environment located".
Unlike to CMAKE_FIND_ROOT_PATH variable, which is used only in find_* commands, CMAKE_SYSROOT is used also as a hint to the compiler (--sysroot option), so compiler will pick correct includes.
In case of cross-compiling, variable CMAKE_FIND_ROOT_PATH is used for provide additional search prefixes for find_* commands. CMAKE_SYSROOT is used as a prefix automatically.
FINAL SOLUTION:
Like explained on the question UPDATE, @Tsyvarev's answer fixed the linking part by providing the --sysroot switch to the compiler pointing to the root filesystem for ARM, but the problem of finding a stubs-32.h from the host system isn't fixed by that. Although it was part of the solution, it is important to note the primary observation had another cause: the include -I switches were being used directly as CFLAGS to the compiler, making it effectively look at the host system, since they where not prefixed with the root filesystem path for the ARM board (remember the flags were output directly by the net-snmp-config script, which was reporting flags for the native ARM build, so with "normal" paths instead). To fix that I used a CMake string command to remove all the -I switches from the NETSNMPCFLAGS variable:
# Gets compiling flags and libs linked to Net-SNMP
execute_process(COMMAND "${NETSNMPCONFIG}" "--base-cflags" OUTPUT_VARIABLE NETSNMPCFLAGS)
execute_process(COMMAND "${NETSNMPCONFIG}" "--agent-libs" OUTPUT_VARIABLE NETSNMPLIBS)
# removes the include dir switches "-I" from the NETSNMPCFLAGS, since we don't want
# the compiler to include paths relative to the host system in the compilation
string(REGEX REPLACE "-I[a-zA-Z0-9/]*" "" NETSNMPCFLAGS ${NETSNMPCFLAGS})
set(STRICT_FLAGS "-Wall -Wstrict-prototypes")
set(CFLAGS "-I. ${STRICT_FLAGS} ${NETSNMPCFLAGS}")
and put the root filesystem in an include_directories directive:
# Sets the flags to be used for compiling and linking the executable
set_source_files_properties(${SRCS} COMPILE_FLAGS ${CFLAGS})
include_directories(${CMAKE_FIND_ROOT_PATH}/usr/include)
add_executable(${CMAKE_PROJECT_NAME} ${SRCS})
target_link_libraries(${CMAKE_PROJECT_NAME} ${NETSNMPAGENT} ${NETSNMPMIBS} ${NETSNMP})
来源：https://stackoverflow.com/questions/44559445/why-is-cmake-apparently-referring-to-host-system-files-while-cross-compiling-a-n