为什么CMake在交叉编译Net代理时显然是指主机系统文件，尽管使用的是正确的工具链文件？

Question

我正在开发一个Net-SNMP子代理，它的最终目标将是ARM板，所以我使用CMake使构建本机版本和交叉编译版本变得更容易。我开始在我的主机平台(x86_64)上测试它，它构建了这个x86_64文件并运行得很好：

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})

我使用一个单独的build，这样我就不会混合生成文件和源文件，它们与CMakeLists.txt文件保持在一起，所以这些命令的输出.

cd ~/git/snmp_agent # CMakeLists.txt is in here along with source files
mkdir build
cd build
cmake ..

...is如下：

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

运行make，它构建得很好：

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

此snmp_agent作为agentX子代理运行，连接到主snmpd主代理，并针对entPhysicalEntry标量对象正确地响应snmpd请求。

好吧，到目前为止还不错。问题是，当我试图交叉编译它时，它在我的最终目标上做同样的事情，那就是ARMv7 Allwinner A20板。我已经为它提供了一个跨工具链--这是我使用曲霉-NG创建的。它位于搜索路径中，其工具以前缀armv7-a20_allwinner-linux-gnueabihf开头。为了使用它，我设置了以下工具链文件，名为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)

对于ARM版本，使用不同的build，CMake应该选择指向ARM根文件系统的正确路径，这是目标板的SDcard上的一个副本：

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 ..

注意，显然CMake运行良好，因为找到的路径都相对于ARM根文件系统：

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

但是当运行make时，它在寻找/usr/include/gnu/stubs.h时会崩溃。如果我们尝试在make中重复VERBOSE=1，我们可以看到gcc调用使用的是相对于主机的路径：

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

奇怪的是，来自ARM根文件系统的/usr/include/gnu/stubs.h与stubs-32.h无关。

/* 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

但是，如果您查看来自主机系统的同一个文件--这是一台x86_64机器--我们可以猜到它为什么要查找stubs-32.h (请记住，我们正在为ARM编译，这样它就找不到__x86_64__符号定义了)：

/* 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

为什么会发生这种情况，因为工具链文件明确指定只在CMAKE_FIND_ROOT_PATH设置的路径中搜索库和包含？

更新(问题尚未完全解决！)：

在接受了@Tsyvarev的回答之后，我再次检查了我的CMakeLists.txt文件，在尝试使其工作时，我通过手动将CMAKE_FIND_ROOT_PATH设置为net-snmp-config，返回的每个编译器包含开关(-I)的前缀进行了黑客攻击，这显然不是理想的做法。这与CMAKE_SYSROOT一起使其工作，但仅CMAKE_SYSROOT本身并不是包含路径的前缀：

# 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}")

Answer 1

最终解决方案：

与关于UPDATE问题的解释一样，@Tsyvarev的回答通过向指向ARM根文件系统的编译器提供--sysroot开关来修正链接部分，但是从主机系统中查找stubs-32.h的问题并没有得到解决。尽管它是解决方案的一部分，但重要的是要注意到主要的观察还有另一个原因: include -I开关被直接用作编译器的CFLAGS，这使得它能够有效地查看主机系统，因为它们不以ARM板的根文件系统路径作为前缀(请记住这些标志是由-I脚本直接输出的，它报告了本机 ARM构建的标志，因此使用了“普通”路径)。要解决这个问题，我使用了一个CMake字符串命令来从NETSNMPCFLAGS变量中删除所有的-I开关：

# 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}")

并将根文件系统放在include_directories指令中：

# 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})

Answer 2

您需要设置赛鲁特变量以引用“这里是所处的目标环境”。

与路径变量不同，它只在find_*命令中使用，CMAKE_SYSROOT也用作编译器的提示(--sysroot选项)，因此编译器将选择正确的包含。

在交叉编译的情况下，变量CMAKE_FIND_ROOT_PATH用于为find_*命令提供额外的搜索前缀.CMAKE_SYSROOT自动用作前缀。

Answer 3

cmake引用主机文件的原因可能与pkg-config有关。

参见本文件关于交叉编译和pkg-config。

我用以下内容更新了我的toolchain.cmake文件：

set(CMAKE_SYSROOT /home/windel/rpi-sysroot)

# Pkg config tweaks:
SET(ENV{PKG_CONFIG_LIBDIR} ${CMAKE_SYSROOT}/usr/lib/aarch64-linux-gnu/pkgconfig)
SET(ENV{PKG_CONFIG_SYSROOT_DIR} ${CMAKE_SYSROOT})