我正在将Linux应用程序移植到macOS,并且链接行为有所不同,这花费了我一些时间来展示自己。该项目使用基于CMake的两阶段构建过程:一棵CMake树创建一个动态库,该库链接到在第二个树中创建的静态库,该静态树随后创建。创建动态库时,静态库尚不存在。这在Linux上有效:动态库是使用静态库中的符号创建的,并且它们已预先声明。当构建第二棵树时,动态库将链接到可执行文件,该可执行文件也链接静态库,因此一切正常。这在macOS上不起作用,因为在第一棵CMake树中,编译器在动态库的链接步骤失败,因为第二棵树中的静态库尚不存在。
我已将我的应用程序简化为一个最小的示例(代码可以在我的问题结尾处找到)。
设置如下:
如果我们停止将静态库链接到动态库:
# This target_link_libraries is intentionally commented out.
#target_link_libraries(dynamic_lib static_lib)
当然,我们在构建程序时会出错。但是错误在macOS和Linux上有所不同:
macOS / clang较早失败在动态库被链接与Linux / gcc稍后失败在程序被链接时执行的步骤
我确实认识到区别可能在于我在macOS上使用clang而在Linux上使用gcc,但这并不能向我解释这个问题。
我想知道:
该示例已发布到Github:Linking a dynamic library that links in symbols from a static library (macOS vs Linux)。
这些是我在Github上的示例中的关键文件:
CMakeLists.txt
cmake_minimum_required(VERSION 3.14)
project(untitled1 C)
add_compile_options("-fPIC")
set(CMAKE_C_STANDARD 99)
add_library(static_lib static_lib.c)
add_library(dynamic_lib SHARED dynamic_lib.c)
# THE ISSUE IS HERE:
# This target_link_libraries is intentionally commented out.
# on macOS the build process fails when linking dynamic_lib
# on Linux the build process fails when linking the
# 'untitled1' program.
#target_link_libraries(dynamic_lib static_lib)
add_executable(untitled1 main.c)
target_link_libraries(untitled1 dynamic_lib)
dynamic_lib.c
#include "dynamic_lib.h"
#include "static_lib.h"
void dynamic_lib_func() {
static_lib_func();
}
static_lib.c
#include "static_lib.h"
void static_lib_func() {}
macOS输出
[ 25%] Building C object CMakeFiles/dynamic_lib.dir/dynamic_lib.c.o
/Library/Developer/CommandLineTools/usr/bin/cc -Ddynamic_lib_EXPORTS -g -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -fPIC -fPIC -std=gnu99 -o CMakeFiles/dynamic_lib.dir/dynamic_lib.c.o -c /Users/stanislaw/workspace/code/Examples/untitled1/dynamic_lib.c
[ 50%] Linking C shared library libdynamic_lib.dylib
/Applications/CLion.app/Contents/bin/cmake/mac/bin/cmake -E cmake_link_script CMakeFiles/dynamic_lib.dir/link.txt --verbose=1
/Library/Developer/CommandLineTools/usr/bin/cc -g -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -dynamiclib -Wl,-headerpad_max_install_names -o libdynamic_lib.dylib -install_name @rpath/libdynamic_lib.dylib CMakeFiles/dynamic_lib.dir/dynamic_lib.c.o
Undefined symbols for architecture x86_64:
"_static_lib_func", referenced from:
_dynamic_lib_func in dynamic_lib.c.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Linux输出
[ 16%] Linking C shared library libdynamic_lib.so
[ 33%] Built target dynamic_lib
Scanning dependencies of target untitled1
[ 50%] Linking C executable untitled1
libdynamic_lib.so: undefined reference to `static_lib_func'
collect2: error: ld returned 1 exit status
我设法找到了一个解决问题,当我将同一项目从Linux移植到macOS时也遇到了相关问题:How to share a global variable between a main process and a dynamic library via a static library (macOS)?。>
事实证明,在macOS上可以使用这种库符号的“转发声明”:
添加-undefined dynamic_lookup
标志使macOS传递原始错误。
将其添加到示例的CMakeLists.txt文件中可解决此问题:
target_link_options(dynamic_lib PRIVATE -undefined dynamic_lookup)