CMake seems to prepend linker flags at the front of a GCC compilation command, instead of appending it at the end. How to make CMake append linker flags?
Here is a simple example to reproduce the problem.
Consider this C++ code that uses clock_gettime
:
// main.cpp
#include <iostream>
#include <time.h>
int main()
{
timespec t;
clock_gettime(CLOCK_REALTIME, &t);
std::cout << t.tv_sec << std::endl;
return 0;
}
This is a CMakeLists.txt to compile the C++ file above:
cmake_minimum_required(VERSION 2.8)
set(CMAKE_EXE_LINKER_FLAGS "-lrt")
add_executable(helloapp main.cpp)
Note that we have added -lrt
since it has the definition of clock_gettime
.
Compiling this using:
$ ls
CMakeLists.txt main.cpp
$ mkdir build
$ cd build
$ cmake ..
$ make VERBOSE=1
Which throws up this error, even though you can see -lrt
in the command:
/usr/bin/c++ -lrt CMakeFiles/helloapp.dir/main.cpp.o -o helloapp -rdynamic
CMakeFiles/helloapp.dir/main.cpp.o: In function `main':
main.cpp:(.text+0x15): undefined reference to `clock_gettime'
collect2: ld returned 1 exit status
make[2]: *** [helloapp] Error 1
The problem here is the C++ compilation command generated by CMake has -lrt
prepended at the front. The compilation works fine if it had been:
/usr/bin/c++ CMakeFiles/helloapp.dir/main.cpp.o -o helloapp -rdynamic -lrt
How to make CMake append the linker flags at the end?
In general you can't (I think), but in the specific case that you want to link against a particular library, you should be using the syntax
target_link_libraries(helloapp rt)
instead. CMake knows that this corresponds to passing -lrt
on the linker command line.