Linking errors with fmt: undefined reference to `std::string fmt::v6::internal::grouping_impl<char>(fmt::v6::internal::locale_ref)'

Question

In our project, we decided to use the latest fmt version(6.2.0) in our project and use mainly the printf functionality as we had our logging where we are using printf extensively.

I built the libfmt.a on our Linux box using the CMakeLists.txt included in the fmt package. In my process, I included the libfmt include directory and in the target_link_libraries. In the code I only used #include<fmt/printf.h>. Now when I compile the code, the code gets compiled but at the time of linking, I get the errors: There are many more but the following is the first one and I believe if this gets resolved, rest will be resolved automatically

abc.cpp:(.text._ZN3fmt2V68internal8groupingIcEESsNS1_10locale_refE[_ZN3fmt2v68internal8groupingIcEESsNS1_10locale_refE]+0X20): undefined eference to `std::string fmt::v6::internal::grouping_impl(fmt::v6::internal::locale_ref)'

I did some analysis and found that this function's definition is present in format-inl.h. I tried including it in my code but still the same linking issue.

Now, when I defined the macro FMT_HEADER_ONLY in the code the linking worked.

My question is: When I am linking with the library libfmt.a, its not able to find that function. Why? I do not want to use the header-only version.

Please let me know how to get this fixed.

Answer 1

The linkage order is important (see https://stackoverflow.com/a/409470/471164). Make sure to pass the library after the source or object files that use it. For example:

git clone https://github.com/fmtlib/fmt.git
cd fmt
make fmt
cat <<EOF > test.cc
#include <fmt/printf.h>

int main() {
  fmt::printf("Hello, %s!", "world");
}
EOF
g++ test.cc -L. -lfmt -Iinclude
./a.out

This example fetches {fmt} from github, compiles it, creates and a small test source, compiles and runs it giving the following output:

Hello, world!

If you pass -lfmt before test.cc you'll get linkage errors.

I recommend using CMake which will take care of the linkage order for you: https://fmt.dev/latest/usage.html#usage-with-cmake