# Determine flags for linking C++ standard library

**URL:** <https://discourse.cmake.org/t/determine-flags-for-linking-c-standard-library/4373>\
**Category:** Code\
**Created:** [October 30, 2021, 12:31pm UTC](https://discourse.cmake.org/t/determine-flags-for-linking-c-standard-library/4373 "2021-10-30T12:31:39Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![szhorvat](https://discourse.cmake.org/user_avatar/discourse.cmake.org/szhorvat/32/1906_2.png) [@szhorvat](https://discourse.cmake.org/u/szhorvat)\
**Post date:** [October 30, 2021, 12:31pm UTC](https://discourse.cmake.org/t/determine-flags-for-linking-c-standard-library/4373/1 "2021-10-30T12:31:39Z")

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_I have posted this question [on StackOverflow](https://stackoverflow.com/q/69706598/695132) a few days ago, but received no answer so far._

* * *

I am trying to use CMake to generate a pkg-config file for a library with a C interface. This library is partially written in C++, so C programs that use it need to also link in the C++ standard library. How do I determine what the necessary linking flags are, using CMake, so I can include them in the generated pkg-config file?

* * *

To elaborate, suppose I have a C program, `prog`, that uses my library `mylib`. One way to link `prog` is to use the C++ linker:

```nohighlight
c++ prog.o -lmylib -o prog

```

This is what CMake normally does (i.e. when `prog` is built with CMake).

Another way is to use the C linker. In this case, we need to explicitly link in the C++ standard library. The linking command might look like this:

```nohighlight
cc prog.o -lmylib -lstdc++

```

The correct flags for linking the C++ standard library will differ from system to system, and from compiler to compiler. How can I use CMake to determine these linking flags?

I would like to include these flags in the `Libs.private` section of `mylib`'s pkg-config file. As far as I know, `pkg-config` has no way to indicate that the C++ linker should be used (or show the name of the appropriate linking program). All it can do is list the required linking flags. Thus the usual solution would be to include these in the `mylib.pc`.

* * *

In practice, this choice comes down to deciding whether to use `-lstdc++` (most Linux and older macOS) or `-lc++` (newer macOS, maybe some Linux, maybe some BSD).

Right now we basically hard-code `-lc++` on macOS and use `-lstdc++` on all other systems, but this will obviously not be correct in some cases.

**UPDATE** In practice, it would be useful to have a limited solution that simply chooses between “`libstdc++`” / “`libc++`” / “neither” when the compiler is either GCC or Clang. This covers the vast majority of cases when it actually makes sense to generate a pkg-config file.

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**Author:** ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)\
**Post date:** [October 31, 2021, 12:48am UTC](https://discourse.cmake.org/t/determine-flags-for-linking-c-standard-library/4373/2 "2021-10-31T00:48:40Z")

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I think you’ll likely have to detect the stdlib in some `try_compile` (or similar) mechanism to decipher what is going on. CMake generally just lets the compiler deal with the stdlib (though I’ve had discussions about abstracting out the runtime library as follow-on work to [`CMP0091`](https://cmake.org/cmake/help/git-master/policy/CMP0091.html).

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**Author:** ![szhorvat](https://discourse.cmake.org/user_avatar/discourse.cmake.org/szhorvat/32/1906_2.png) [@szhorvat](https://discourse.cmake.org/u/szhorvat)\
**Post date:** [October 31, 2021, 8:50am UTC](https://discourse.cmake.org/t/determine-flags-for-linking-c-standard-library/4373/3 "2021-10-31T08:50:05Z")

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Thanks for the response. Do you mean trying to parse verbose compiler output or similar?

I think looking for `-lstdc++` or `-lc++` in the verbose output of `c++ -v foo.cpp` (where `foo.cpp` is a trivial `main() { return 0; }`) would work in most practically relevant cases (after verifying that the compiler is either Clang or GCC). Can `try_compile` give me this output, or would I need to use something like `execute_process` instead?

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<div class="post-metadata">

**Author:** ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)\
**Post date:** [October 31, 2021, 11:33am UTC](https://discourse.cmake.org/t/determine-flags-for-linking-c-standard-library/4373/4 "2021-10-31T11:33:53Z")

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You can use `check_symbol_exists` for `_LIBCPP_VERSION` (libc++) or ` __GLIBCXX__ ` (libstdc++).