# Linking C shared library - Undefined symbols

**URL:** https://discourse.cmake.org/t/linking-c-shared-library-undefined-symbols/13864
**Category:** Usage
**Tags:** os:macos
**Created:** [April 1, 2025, 3:38pm UTC](https://discourse.cmake.org/t/linking-c-shared-library-undefined-symbols/13864 "2025-04-01T15:38:56Z")
**Posts on this page:** 4
**Page:** 1

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### Author: ![ghenry](https://discourse.cmake.org/user_avatar/discourse.cmake.org/ghenry/32/4888_2.png) [@ghenry](https://discourse.cmake.org/u/ghenry)
#### Post date: [April 1, 2025, 3:38pm UTC](https://discourse.cmake.org/t/linking-c-shared-library-undefined-symbols/13864/1 "2025-04-01T15:38:56Z")

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Hi all,

Looking for a second pair of eyes. My [CI is failing](https://github.com/SentryPeer/SentryPeer/actions/runs/14198375157/job/39778974814) on macOS for my shared lib. I’m not sure why linking is failing.

My [CMakeLists.txt](https://github.com/SentryPeer/SentryPeer/blob/main/CMakeLists.txt)

High level:

1. C binary project builds a lib of itself
2. Rust library uses and links to that lib
3. C project uses Rust library and links to that

All works on my Fedora 41 workstation, but not my MacBook or CI.

I think I need the following, but for the `${CMAKE_PROJECT_NAME}_lib`?

```auto
target_link_libraries(${CMAKE_PROJECT_NAME} -losipparser2)
target_link_libraries(${CMAKE_PROJECT_NAME} -lsqlite3)
target_link_libraries(${CMAKE_PROJECT_NAME} -luuid)
target_link_libraries(${CMAKE_PROJECT_NAME} -lmicrohttpd)
target_link_libraries(${CMAKE_PROJECT_NAME} -ljansson)
target_link_libraries(${CMAKE_PROJECT_NAME} -lcurl)
target_link_libraries(${CMAKE_PROJECT_NAME} -lpcre2-8)

```

as well as my binary (already there).

Thanks.

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### Author: ![LegalizeAdulthood](https://discourse.cmake.org/user_avatar/discourse.cmake.org/legalizeadulthood/32/5282_2.png) [@LegalizeAdulthood](https://discourse.cmake.org/u/LegalizeAdulthood)
#### Post date: [April 1, 2025, 11:32pm UTC](https://discourse.cmake.org/t/linking-c-shared-library-undefined-symbols/13864/2 "2025-04-01T23:32:42Z")

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IMO, you should identify which targets need what things and then use `target_link_libraries` to establish the relationships between targets.

It’s a little bit wrinkly because you’re using dependencies from your environment, e.g. `-lcurl`, instead of a CMake target like `curl::curl`.

The “modern CMake” way of expressing these relationships is to use `find_package` to locate your dependencies on the system and then you should have imported targets on which you can express the relationships.

Starting with the first error:

```auto
[28%] Linking C shared library libsentrypeer.dylib
...
 _MHD_add_response_header, referenced from:
      _finalise_response in http_common.c.o
      _finalise_response in http_common.c.o
      _finalise_response in http_common.c.o
      _finalise_response in http_common.c.o

```

Says that `http_common.c` has a link dependency on `MHD_add_response` and presumably this symbol is supplied by `-lmicrohttpd`.

You’re building a shared library – `libsentrypeer.dylib` – and shared libraries, unlike static libraries, need to resolve all their symbols at link time.

So yes, you need to make your shared library target depend on `-lmicrohttpd`.

Fix the link errors one at a time, identifying the missing library and adding it until linking succeeds.

Once you’ve got it all linking and comitted, consider using `find_package` to locate your dependencies and express the relationships with imported alias targets instead of link command-line options.

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### Author: ![ghenry](https://discourse.cmake.org/user_avatar/discourse.cmake.org/ghenry/32/4888_2.png) [@ghenry](https://discourse.cmake.org/u/ghenry)
#### Post date: [April 2, 2025, 5:59pm UTC](https://discourse.cmake.org/t/linking-c-shared-library-undefined-symbols/13864/3 "2025-04-02T17:59:33Z")

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Thanks for the detailed reply Richard. I’ll have a look at this in the morning. Very much appreciated!

Why does it work on Linux though, e.g. on my Fedora workstation.

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### Author: ![LegalizeAdulthood](https://discourse.cmake.org/user_avatar/discourse.cmake.org/legalizeadulthood/32/5282_2.png) [@LegalizeAdulthood](https://discourse.cmake.org/u/LegalizeAdulthood)
#### Post date: [April 2, 2025, 8:58pm UTC](https://discourse.cmake.org/t/linking-c-shared-library-undefined-symbols/13864/4 "2025-04-02T20:58:40Z")

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> [@ghenry](#):
>
> Why does it work on Linux though, e.g. on my Fedora workstation.

Different systems do different things. It’s what I call “working by accident” as opposed to “working by design”.

On your linux machine, something is picking up the necessary dependencies “automatically”.