tpudlik / weak-symbols

weak-symbols

A minimal example of weak symbol linking problems. See https://issues.pigweed.dev/303846960 for context.

Cloning

After cloning the repository, run git submodule --update init to get the Pigweed submodule (can't be pulled in via WORKSPACE for obscure reasons, https://issues.pigweed.dev/300695111).

Building

If you run,

bazel run :main

Bazel will build the binary with the hermetic clang configured in upstream Pigweed, here, and then run it.

If you run,

bazel build --platforms=//targets:cortex-m4 :main

Bazel will build the binary with Arm GCC, configured here.

Results

In the code-as-checked-in everything looks fine:

1. The binary built with the host clang prints,

   Calling A
   strong A!
   

2. I ran nm --demangle on the binary produced by Arm GCC, via

   $(bazel info output_base)/external/gcc_arm_none_eabi_toolchain/bin/arm-none-eabi-gcc-nm --demangle bazel-bin/main > /tmp/nm.txt
   

   The output includes the line,

   00008164 T A
   

   This indicates that the strong symbol was correctly linked in.

But you can break the program by adding __attribute((weak))__ to the function declaration in weak.h. It will still compile, but at runtime you'll get a segmentation fault (with host clang). And nm on the Cortex-M4 binary shows no entry for A at all.