`mdc_formatter` fails to read MDC values due to `static thread_local context` being duplicated across TUs

[Cansisti]

Summary

When building spdlog as a compiled library (SPDLOG_COMPILED_LIB) and using the mdc_formatter (%&), MDC values inserted via spdlog::mdc::put(...) are not visible in the formatter. This only happens in optimized builds (-O2, -O3); in debug builds (-O0), everything works as expected.

This is caused by static thread_local usage in mdc::get_context() in the header file mdc.h. Under optimization, the compiler generates separate instances of the thread_local context per translation unit, which causes put(...) and the formatter to operate on different contexts — even within the same thread.

---

Minimal reproduction

1. Build spdlog with -O2, -DSPDLOG_BUILD_SHARED=ON and SPDLOG_BUILD_TESTS=ON.
2. Run the test: mdc formatter test-1 in test_pattern_formatter.cpp.

Expected log output:
[logger-name] [info] [mdc_key_1:mdc_value_1 mdc_key_2:mdc_value_2] some message
Actual output:
[logger-name] [info] [] some message

If you insert this line before the formatter call:

REQUIRE(spdlog::mdc::get("mdc_key_1") == "mdc_value_1");

…then the value is found — confirming that put() worked, but mdc_formatter sees a different thread_local context.

Cause

mdc::get_context() is currently defined as:

static thread_local mdc_map_t context;

in a header-only context. This causes multiple copies of the variable to be instantiated across different TUs when optimization is enabled, due to inlining and aggressive code elimination. In particular:

• mdc::put() is compiled into the test TU,
• mdc_formatter::format() is compiled into the spdlog library TU,
• and they each operate on a different thread_local instance.

This does not occur at -O0 because inlining is disabled, and symbol resolution happens normally.

Suggested fix

Move the definition of get_context() to a .cpp file to guarantee a single instance of the thread_local variable per program (per thread)

This avoids duplication across TUs and ensures that all MDC-aware components share the same storage.

Notes

• Tested on macOS (Clang 14, Apple Silicon).

[tt4g]

PR is welcome.