Structured logging for embedded systems in portable C99.

No third-party runtime dependencies. Zero dynamic allocation. Pluggable backends. Predictable output.

• Five runtime levels: TRACE, DEBUG, INFO, WARN, ERROR
• Compile-time stripping via MLOG_LEVEL_MIN
• Runtime global and per-backend filters
• Multiple backends with per-backend color selection
• Optional timestamps via one clock callback sample per event
• Deterministic truncation with newline termination
• C99 logging macros that compile without GNU variadic extensions
• Hex dump helper for byte-oriented diagnostics

#include "mlog.h"

static void uart_write(const char *buf, uint16_t len, mlog_level_t level, void *ctx)
{
    (void)level;
    (void)ctx;
    HAL_UART_Transmit(&huart2, (uint8_t *)buf, len, 100);
}

void logging_init(void)
{
    mlog_backend_t backend = {
        .write = uart_write,
        .ctx = NULL,
        .level = MLOG_INFO,
        .color = false,
    };

    mlog_t *log = mlog_global();
    mlog_set_clock(log, HAL_GetTick);
    mlog_add_backend(log, &backend);
}

void app_run(void)
{
    MLOG_INFO("BOOT", "ready");
    MLOG_WARN("BATT", "Low battery: %u%%", 17u);
}

• Every emitted record ends with \n.
• If ANSI color output is enabled for a backend, microlog reserves room for the reset sequence before the newline.
• Long tags and messages are truncated deterministically to preserve a terminating NUL and the final line ending.
• MLOG_NONE is a filter-only level. It must not be used as an emitted message level.
• Backend callbacks are synchronous. The callback buffer is borrowed stack storage and must be copied before deferred use.
• Backend write failures are not propagated through the API.
• The library is not internally synchronized. Concurrent access and recursive logging require external discipline.

Manual build:

cc -std=c99 -Wall -Wextra -Wpedantic -Iinclude src/mlog.c tests/test_all.c -o test_all
./test_all

Local make-based test flow:

make -C tests
make -C tests compile-checks

CMake package flow:

cmake -S . -B build -DMICROLOG_BUILD_TESTS=ON
cmake --build build
ctest --test-dir build --output-on-failure
cmake --install build --prefix install

• API Reference
• Cookbook
• Design
• Porting Guide
• Troubleshooting
• Verification
• Contributing
• Security
• Changelog
• Release Workflow

Recommended reading order for new consumers:

1. README.md
2. docs/COOKBOOK.md
3. docs/API_REFERENCE.md
4. docs/TROUBLESHOOTING.md
5. docs/VERIFICATION.md

Releases are tag-based. Branch pushes do not create releases; only pushed v* tags trigger .github/workflows/release.yml.

MIT License. See LICENSE.