Structured logging for embedded systems in portable C99.

Zero dependencies. Zero dynamic allocation. Pluggable backends. Predictable output.

Embedded projects usually start with scattered printf calls and quickly grow into an unstructured mix of debug noise, warnings, and production diagnostics. microlog provides a compact logging layer with levels, tags, timestamps, and multi-backend routing while staying simple enough for firmware and low-footprint systems.

MLOG_INFO("MQTT", "Connected to %s:%d", host, port);
MLOG_WARN("SENS", "Temperature out of range: %.1f", temp);
MLOG_ERROR("CONF", "CRC failed, using defaults");

Example output:

12.345 [I] MQTT: Connected to broker.local:1883
12.350 [W] SENS: Temperature out of range: 85.3
12.351 [E] CONF: CRC failed, using defaults

• Five log levels: TRACE, DEBUG, INFO, WARN, ERROR
• Compile-time filtering via MLOG_LEVEL_MIN
• Runtime global and per-backend level filtering
• Up to MLOG_MAX_BACKENDS simultaneous outputs
• Optional millisecond timestamps through a clock callback
• Optional ANSI colors per backend
• printf-style formatting through vsnprintf
• Hex dump helper for protocol and transport debugging
• Global singleton and explicit logger instances
• No heap allocations and no external dependencies

include/mlog.h        Public API
src/mlog.c            Implementation
tests/test_all.c      Test suite
tests/Makefile        Local test runner
docs/API_REFERENCE.md Complete API reference
docs/DESIGN.md        Design rationale
docs/PORTING_GUIDE.md Integration patterns

Add these files to your project:

• include/mlog.h
• src/mlog.c

#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_t *log = mlog_global();
    mlog_set_clock(log, HAL_GetTick);

    mlog_backend_t uart = {
        .write = uart_write,
        .ctx = NULL,
        .level = MLOG_DEBUG,
        .color = false,
    };

    mlog_add_backend(log, &uart);
}

MLOG_INFO("BOOT", "Firmware v%d.%d.%d", MAJOR, MINOR, PATCH);
MLOG_DEBUG("WIFI", "Scanning for networks...");
MLOG_WARN("BATT", "Low battery: %d%%", percent);
MLOG_ERROR("MQTT", "Publish failed: %s", err_str);

uint8_t packet[] = { 0x10, 0x0E, 0x00, 0x04, 'M', 'Q', 'T', 'T' };
mlog_hexdump(NULL, MLOG_DEBUG, "PKT", packet, sizeof(packet));

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

cd tests
make

GitHub Actions runs this test flow automatically through .github/workflows/ci.yml.

Production build example:

gcc -DMLOG_LEVEL_MIN=2 -DMLOG_ENABLE_COLOR=0 -std=c99 -Iinclude src/mlog.c app.c

Full API reference: docs/API_REFERENCE.md

• API Reference
• Design Rationale
• Porting Guide
• Contributing Guide
• Changelog

microlog is intended for small systems and companion firmware libraries where deterministic behavior matters. Typical integrations include:

• UART or RTT console output during development
• Ring buffers for post-mortem diagnostics
• Transport/protocol tracing during bring-up
• Shared logging across small embedded support libraries

MIT License. Copyright (c) 2026 Vanderhell. See LICENSE.